CA1304453C - Blood pressure monitoring method and apparatus - Google Patents
Blood pressure monitoring method and apparatusInfo
- Publication number
- CA1304453C CA1304453C CA000553722A CA553722A CA1304453C CA 1304453 C CA1304453 C CA 1304453C CA 000553722 A CA000553722 A CA 000553722A CA 553722 A CA553722 A CA 553722A CA 1304453 C CA1304453 C CA 1304453C
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- Prior art keywords
- hold
- pressure
- down pressure
- polynomial
- pulse amplitude
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Abstract
ABSTRACT
Intraarterial blood pressure is measured noninvasively by an electromechanical transducer. The correct hold-down force to be applied to the transducer for obtaining accurate blood pressure measurements is determined by obtaining a set of at least one of the diastolic pressure, systolic pressure, and pulse amplitude versus hold-down pressure values over a range of hold-down pressures between which the underlying artery is unflattened and it is occluded. A polynomial is fitted to at least one of the sets of values, from which polynomial the correct hold-down pressure is determined. The hold-down pressure at the point of minimum slope of graphs of the polynomials fitted to the systolic and diastolic versus hold-down pressure values provides an indication of the correct hold-down pressure. An indication of the correct hold-down pressure using the pulse amplitude measurements is provided by locating the midpoint of a pair of hold-down pressures at which the pulse amplitude is substantially sixty percent of the maximum pulse amplitude on the graph of the polynomial fitted to the pulse amplitude versus hold-down pressure values. An alternative method determines the correct hold-down pressure directly from the pulse-amplitude polynomial coefficients. A measure of the compliance of the underlying artery is obtained from the ratio of the minimum slope of the graph of the polynomial fitted to one of the systolic and diastolic versus hold-down pressure values to the slope of a straight line fitted to a subset of one of the systolic and diastolic versus hold-down pressure values over a range of hold-down pressures below which flattening of the underlying artery occurs.
Intraarterial blood pressure is measured noninvasively by an electromechanical transducer. The correct hold-down force to be applied to the transducer for obtaining accurate blood pressure measurements is determined by obtaining a set of at least one of the diastolic pressure, systolic pressure, and pulse amplitude versus hold-down pressure values over a range of hold-down pressures between which the underlying artery is unflattened and it is occluded. A polynomial is fitted to at least one of the sets of values, from which polynomial the correct hold-down pressure is determined. The hold-down pressure at the point of minimum slope of graphs of the polynomials fitted to the systolic and diastolic versus hold-down pressure values provides an indication of the correct hold-down pressure. An indication of the correct hold-down pressure using the pulse amplitude measurements is provided by locating the midpoint of a pair of hold-down pressures at which the pulse amplitude is substantially sixty percent of the maximum pulse amplitude on the graph of the polynomial fitted to the pulse amplitude versus hold-down pressure values. An alternative method determines the correct hold-down pressure directly from the pulse-amplitude polynomial coefficients. A measure of the compliance of the underlying artery is obtained from the ratio of the minimum slope of the graph of the polynomial fitted to one of the systolic and diastolic versus hold-down pressure values to the slope of a straight line fitted to a subset of one of the systolic and diastolic versus hold-down pressure values over a range of hold-down pressures below which flattening of the underlying artery occurs.
Description
~30~
BLOOD PRESSURE MONITORING METHOD ~ND ~PPARATUS
ORIGIN OF THE INVENTION
The ln~ention described herein wa~ made ln the course of work under a Brant or award from th~
5 Department of ~ealth and Human Services.
TECHNICAL FIELD
This invention relates to method and app~ratus for non-in~a~i~ely ~onitoring blood pres~ure through ~se of a tran6ducer ~rray of individual pressure or force senslng elemeDts, ~nd to method and means for ascertalning the correct transducer hold-down pressure required for obtaining accurste blood pre~sure mea~urements.
BAC~GROUND OF THE INVENTION
The contlnuous measurement of blood pressure by use of srterlal tonometer traQ~ducers i~ well known ~B ~hown in U~S. Pstent Numbers 3,123,06B to R.P.
Bigliano, 3,219,035 to G.L. Pre6sman, P.M. Newgard and John J. ~ige, 3,880, 145 eO E.F. Bllck, 4,269,193 to the present i~entor, and 4~423,738 to P. M. New~ard, and in e~ ~rticle by G.L. Pres~msn and P.M. Newgsrt entltled "A
Tran~ducer for the Contlnuous External Mea~ure~ent of Arteridl Blood Pressure" (IEEE Tran~. Bio Med. ~lec., Apr~l 1963, pp. 73-81).
In a typicfll tonometrlc technique for ~onitorlng blood preseure, a transducer ~hich includea ~n crray of pres~ure ~e~siti~e element~ 15 positioned o~er a super~lcial artery, ~Dd a hold-down force i6 2 ~l30~Æ53 applied to the tran6ducer BO ~8 to flatten the wall of the underlylng artery ~lthout occlutlng the artery. The pres~ure sen~lti~e element6 ln the array typicslly have at least one dimension ~maller th~n the lumen of the 5 underlyin~ ~rtery ln which blood pressure is measured, and the transducer i6 positioned ~uch thst ~t lea~t one of the indi~idual pres~ure-sensitiYe element6 iB oYer ~t lea~t a portlon of the underlylng artery. The output from one of the pre~sure ~ensitive elementB iB ~elec$ed for monltoring blood pre~6ure. In some prior art arrangements, the pressure sen~iti~e element having the m~imum pulse amplitude output is selected, ~nd in other arrsnge~ent~ the element haYing a local minimum of diastolic or systolic pressure which element i~ within substantially one artery diameter of the element which generates the ~a~eform of maximum pulse amplitude i~
selected . , - .
The pre~sure mea6ured by the selected pressure sensitive element, i.e the element centered over the artery, wlll depend upon the hold-down pressure used to press the transducer ~gain6t the skin of the sub~ect.
Although fairly accurate blood pressure measurements ~re obtained when 8 hold-down pres6ure within 8 rather wide pre3sure ran8e ls employed, lt has been found that there exists a sub~t~ntlslly unique ~lue of hold-down pressure within said range for which tonometrir measurements are most accurate. Thi6 so-called "correct"
hold down pressure varies among sub~scts. Wlth prior art tonometrlc type transducerR ~he correct hold-down pressure often 18 not determlned thereby leading to inaccur~cles ln the bloot pres~ure ~ea~urements.
SUMMARY ~ND OBJECT~ OF T~E INVENTION
A~ ob~ect of this ln~ention 18 the pro~ision of an improYed ~onometric type method and appar~tus for 3 ~ 4~3 non-in~a6i~ely monitoring blood pres6ure wl~h a hiph de8ree of accur~cy.
Another ob~ect of thi3 lnventlon ls the pro~i6ion of ~uch a blood pressure messuring method and 5 appur~tu6 which includes the use of a transducer h~ving ~n arrsy of ind~idual arteri~l rlders (pressure se~sltive elsment~) and wherein mesns sre provided for determining the correct pres6ure required to hold the tr~nsducer ug~inst the sub~ect to assure ~ccuracy of the blood pressure readin~s.
The present lnvention includes a tr~nsducer arr~y for gener~tion of electrical waYeform~ indicati~e of blood pres~ure ln an ~rt~ry. Using the ~elected pressure sensing element that 18 determined to be poæitioned substsntially o~er the center of the unterlying artery, n set of st least one of the diastollc pressure, eystolic pressure, or pulse amplitude pre~sure ~ersus hold-down pressure values o~er a ran8e of hold-down preBsures between which the underlying 8 rtery i~ unf 18 ttened and ls occluded is obtained. ~ polynomial iB fitted to at least one of the set6 of ~alues from which polynomial the correct hold-down preBsure 1B tetermined. The hold-down pressure at the polnt of minimum slope of graphs of ~he polynom~al6 fitted to the ~ystolic or diastolic ~ersus hold-down pressure ~alues provides An indication of the correct hold-down pressure. ~n indication of the correct hold-down pre~sure using the pulse 0mplitude meB8Urement~ 16 proYided by loc~ting the polnt where the slope of the polynomi~ zero or the midpoint of ~
pair of hold-down pre6sures at whith the pul~e smplltude 1~ sub6ts~tiHlly ~isty percent of maslmum pul~e zmplltude of the ~raph of the polynomial fitted ts the pulse a~plltude qer~us hold-down pres~ure ~alues. A
~easure of compllance of the underlying ~rtery ls 4 ~l30~53 obtai~ed fro~ the ratio of tbe mini~um 810pe of the graph of the polynomial fltted ~o one ~f the ~y6tolic or diastollc pres~ure ~er~us hold-down pre~sure values to the ~lope of a straight line fitted to a ~ubset of one of the systolic or tlastolic ~ersus hold-down pressure Yalues oYer a rsnge of hold-down pressures below wh~ch flattenlng of the underlying artery beBin6 or it may be obtained dlrectly from the polynomial coefficlents. The hold-down presæure st which fla~tening of the underlying artery be8ins i8 taken as the lowest of the abo~e-mentlonea pair of hold-down pressures at whlch the pul6e amplitude is subst2ntially sixty percent of maximum pulse amplitude of the graph of the polynomial fitted to the pulse amplltude versus hold-down pressure ~alue6.
BRIEF DESCRIPTION OF THE DRAWINGS
The in~ention, ~ogether with the abo~e and other ob~ects snd adYantages thereof will be better understood fro~ the followins description when considered with the accompanying drawings. It vlll be understood, howe~er, that the lllustrated embodiments of the invention are by way of example only and that the ln~entlon is not limited thereto. The noYel fe~tures which are believed to be ch~racteristic of the inYention are set forth wlth particularity in the appended claims.
In the drawlngs, whereln like reference character6 refer to the same parts in the se~eral ~iews:
Flg. 1 shows the external appearance of a blood pres6ure transducer csse, typically po itioned o~er a superficial artery auch a6 the radial srtery, f~r pro~id$ng ~ continuouB external measurement of arterial blood pres~ure;
Fig. 2 1~ a schematic dlH~ram lllustratin~
the force balance bet~een thP artery and the multiple transducer elem~nts tarter$al riders), with the artery .~3~4~53 wall properly depre~aed ~o giYe accur~te blood press~re readin~6;
Fig. 3 1~ a combination 6implified ~ertlcal sectional ~iew taken through the transducer case of Fig.
1 and block dlagram of a system wh$ch may be employed there~ith ln the practice of thi~ ~n~ention;
Fi~. 4 is & waYeform of human blood pres~ure versus tlme of the type which may be obtained using ~he present inve~lon for illustrating syctolic and 10 disRtolic pre66ures and pulse amplitude of the blood presBure wsve;
Figs. 5A and SB to~ether show a flow chart for use in explaining o~erall operation of this lnvention;
15Fig. 6 ~ho~ plots of diastolic pressure snd pulse amplitude ~er6us hold-down pressure for a typical ~ub~ect; and Fi~. 7 iB a flow chart showin~ details o~
the computstion of a correct hold-down pressure.
20A typical appllcation of the transducer array f or arterial tonometry 1B illustrated in Fig. 1 whereln the transducer housiDg, or case, 10 whlch may ha~e the appearance of ~n ordinary wristwatch c~se, is held in place over the radial artery ln a human wrist 12 by a band 14. ~ cord 16 extends from the tran~ducer housl~g 10 through which electrical wiring for the transducer array~wlthin the housing, together with a 8m811 tube that conDects the housing to an ~ir pressure ~ource, e~end. The wirln~ 18 and tube 20 are 6hown in Fi~. 3, but not ln Fig. 1.
Reference now i8 made to Fi8. 2 wherein a diagr~mmQ~lc mechsnic~l model 18 shown which is representstl~e of fsctor~ to be considered in the physical syste~. The illu~trated model is that shown in the ~boYe-mentioned J.S. Eckerle p~tent number 4,269,193 6 1~04~5i3 ~hlch ~as ~dap~ed from the G.L. Pre~man and P.M.
Newgard ~rtlcle entitled "A Trsnsducer for the Continuous External Messurement of ~rter$al Blood Pressure~. In brlef, an ~rray 22 of lndi~dual pressure sensiti~e element6 or transdu~ers 22A through 22J which con~titute the ~rterlal riders, i8 po61tioned ~o that one or more of the rider~ ~re entirely o~er ~n artery 24. The lndi~idual riders 22A-223 ~re ~mall relatlve to the diameter of the srtery 24 thus assurin~ that at least one of the rider~ in it~ entirety ls over the artery. The skln surface 26 and artery underlying the transducer must be flattened by flppl~cation of a hold-down pres~ure to the transducer. One rider overlying the center of the artery is ldentlfied as the "centered"
srterial rider, from wh~ch arterial rider pressure readin86 for ~onitoring blood pressure are obtained.
Mean6 for selecting the arterial rlder are disclosed in the above-mentioned J.S. Eckerle ps~ent and G.L.
Pressman and P.M. Newgard ~rticle. Using the sbove-ment~oned rider selecting means, rider 22E, for example,may be selected as the ~centered" srterial rider, in ~hich csse the remainder of the riders, here riders 22A-22D and 22F through 22J comprl~e side plates which ~erve to flatten the underlylng ~kin snd srtery. Depending upon the pO8~ tioning of the transducer on the sub~ect, a different trsnsducer element may be positioned oYer the center of the artery and thereby be ~elected 3S the "centered" arterlsl rider.
Superficial arteries, such as the radial artery, are ~upported fros below by bone which, in Fig.
2 1B illu~trated by ground symbol 28 under ~he srtery~
The w~ll of srtery 24 behs~es ~ubstantlally like a ~embrsne ~n that it transmlts tension force6 but ~ot bendlnR ~o~snts. The ~rtery wnll re6ponds to the loading force of the transducer, snd during blood :: .
7 ~3~53 pressure measurements 8Ct6 as lf it i8 resting on the firm ba~e 28. The effecti~e ~iffness of an srtery wall i~ ~mall And differs between 6ub~ects. In prior art mech~nlcsl models of the physical ~ystem, the effecti~e stiffDess of the ~rtery wall i6 taken ~s zero, ln which case the actual hold-down pressure employed iB not consldered to affect ~ccur~cy of the blood pressure readingB BO lon~ as the transducer i6 pressed agalnst the 6kin surface with sufficient force to cause compre~sion but not occlusion of the underlying ~rtery.
Appllcant ha~ found thst not only are blood pre6sure readings dependent upon hold-down pressure within the ran~e of hold-down pressures that the sr~ery is flsttened but not occluded, but thst most accurate blvod pres~ure resdings ~re obtained where a hold~down preasure is selected thst iB subætsntially midway between the pressure where flattening of the ~rtery begln6 and the minimum pressure required for occluding the ~ame. Novel steps in~olved ln computing the correct hold-down pressure are described ln detail hereinbelow following completion of the description of the mechanic~l model of Fi8. 2 snd the oversll system 6hown in FiB. 3.
With the illu~trsted system, the trRnsducer csse 10 ~nd mounting str~p 14 together with sir pressure applied to ~ ~ellows, 54, supply the required compres6ion force and hold the riders 22A-22J in such a manner ~hat ~rterisl pressure chsnges are transferred to ehe riders ~hich oYerlie tbe ~rtery 24.
Di2grAmmat~c~lly thls 18 illustrated by showing the lndi~idual riders 22~-22J backed by rlder sprlng me~ber~ 30~-30J, re~pec~i~ely, a rigld ~pring bscking plste 32, aDd ~ hold-down force generator 36 between the b~cking plate 32 and the ~ounting strap system 38.
If, without force generator 36, the ooupling 4~53 between the mountlng s~Lrap 8y8tem 38 and sprlng backing pl~te 32 ~ere infinltely ~tiff ~o restrain the riders 22A-22J rigldly with respect to the bone structure 2B, the riders would be maintained in a fi~ed positlon relati~e to the artery. In practice, bowe~er, 6uch a ~y~tem i8 not practic~l, and hold-down fo~ce ~enerator 36, comprising a pneumatic or other 6uitable loadin~
system, is lncluded to keep const3nt the force applied by the mountin~ ~trap system 38 to riders 22A through 22J. In the mechanical model the 6prlng con~tant) k (pressure ~er unit of deflection) of the force generator 36 i8 nearly zero. Suitable pneumatic loading systems are shown and described in the abo~e-reerenced U.S.
Patent number~ 3,219,035, 4,269,193 and the Pressman-Newgard IEEE article.
In order to insure that the riders 22A through22J flatten the artery and pro~ide a true blood pressure measurement, they must be rigidly mounted to the backing plate 32. Hence, the rider ~prings 30A through 30J of the model ideally are infinitely ri~ld (~pring constant k .O~ ). It i8 found that as long as the sy~tem operates in such 8 mBnner that it can be ~odeled by rider spring6 30A ~hrou~h 30J having a spring constant on the order of about ten t~mes the value f~r ~he artery-~kin syBtem~ 80 that the deflection of rider~ 22A
throu~h 22J i6 small; 8 true blood pressure messurement may be obtained when the correct hold-down pressure is employed.
The octual physical structure of a prsctical transducer of a type which may be employed for transtucer ~rray 22 in the pre~ent ~ystem i 6hown ln the aboYe-mentioned J.S. Eckerle patent No. 4,269,193.
There, ~ transducer array i8 shown in which the ~ndl~idusl tran~ducer~ (riders) are formed in a thin ~onocrystslline silicon ~ubstrate which 18 made using ~: , ..
9 ~ 3 integrated circuit technlques. In Fig. 3, to which reference now i~ made, a slmplified ~ho~ing of trsnsducer 22 iB shown co~prislng a chlp 40 which lncludes an array of individual tr~nsducers, not shown.
Electrical conductors 42 connect the indivldual tr~n~ducers to the wlring 18 for connect~on thereof to a multiple~er 43.
A6 seen in Fig. 3, ca~e lO compri~es a gener~lly cylindrlcal, hollow, contsiner having rigid back snd aide walls 44 and 46, respecti~ely. The silicon tr~nsducer chip 40 i6 moun~ed within the face 48 of the ca~e ~de~ignated as the front or operati~e face) ln ~ cyli~drical cup-like tran6ducer housing 50. The operati~e foce 48 lncludes the silicon transducer chip 40 along with its included lndlvidual transducers and arterial riders. Chip 40 may be affixed to a conventional ceramic dual in-line package that i~
plugged into an associated dual in-line socket, neither of whlch are shown in the drawings. A silicone rubber filler 52 iB proYided lnside the housing 50 and sround the dual in-line package and 60cket to provide a good sesl, pre~ent electrical leakcge between the transducer circuits and housing 50, and provide a flat fiurface to press ag~inst the sub~ect. The front face 48 of the transducer includes the lower face6 of hou~lng 50 and filler 52.
The transdu~er hou~ing 50 i~ fixed to the inside of the tranaducer CBBe 10 by me~ns of a cup-like sllicone rubber bellows 54 whlch i8 sealed ~round the lower outalde lip of the cup-shaped transducer housing 50, e~tends upwardly inside the outer wall of the tr~n~ducer case 10~ and is ~ealed to a ring 56, whlch in turn iB fl~ed and se~led to the inside back of the trsnsducer cs~e lO. A chamber i8 formed $nside the bello~ ~hich iB connected to on ~ir pressure source 58 \ I
lo ~30A~53 through tube 20. A pres6ure controller 58A may be lncluded ln the pre~sure ~ource. Slnce the flexible bellows 54 1~ ~ealed both to the ~ran~ducer hou~ing 50 and the inside of the transducer case 10, flir under presxure from source 58 pneumstically losd~ the operatlYe face 48. With ~he trsnsducer ~trapped to the subject'~ wri6t, the hold-down force Fl exerted by the transducer array a8s~nst the skin of the 6ubject is ad~ustsble by control of the ~ir pres6ure. tIn the diagrammatic ~echanical model 6hown ln Fig. 2 the hold-down force Fl i8 generated by hold-down force generator 36.) As ~oted above, electrical signals related to pressure 6ensed by the indi~idual transducers 22A-~2J of transducer 22 are supplied a~ inputs to an analog multlplexer 43. From the multipleser, the signals are digitized by ~n analog-to-digital (A-D) converter 60, and the digitized slgnals sre 6upplied to B digital computer 62 hs~ing memory 62A and a clock 62B. Other information, ~uch a6 the sub~ects name, sex, weight, hei~ht, s~e, arm/wrist dimensions, and the like, may be fiupplied to the computer throu~h 8 keyboard 64. Output from the computer i~ supplied to data dlsplay and recorder means 66 which may include a recorder, cathode ray tube monltor, a sQlid state di6play, or the like.
If desired, at least a portion of the ~isual display may be included in transducer case 10. In fact, all of the components ~hown ln Figure 3 may be lncluded in the case lO witbout depar~ing from the prlnciples of thi6 lnventlon. Obviously, the computer output may be supplied to a printer, un oudible slarm, or the ~ike, a~
desired. ~1BO~ an output from the computer iB ~upplied o~er llne 68 to the pressure controller for control of the transducer hold-down pressure.
In Fi8. 4, to whicb reference now 16 msde, the 1 1 ~30~53 signsl waYeform of the output from one of the pre sure sensitive elemen~ 22A through 22J which overlles ~rtery 24 i8 ehown. Other element~ of the ~ransducer arrsy which oYerlie the ar~ery ~ill have wAveforms of almilar shape. ~ith a correct hold down pressure and correct selection of the ~centered" arterial rider (i.e. the rlder ~ubstantislly centered o~er the artery~ the ~aveform i~ representstiYe of the sub~ect'6 blood pressure ~ithiD the underlying artery. Systolic, diastollc and pulse ampl~tude pressures are indicated on the wa~eform, wherein pulse amplitude ls the difference between the ~ystolic and diastolic pressures for a giYen heartbeat.
OVERALL SYSTEM OPERATION
Fi~s. 5A and 5B, together ahow a flow ch~rt of an algorithm for gener~l, overall, operation of the blood pressure monitorlng system. Some of the operstions indicated there~n are under control of computer 62 responsive to programming instructions contained in memory unit 62A. Obviously, one or more progr~mming ~tep~ may be in~olved in the actual implementation of the lndicated operations. Since the progrsmming of ~uch 6tepB iB ~ell within the skill of the s~ersBe programmer, a complete program listin~ is not required ~nd i8 not included herein.
Preparation for monitoring is begun at START
~tep 100 ~t which time ~ystem power 1B turned on or a re~et operation læ per~ormed, by mesn6 not shown, and counter~, regi~tere, tlmera, and the like ln computer 62 ore lDitialized. At ~tep 102 information concerning the ~ub~ect, ~uch A~ the sub~ect'6 name, ~e~, weight, height, ~ge, arm ond/or ~rist dimensiona, ~nd the like, is entered into the compute~ memory thrsu~h U8e of the keyboard 64. Ne~t, at 6tep 104, ~ nominsl hold-down pres~ure (~-D.P.) i8 applied wherein ~ir under pressure ~3C)~53 from source 58 ls 6upplied to the transducer. For example, 8 hold-down pressure of ~ay 120 mmHg may be supplied to the transducer~ which pressure ser~es to extend the bellows 54 whereby operati~e face 48 extends out~rdly a 6hort d~stance from the bottom of the case lO. The tr~nsducer i8 attached to the ~ub~ect at ~tep 106 at a location wherein ~ centraily locsted transducer element, ~uch ~s element 22E of transducer srray 22 o~erlies the center of artery 24. Of course, the exact posltion of the transducer srrsy relati~e to the underly~ng artery generally 1~ not ~isually apparent to the subject, or operator, and repo.sitioning Qf the transducer may be required to properly position tne same .
~ith the transducer attached to the subject, step 108 ls enterea (select centered transducer element) at which point the transducer element which overlies.the center of the artery is identified. The location of the selected trsnsducer element is displsyed at step llO. If desired, the exact transducer element 22A
through 22J selected at step 108 may be di~plsyed at 6tep llO. Alternati~ely, a l~near array of, say, three lights may be provided wherein energization of the center light indicates that a centrally located transducer element wa6 Belected st Btep 108.
Illumlnation of elther of the end li~hts would lndicate thnt NoYement of the transducer to the right or left is required for proper posltioning of the transducer array relati~e to the underlyln~ artery. ~8 noted above, Yelection step 108 may lnclude means for selecting the one pressure sensiti~e element ~hat has a local mlnimum of at least one of the diastollc and ~ystolic pre~sures that ~B ~i~h~n substantially one artery diameter of the o~e pressure sensitl~e element which generates the wa~e~orm of ~aximum pulse amplitude, as disclosed in '~ : . '- ': '' ' U.S. P~tent Number 402S9,193. Processes whlch ~ay be ~mployed 1~ ~election ~ep 108, including identifying 6y~tolic and diastol~c pressure~, pul6e ~mplitude, ~s~im~, loc&l minima, ~nd the like, from the transducer outputs sre resdily ~mplemented u6ing diglt~l computer 62.
After the trsnsdueer ele~ent directly o~er ~h~
artery 16 ~elected and it~ location dlsplayed, decision step 112 i8 entered to determine whether sr not the selected element 18 nesr the center of the tran6ducer ~rr~y. If it i8 not, the transducer i6 repositioned on the subJect at 6tep 114, and ~tep 108 is reenterPd. The proce6~ is repeated until the transducer is properly loc~ted on the subject.
When tec~6ion step 112 i6 affirmative, the hold-do~n pre~sure to u6e i6 computed ~t 6tep 11~. An a$firmatlve decl~ion at 6tep 112 may require the operator to actuate an "Adjust Pressure" but~on, or the like, to enter ~tep 116 from 6tep 112. No~el algorithms which may ~e used in computing the correct hold-down pre6sure (step 116) are shown in Flg. 7 snd described below. For present purpoæes it will be under~tood that ~ correct hold-down pressure for accur~te blood pre~ure monitoring i6 computed at step 116, follo~in~ ~hlch, at step 118, the computed hold-down pressure i6 set by control of pre6sure con~roller 58A by the computer 62.
Data obtained and u6ed to compute the correct hold-down pre6sure (step 116) may be u~ed in the calculatlon of an ~ccur~cy index ~t Etep 119, which lndex ~l~ply co~prises ~ measure of the complisnce o~
the underlyi~g crtery 24. Generally, ~he more compli~Dt, or le86 ~tlff, the underlying artery, the better will be the ~ccurecy of the blood pres6ure ~eHsurement~. Means for computing the compliance and i 14 ~3~S3 accuraoy index are deRcribed ln deta~l hereinbelow. For pre6ent purpo~es, lt ~111 be understood that the accuracy indeY m~y be cslculated ~t s~ep 119 (Fi~. 5B) ~nd the ~lue thereof stored for di6plsy at B later step 5 with blood pressure messurements.
With the transducer properly positioned on the sub~ect and the correct hold-do~n pressure supplled thereto~ the syste~ ie ln condition for obtaining accurate blood pres~ure readlngs. At step 120 an ~ndication that the system i6 operati~e i6 provided, as by di6pl~y of the ~ords "Readings Val~dn. ObYiously, other d~spl~ys, such 8S a 8reen indicator light, ~ay be employed for indic~ting the opersting state of the ~y6te~.
From the output from the 6elected tran~ducer element, systolic and diastolic pres6ure ~alues toge~her with pulse amplitude values are readily determined in step 122. Also, pulse rate i6 readily calculated by determinlng the time between 6ucces6ive diastolic or 6y6tolic pressures. At 6tep 124, ~slue6 calculsted and determined ln step6 119 and 122 are displayed and/or recorded nlong with the actual waveform.
ObYiously, the Yalues whlch are calculated and di6played depend upon the use to be made of the blood pre6sure monitor, a dl~play of all of the ~alues not being requlred in ~any lnst~Dce6. For example, the blood pressure waveform could ~e recorded without cslculaeion ~nt di~play o~ any of the ~lues identlfied ln ~teps 119 and 122.
After the value6 ldenti$ied in step 124, such a~ systollc Dnt/or dla~tolic pres6ure, 3re displayed, step 126 i8 entered ~erein the 6ystem ~alt~ for the next heart~eat cycle. Dia~tolic or systolic pressure point~ 0ay be used to ldenti~y reference point6 ln the 35 hesrtbe~t cycle6. Deci6ion ~tep 128 then ls entered at ~ 4~53 ~hich time 6 timer ln computer 62 18 tested to determine ~hether or not it ha~ reached ~ predetermined time "M", where M 1B a tlme period of, 8~y, 30 minu~e6. If the el~psed tl~e exceeds the predetermined time period, M, the deci6ion i6 affirmatiYe, the timer 1~ re~et at step 130, hold-down pressure i~ reduced to appro~im~tely 120 ~mH8 at step 131, and step 116 i6 reentered for recomputstion of ~he correct hold-down pressure and resetting thereof, if required. Periotic checking and resettlng of hold-down pressure helps to ~ssure long-term accuracy of the blood-pressure readings.
If the predetermined time period has not been exceeded, 6tep 132 is entered for qelection of the centr~l transducer element, which step is the ~ame as step 108 described abo~e. Decision step 134 then ~s entered in which the selected transducer element deter~ined ln ~tep 108 1~ compAred to that determined in step 132. If there ls no change in the 6elected transducer element, step 120 is reentered indicsting thst the readln~s are ~alid. However, if there has bPen a change in the ~elected transducer element, ~uch that decl~lon step 134 ls ~ff~rmati~e, then step 136 is entered wherein the newly-determined eelected transducer is displayed. Thi~ ~tep corresponds to step llO wherein either the actusl ~electad trsn~ducer element iB
identified, or arrow6 or llghts indlcate the direction tha~ the tran~ducer needs to be mo~ed to recenter the ~ame over the underlying artery. A ~arnlng is l~sued at ~tep 138 indlcatlng to the ~ub~ect or operator that movement of the trsn~ducer relatiYe to the artery hs~
taken place. If deslred~ the transducer then m~y be reposltioned and the process researted. If the tran~tucer i~ not repositloned, end ~he process 18 no~
termlnsted and restarted, step 120 i~ reentered for contlnuatlon of the monltoring process, but now u ing . .
16 iL;304~5~
the output from the newly selected tran~ducer element.
DET~RMINATION OF OLD-DOWN PRESSUP~E
1) Di~stolic pressure vs ~old-Down Pres~ure Method Reference now 18 made to Fig. 6 wherein plots of diastolic pres6ure and pulse smplitude ver~us hold-down pres6ure are shown which wlll facllitste an under6tanding of novel meAn~ for determinin~ correct hold-down pressure for accurs~e blood pressure measurement6. The method for obt~ining the d~ta points in this plot wlll be described below. A third-order polynomisl i~ fitted using, for example, least squares techniques to the Fig. 6 series of diastolic pressure points to pro~ide a curve 140 which has the typical shspe 6hown reg~rdless of physical characteristics of the sub~ect.
A thlrd-order polynomisl fitted to the measured data may be written aR follows:
Pm - aO + 8lPh ~ a2Ph ~ a3ph (1) whereln:
Pm ' measured diastolic pressure, Ph - hold-down pressure, and aO, al, e2~ and a3 are coefficients of the polynomisl.
For hold-down pressures between zero and Pl, the uDderlyin~ artery remains unflsttened, and the measured pre88Ure iB pri~arily dependent upon the hold-down pressure snd secondsrily upon the intraarterial pressure, Pa. The graph of the polynomial ls a relatively ætrai8ht line o~er this range. Up to pre~ure Pl the effecti~e spring const~nt of the artery, uslng the mechanical model of the system shown in FiR.
2, i6 large.
Between hold-down pressures Pl and P2 the hold-do~n pres6ure ls 8rest enough to part~lly flstten ~ . ~
. . . - .
~04~53 the underlyiDg artery, but no~ 8reat enough to occlude lt. Experiment hss shown thBt ~DoBt accurate blood pre3sure mea6urement~ are sbtained when a hold-down pre6sure that i~ Rub~tsnti~lly mitway between presRures 5 Pl and P2 ls employed. Between pressures Pl and P2 the effectiYe ~pr$n~ coDstant of the artery using the olechan~cal model of Fig. 2, i6 relDtively small.
At hold-down pressures greater than P2, the underlying artery i6 completely occluded, and the effectiYe spring con~tant of the underly~n~ artery is a~in relatl~ely l~rge. Consequently, the me~sured pres~ure i6 again ~ubstantially independent of the intraarterlsl pre~ure, Pa, and the curYe ls sub~tantially 8 strai8ht line above pressure P2. As ~een in Fig. 6, the ~lope of curve 140 i~ lowest between pressure~ Pl and P2 where the underlying ~rtery is flatte~ed but not occluded. A~ noted above, ~ubstantislly the center, or midpoint, of this region of lowest slope, bet~een Pl and P2, i6 the corr ct hold-down pressure for obtainin~ accura~e blood pressuremeasurement6. This midpoint also substantially coincldes wlth the point ~here the slope of the grsph of the polynomial (equation 1) is minimum. Therefore, the correct hold-do~n pressure value iB readily determlned 25 by locating the mini~um slope point using coefficients of the graph 140 of the polynomial fitted to the di~stolic pre~sure point~. In particulsr:
p3 , ~2 ~2) where: P3, the polnt of mlnimum ~lope, is the correct hold down pressure, and ~ 2 ana a3 are the roefficlents of the second and third degree terms of the third-order polynomial.
- ~
18 ~ 53 For curYe 140 of Fl~. 6, a2 ~~ 0.084760 and a3 . 0.00014431 whereby, ~rom equat~on (1) a correct hold-down prçssure of approxlmately 196 mm~g i~
indlcated.
52) Systolic Pressure vs Hold-Down Pressure Method Instead of using a plot of diastolic pressure ~s hold-down pressure to determ~ne the correct hold-down pressure, a plot of systolic pressure versus hold-down pressure points msy be employed. The method is the same a~ that described sbo~e except that a third order polynomlal i6 fitted to the series of systolic pres~ure points, and equatlon (2) $s appliea to the re~ultant polynom i81 to pro~ide an lndioation of the correct hold-down pres6ure.
153) Pul~e Amplltude Y8 Hold-Down Pressure Method A. A third method of determining the correct hold-down pressure to u~e in monitoring blood pressure lnYol~es the use of the plot of pulse smplitude vs hold-down pressure point3 shown in Fig. 6. As with the plot of dlastolic pre6sure ~s hold-down pressure, a third order polynomial is fitted to the series of pulce amplitude polnt~ which results in a generally ln~erted U-shaped cur~e 1~2. It has been found that value~ of hold-down pressure corre~ponding roughly to pressures Pl ~nd P2 shown in Fi8. 6 msy be iound by taking the pre~sures where the pul~e amplitude i~ 0.6 times the maxlmum pulse amplltude on the graph of the polynomi~l, 142 . In Fi 6, these 0.6 maximum hold-down pressure points ore ldentified as pll ~nd P2~. The correct hold-down pressure, P3', i8 the Yalue substantlally midwaybet~een these points. Uslng thl~ method, the correct hold-to~ press~re IB
- ~ .
: ' .
- . :
- - ' .
.~ ' ~ .'' :', ' .
. ~ . ~'-:
~30~5;3 2 (3) For cur~e 14~ of F~8. 6 ~ correct hold-down pres~ure of ~pproYimately 195 mmHg B indicated by Eq. (3). For the ~ub~ect fro~ ~hich the diastolic snd pulse amplitude 5 plot8 of Fig. 6 were obt~lned, the correct hold-down pressure determined u8ing equa~ion~ (2) ~nd (3) differ by only one mmHg. If "correct" hold-down pressure ~alue~ celculated u~ing the three abo~e-described methods agree within approsimstely 10 mmHg, then ~ub~tantially correct blood pre~sure measurements ~re obtaiDed usin~ 6ny one of 6cid calculated values.
B. In a variation of ~he pulse amplitude VB H-D.P.
method, the pressure, P3', may be found directly from polynomial coefficients. For example, if a 6econd-order polynomisl is fltted to the pulse amplitude ~s N-D.P.
data, ~ith coefficients aO, al, and a2, then P3' is gi~en by -al (4) 2~2 This corre~poud~ to the m~ximum of the polynomial.
Si~ r expressions may be used for third ~nd hi~her-order polynomials.
ARTERY ,COMPLIANCE - ACCURACY INDE:R CALCULATIONS
The accuracy of tonometric methods of the pre~ent type for measuring blood pressure i8 dependent upon the compllance, or ~tlf~ness, of the underlying artery; the ~ccuracy of measurement decreasing with incressed stiffness of the artery. A messure of the stlffnecs, or co~pliance, of the srtery may be obtained usln~ lnfor~Dtion contained in the diastollc (or .. ..... .. , , , _ _ __ _ 2~ 13~53 systollc) pre66ure curve 140 and the pulse amplitude curYe 142 of Fi~. 6. In particulsr, a meaeure of complisnce of the artery i8 provided by the ratlo of the slope,S2, of the tia~tolic tor Rystolic) pres~ure curve at the correct h~ld-down pres6ure P3, and the slope, Sl, thereof bet~een zero snd ~old-d~wn pressure Pl.
From the abo~e, lt will be aeen that a measure of compliance, C, of the underlying srtery ia C ~ S2 (5) ~s noted ~bo~e, pressure~ Pl' and P2', equal to 0.6 of the maximum pulse amplitude value of ~raph 142, zubstantially correspond to hold-down preæsures Pl snd P2. Slope Sl ia determined by fittlng a straight l~ne 144 to the messured diastolic ~ersus hold-down pressure points for hold-down preRsures less than Pl.
As noted sbo~e, flattening of the underlyi~g artery does not beg~n until hold-down preasure Pl iB reached. Slope S2 at the correct hold-down pressure P3 is re~dily obtainsble using coefficients of the polynomial 20 (equation 1) identlfylng curYe 140. In particular, S2 ~ Bl ~ (6) If the mea6ure of compliance, C, i6 Bmall compared to unlty (aay C<0.3) the srtery ~tiff~ess is relatl~ely low, ~nd lntr~arterial blood pressure wlll be measured relatiqely ~ccurately by the present tonometric method. Lar8er v~lues of C, appro~ching unity, inticate that t~e artery atiffnes6 ls relati~ely large, which may le~d to relati~ely lnaccurate measurements. The compllance psrame~er obtained from equation (5) i8 . . ' ' ' ' .
`: , :
~L3~ ;3 calculated at ~tep 119 after the polynomials for the ~y~tolic, di~stolic ~nd pulse a~plitude qersus hold-down pressure curYes are determined, and the calculated compliance parameter ~alue i~ 6tored for ti~play ~t step 124. The ~alue it~elf may be diæplayed, or indicstion6 that arCur~cy i8 "goodn, "fsir~, ~poorn, or the like, may be displ~yed, dependent ~pon the calculsted value.
Reference now i~ made to tbe flow ehart of Fig. 7 wherein deteils of ~tep 116 of Fi8. 5A for computing hold-down pressure are shown. A~ no~ed sbove, when deci~ion step 112 of Fig. SA i6 affirm~tive, indicsting thst the selected trsnsducer element i~ near the center of the tran~ducer array, the correct hold-dow~ pressure iB computed at step 116. As seen in Fi~.
7, thi~ ~tep (step 116) ~ncludes waiting for the next hesrtbeat at Btep 150, following which the centered transducer element 15 ~elected at gtep 152. At 3tep 154 syætolic snd dis6tolic pre sures are determined from the blood pressure measurement6 obtaiDed from the selected trsnsducer element, or rlder, snd the systolic and diastolic pressure ~slue~, along with the hold-down pressure employed, are stored in computer memory 62A.
It will be recalled that at step 104 (Flg. 5A) a nominal hold-do~n pressure of approsimately 120 mm~g was Z5 applied. Therefore, the first sy~tolic snd diastolic pressure values are obtained using the nominal, 120 mmHg, hold-down pre~sure.
~ t step 156 the hold-down pres~ure is lncreased by ~n incrementsl amount of, 6ay, 5 mmHg.
Decision ~tep 158 i~ then entered to determine whether or ~ot ~he hold-down pressure i~ greater than, say, 300 mmHg. If the dn~wer is negati~e, ~tep lS0 i6 reentered whereupo~ another ~et of sy~tolyr snd diastolic pressure6 are obtained end ~tored for thls lsrger hold-down pressure.
.
, .. . . .
, ~()4~53 After an entire set of ~y6tolic and dlastolicpre~ure ~alue& have been obtained for a range of hold-down pre~sure6 betwee~ 120 and 300 mmH~, decis~on ~tep 158 is affir~tiYe and ~tep 160 i~ entered wherein a set of pulse ~mplitude v~lues are calculated by subtraction of d$~stolic pre~sure from the a~sociated ~ystolic pres~ure Yalue. The pulse amplitude ~alues Are stored in memory along with the ~e~ of systolic aDd diastolic pressure ~alues.
At ~tep 16Z, a polyDomial fit (typically a third-order fit~ is computed for each of the syætolic, diAstolic and pul~e amplitude ~ersus hold-down pressure ~ets of dst~ obtained at ~teps 154 and 160. The shape of curve 140 ln Fig. 6 is representative of the shspe obtsi~ed for both disstolic and systolic pressure ~s hold down pre6sure plots, and cur~e 142 in Fig. 6 has a ~hape that 1~ representati~e of pulse amplitude versus hold-down pre6sure plots. The constants and coefficlent6 of the polynomials obtained at ~tep 162 are stored for u~e in step 164 where a correct hold-down pressure i8 calculated for each polynomial.
As described above, the correct hold-down pressure using the 8y6tolic ~nd diastolic versus hold-down pres~ure points i6 obtained using equatlon (2) which locates the point of minimum ~lope of the plot of the thlrd-order polynomial fltted to the points. In p~rticular, the hold-down pressure is taken ~6 the negatl~e of the coefficient o the ~econt degree term di~lded by three time~ the eoeffic$ent of the third degree term of the polynomial.
Uslng the set the pul~e amplltude ~ hold-down pressure point~, the correct hold-down pres~ure i8 deter~inet b~ first calculatlng a pul~e amplitude Yalue substanelally equal to slxty percent of maximum pulse empl~tude on the graph of the polynomial fitted to the - .
.
: ; :
.
, : :
.
23 ~.304~i3 polnt~. The two hold-do~n pre~sure ~alue~ along the graph ~t ~hich the pul6e ~mplltude ~alues ~re sub6tantlally equal to ~aid BiXty percent of ~axi~um pul6e amplitude are identified, and the mean ~lue of the~e two hold-down pre66ure ~lues i6 taken as the correct hold-down pre~sure. AlternatiYely, the maximum of the polynomial may be determlned directly from the polynom~al coefficients, which maximum value i6 tQken 8S
the correct hold-down precsure.
At decision ~tep 166 the three (or four) hold-down pressure ~alues calcul~ted in 6tep 164 ~re comp~red to determine if the Yalues substantially Q~ree, e.g., if they agree within, ~Ry, 10 mmHg of each other. If the correct hold-down values do subs~al~tlslly agree, the deci~ioD ~s affirmati~e and step 118 (F$g. 5A) is entered where the hold-down pressure i5 set within the range of computed ~alue~. If they do not ~ubstantially agree, the hold-down pressure 18 reduced to ~ low ~alue, say 120 ~mHg, at step 168 snd step 150 is reentered for redetermination of a correct hold-down pressure ~alue.
Although the operation of the blood-pressure monltoring system is belie~ed to be apparent from ~he sbove-de~cription, ~ brief descrlption thereof now will be pro~lded. After turning on or resettlng the 6ystem (6tep 100) infor~ation regarding the sub~ect $s eDtered into computer memory 62A through keybo~rd 64 (~tep 102).
A hold-down pressure of apyroximately 120 mmHg is supplied to the transducer ~hrough tube 20 from pre~sure ~ource SB (s~ep 104) sfter ~hich the transducer i~
~ttached to the ~ubJect (step 106). Outputs from the tr~nsducer elements 22A through 22J are digltized at anslog to digital con~erter 60 ~nd are supplied to digital co~puter 62 for proce6~ing. Us$ng outpu~6 from eac~ of the trsnsducer ele~ent~ 22A through 22J, the transducer element thst i8 sub6tant~ally centered oYer ' :
the underlylDg artery 1B selected as thst element from whl h blood pree~ure mes~urement6 are to be obtalned (step 108). A method of ~electing the centered transducer element employing analog circuitry ifi shown ln U. S. Pateht 4~26~,193, which method iR readlly implemented digit~lly u6ing digital computer 62. The locat~on of the selec~ed transducer element i~ diRplayed (~tep 11~) and if the selected element 18 not nesr the center of the ~rray, the array may be repositloned (steps 112 and 114) and the process of selectin~ the centered element i6 repested.
With the transducer properly positioned on the subject, the correct hold-down pressure to empl~y for obtaiDl~g accur~te blood pressure measurements is determi~ed (step 116). Four dlfferent methods of computing the correct hold-down pressure are disclosed, one or more of which may be employed in a monitoring ~ystem. The first, second, and third methods use ~y6tolic pressure, diastolic pressure, and pulse amplitude ~ersus hold-down pressure mea6urements, respectl~ely, which sre obt~ined o~er hold-down presQureR which rsnge from a pressure where the underlging artery i6 unflattened to a pressure where it iB occluded. The fourth method is a ~sristion of the third, using a formula of polynomial coefficie~ts rather thsn the aforementioned 60% polnt6.
After 8 heartbeat ~step 150, Fig. 7) the tran~ducer e~e~ent centered over ~he underlying artery le selected (step 152) ~nd systolic and dia~tolic preasure~ are determined from the blood pres~ure waYe~orm (Flg. 4) from the eelected transducer element.
These maximum and minimum pre6sure points sre readily determlned by digital computer 62 UB~ n8 known progr~mming methots. The sy6tolic and diastolic ~ressures sre 6tored in computer memory 62A together '' .. . ' . ' ' -- ,' :
~ 4S3 wlth the associated hold-down pre~sure. The hold-down pressure then i~ lncrea6ed ~n incremental amount (s~y by 5 mmHg) ~ ætep 156, and, if the hold-down pre sure i8 le6s thsn about 300 mmHg, the ~y6tollc ~nd di~stolic pressure ~alues for the new hold-down pres~ure are obtained aDd stoTed in computer memory 62A. If the hold-down pressure iB ~reater than 300 mmHg (~tep 158),whlch i6 ~ pressure greater than that required for occlusion of the underlyin~ srtery, the ~cquisition o systolic and di~tollc pre6sures ~er~us hold-down pressure ~alues is 6topped, and pulse amplitudes for the collected dat~ ~re calculsted and stored. As ~een in Fig 4, pulse ampl~tude simply comprises the difference ln systolic and diastolic pressures for a given cycle of the blood pressure wa~eform.
Third order polynomials using a le~st ~quares method, for example, are fitted to the ~ystolic, disstolic and pulse amplltude ~er~u~ hold-down pressure ~slue6 (6tep 162). From the~e cur~es, correct hold-down 2f) pressure6 are c~lculated (step 164). From the ~ystolic and di~Etolic pre6sure cur~es, the point of mlDimum ~lope i~ determined to provide an indication of correct hold-down pressure. This minimum slope point is located substsnti~lly midwsy between hold-down pressures at which flattenlng of the underlying artery begins, and at which the artery is occluded. From the pulse amplitude curve, hold-down pres~ure Yalue~ at which the pulse amplltude ls si~ty percent of maximum are determlned9 which ~Alues sub~tantislly equal hold-down pres~ure~ at 30 which flattenln~ of the underlylng artery be8in~ and ~t which the artery ~ occluded. Hold-down pre~sure at the midpoint between the two si~ty percent point6 i8 tAken a~ the po~ne at which ~he hold-dovn pre~6ure i8 correct.
Also, correct hold-do~n pressure i6 determlned from coefflcients of the polynomial fitted to the pul~e .
~ , 26 ~30~3 ~plitude ~alue6 using, for esample, equ~tlon ~4).
If the plural~ty of "correct" hold-down pre6aure8 calculated at step 164 a8ree to vithln, 6~y, 10 mm~g, then the hold-do~n pre~Bure ~8 Bet at a ~alue ~ithln the calculated ran3e of ~alues (6tep 118) and an lndlcatton that blood pre~fiure resdin8s now sre ~alid 1B
pro~lded at ~tep 120. The blood pre6sure va~eform from the aelected tran~ducer element ~ay be di6played or recorded (step 124), ~nd the systolic and diastolic pressures, as ~ell BB pulse amplitute and pulse rate m~y be obtained $rom the blood pre6sure waveform (~tep 122) ~nd dl~plsyed at step 124.
~ t the ne2t heartbeat (step 126), the el~psed tlme of operstlon is eompared to a predetermined time perlod, M, ~uch as 30 ~inu~e6, and if M minutes have not elapsed, the process of selecting the central transducer element iB performed (atep 132). If there i~ a change in the selected trsnsducer element from that which was 18 t-determined, the newly selected ele~ent is displayed (~tep 136) ~Dd e ~arnln~ lssued (step 138). The transducer oay be repositloned ln response to the w~r~n~, or the monitoring proce6s may be con~lnued without reposltlonlng of the transducer, but u61ng the newly-~elected tr~Dsducer ele2ent. If, at atep 134, there hss been no change ln the selected transducer element, the oonltorl~ process contlnue~ without l~susnce of ~ ~arnlDg.
If, ~t declslon step 128, the respon6e 1B
~f~lrmstiYe lndlcatlng thst M ~inutes haYe elapsed, ~he tlmer 1B reBet ~t step 130, tbe hald-down preseure i~
decrecsed to ~ub~taDtially 120 ~8 ~t step 131~ and correct kold-do~n pres~ure to be u~cd 1~ recomputet by reentry Into tep ~16.
~t ~tep 119 ~n accuracy lndex may be calculated, vhich then i8 d~played at 6tep 124. The accuracy indes 2~ 53 18 tsken ~ the ratl~ of the ~lope of the ~y6tolic (or ti~ollc~ pressure ~erau~ hold-down curve ~here the ~lope ~B ~lnioum, to the alope of 8 6traight llne fitted to the cur~e befor~ fl~te~in~ of the artery be~in6.
The ~inl~um slope of the cur~e 16 readi?y deter~ined from coefficients of the polyno~l~l fitted tc the curve, ond the slope of the strai~ht llne i~ determlned by fitting a atr~ight llne to the lnitlal portlon of the cur~e. The hold-down pres~ure Pl' at ~hlch flsttening of the underlying artery begins i8 determlned from the lo~er BiYty percent o~ ~a~imum pulse smplitude point of the pulse amplltude YersUs hold-down pressure curYe 142.
The ~nvent~on ha~in8 been described in detail 1D accordance ~ith re~uirements of the P~tent St~tutes, varlou6 other ch3nge6 und modific~tions will 6uggest themsel~es to those skilled this ast. For example, not all ~our of the abo~e-described methods of determlning correct hold-dowD pres~ure Deed be employed 6ince the four methods 8enera~1y result in hold-down pres~re6 that Are 6ubstantially the same. If only one of the methods i6 employed, then &teps 166 snd 168 of Fig. 7 would be ellminated from the process. Also, once the transducer i6 properly positloned ~nd the correct hold-do~n pre~sure determlned aDd applled to the transducer, theD the waYe~orm fro~ the ~elected transtucer element ~ESy be di6pl~yed, recorded, or the like, ond/or any de~lret value derl~ed there~rom may be ti6pl~yed, recorded, or the l~ke. ~ny de6ired use ~y ~e ~ade of the bloDt ~r2~sure ~cYe~orm, the in~entlon not being l~mited ts an~ partlcular uae. ~1 BO ~ ~n a10~ c~rcult ~esns ~ be employed for proces~lng the blood pres~ure w~Yeform ln ~lsce of the ~llustrated dlgital proce~lng meaD~. Flhall~, tronsducera of ~arioua conatructions (Includ~n~ ~lngle-element tran6ducer6, capaci~lYe, fiber-optlc, plezoresl~tl~e ~nd other types of force or 2&
3~5i3 pre~sure tren6ducers) may be used to obtEIln pre~sure waveforms from the sub~ect. It ls lntended that the sbove and other such chsnges and modiication~ shall ~all w~thin the ~plrlt and scope of the invention defined ln the appended claims.
:
BLOOD PRESSURE MONITORING METHOD ~ND ~PPARATUS
ORIGIN OF THE INVENTION
The ln~ention described herein wa~ made ln the course of work under a Brant or award from th~
5 Department of ~ealth and Human Services.
TECHNICAL FIELD
This invention relates to method and app~ratus for non-in~a~i~ely ~onitoring blood pres~ure through ~se of a tran6ducer ~rray of individual pressure or force senslng elemeDts, ~nd to method and means for ascertalning the correct transducer hold-down pressure required for obtaining accurste blood pre~sure mea~urements.
BAC~GROUND OF THE INVENTION
The contlnuous measurement of blood pressure by use of srterlal tonometer traQ~ducers i~ well known ~B ~hown in U~S. Pstent Numbers 3,123,06B to R.P.
Bigliano, 3,219,035 to G.L. Pre6sman, P.M. Newgard and John J. ~ige, 3,880, 145 eO E.F. Bllck, 4,269,193 to the present i~entor, and 4~423,738 to P. M. New~ard, and in e~ ~rticle by G.L. Pres~msn and P.M. Newgsrt entltled "A
Tran~ducer for the Contlnuous External Mea~ure~ent of Arteridl Blood Pressure" (IEEE Tran~. Bio Med. ~lec., Apr~l 1963, pp. 73-81).
In a typicfll tonometrlc technique for ~onitorlng blood preseure, a transducer ~hich includea ~n crray of pres~ure ~e~siti~e element~ 15 positioned o~er a super~lcial artery, ~Dd a hold-down force i6 2 ~l30~Æ53 applied to the tran6ducer BO ~8 to flatten the wall of the underlylng artery ~lthout occlutlng the artery. The pres~ure sen~lti~e element6 ln the array typicslly have at least one dimension ~maller th~n the lumen of the 5 underlyin~ ~rtery ln which blood pressure is measured, and the transducer i6 positioned ~uch thst ~t lea~t one of the indi~idual pres~ure-sensitiYe element6 iB oYer ~t lea~t a portlon of the underlylng artery. The output from one of the pre~sure ~ensitive elementB iB ~elec$ed for monltoring blood pre~6ure. In some prior art arrangements, the pressure sen~iti~e element having the m~imum pulse amplitude output is selected, ~nd in other arrsnge~ent~ the element haYing a local minimum of diastolic or systolic pressure which element i~ within substantially one artery diameter of the element which generates the ~a~eform of maximum pulse amplitude i~
selected . , - .
The pre~sure mea6ured by the selected pressure sensitive element, i.e the element centered over the artery, wlll depend upon the hold-down pressure used to press the transducer ~gain6t the skin of the sub~ect.
Although fairly accurate blood pressure measurements ~re obtained when 8 hold-down pres6ure within 8 rather wide pre3sure ran8e ls employed, lt has been found that there exists a sub~t~ntlslly unique ~lue of hold-down pressure within said range for which tonometrir measurements are most accurate. Thi6 so-called "correct"
hold down pressure varies among sub~scts. Wlth prior art tonometrlc type transducerR ~he correct hold-down pressure often 18 not determlned thereby leading to inaccur~cles ln the bloot pres~ure ~ea~urements.
SUMMARY ~ND OBJECT~ OF T~E INVENTION
A~ ob~ect of this ln~ention 18 the pro~ision of an improYed ~onometric type method and appar~tus for 3 ~ 4~3 non-in~a6i~ely monitoring blood pres6ure wl~h a hiph de8ree of accur~cy.
Another ob~ect of thi3 lnventlon ls the pro~i6ion of ~uch a blood pressure messuring method and 5 appur~tu6 which includes the use of a transducer h~ving ~n arrsy of ind~idual arteri~l rlders (pressure se~sltive elsment~) and wherein mesns sre provided for determining the correct pres6ure required to hold the tr~nsducer ug~inst the sub~ect to assure ~ccuracy of the blood pressure readin~s.
The present lnvention includes a tr~nsducer arr~y for gener~tion of electrical waYeform~ indicati~e of blood pres~ure ln an ~rt~ry. Using the ~elected pressure sensing element that 18 determined to be poæitioned substsntially o~er the center of the unterlying artery, n set of st least one of the diastollc pressure, eystolic pressure, or pulse amplitude pre~sure ~ersus hold-down pressure values o~er a ran8e of hold-down preBsures between which the underlying 8 rtery i~ unf 18 ttened and ls occluded is obtained. ~ polynomial iB fitted to at least one of the set6 of ~alues from which polynomial the correct hold-down preBsure 1B tetermined. The hold-down pressure at the polnt of minimum slope of graphs of ~he polynom~al6 fitted to the ~ystolic or diastolic ~ersus hold-down pressure ~alues provides An indication of the correct hold-down pressure. ~n indication of the correct hold-down pre~sure using the pulse 0mplitude meB8Urement~ 16 proYided by loc~ting the polnt where the slope of the polynomi~ zero or the midpoint of ~
pair of hold-down pre6sures at whith the pul~e smplltude 1~ sub6ts~tiHlly ~isty percent of maslmum pul~e zmplltude of the ~raph of the polynomial fitted ts the pulse a~plltude qer~us hold-down pres~ure ~alues. A
~easure of compllance of the underlying ~rtery ls 4 ~l30~53 obtai~ed fro~ the ratio of tbe mini~um 810pe of the graph of the polynomial fltted ~o one ~f the ~y6tolic or diastollc pres~ure ~er~us hold-down pre~sure values to the ~lope of a straight line fitted to a ~ubset of one of the systolic or tlastolic ~ersus hold-down pressure Yalues oYer a rsnge of hold-down pressures below wh~ch flattenlng of the underlying artery beBin6 or it may be obtained dlrectly from the polynomial coefficlents. The hold-down presæure st which fla~tening of the underlying artery be8ins i8 taken as the lowest of the abo~e-mentlonea pair of hold-down pressures at whlch the pul6e amplitude is subst2ntially sixty percent of maximum pulse amplitude of the graph of the polynomial fitted to the pulse amplltude versus hold-down pressure ~alue6.
BRIEF DESCRIPTION OF THE DRAWINGS
The in~ention, ~ogether with the abo~e and other ob~ects snd adYantages thereof will be better understood fro~ the followins description when considered with the accompanying drawings. It vlll be understood, howe~er, that the lllustrated embodiments of the invention are by way of example only and that the ln~entlon is not limited thereto. The noYel fe~tures which are believed to be ch~racteristic of the inYention are set forth wlth particularity in the appended claims.
In the drawlngs, whereln like reference character6 refer to the same parts in the se~eral ~iews:
Flg. 1 shows the external appearance of a blood pres6ure transducer csse, typically po itioned o~er a superficial artery auch a6 the radial srtery, f~r pro~id$ng ~ continuouB external measurement of arterial blood pres~ure;
Fig. 2 1~ a schematic dlH~ram lllustratin~
the force balance bet~een thP artery and the multiple transducer elem~nts tarter$al riders), with the artery .~3~4~53 wall properly depre~aed ~o giYe accur~te blood press~re readin~6;
Fig. 3 1~ a combination 6implified ~ertlcal sectional ~iew taken through the transducer case of Fig.
1 and block dlagram of a system wh$ch may be employed there~ith ln the practice of thi~ ~n~ention;
Fi~. 4 is & waYeform of human blood pres~ure versus tlme of the type which may be obtained using ~he present inve~lon for illustrating syctolic and 10 disRtolic pre66ures and pulse amplitude of the blood presBure wsve;
Figs. 5A and SB to~ether show a flow chart for use in explaining o~erall operation of this lnvention;
15Fig. 6 ~ho~ plots of diastolic pressure snd pulse amplitude ~er6us hold-down pressure for a typical ~ub~ect; and Fi~. 7 iB a flow chart showin~ details o~
the computstion of a correct hold-down pressure.
20A typical appllcation of the transducer array f or arterial tonometry 1B illustrated in Fig. 1 whereln the transducer housiDg, or case, 10 whlch may ha~e the appearance of ~n ordinary wristwatch c~se, is held in place over the radial artery ln a human wrist 12 by a band 14. ~ cord 16 extends from the tran~ducer housl~g 10 through which electrical wiring for the transducer array~wlthin the housing, together with a 8m811 tube that conDects the housing to an ~ir pressure ~ource, e~end. The wirln~ 18 and tube 20 are 6hown in Fi~. 3, but not ln Fig. 1.
Reference now i8 made to Fi8. 2 wherein a diagr~mmQ~lc mechsnic~l model 18 shown which is representstl~e of fsctor~ to be considered in the physical syste~. The illu~trated model is that shown in the ~boYe-mentioned J.S. Eckerle p~tent number 4,269,193 6 1~04~5i3 ~hlch ~as ~dap~ed from the G.L. Pre~man and P.M.
Newgard ~rtlcle entitled "A Trsnsducer for the Continuous External Messurement of ~rter$al Blood Pressure~. In brlef, an ~rray 22 of lndi~dual pressure sensiti~e element6 or transdu~ers 22A through 22J which con~titute the ~rterlal riders, i8 po61tioned ~o that one or more of the rider~ ~re entirely o~er ~n artery 24. The lndi~idual riders 22A-223 ~re ~mall relatlve to the diameter of the srtery 24 thus assurin~ that at least one of the rider~ in it~ entirety ls over the artery. The skln surface 26 and artery underlying the transducer must be flattened by flppl~cation of a hold-down pres~ure to the transducer. One rider overlying the center of the artery is ldentlfied as the "centered"
srterial rider, from wh~ch arterial rider pressure readin86 for ~onitoring blood pressure are obtained.
Mean6 for selecting the arterial rlder are disclosed in the above-mentioned J.S. Eckerle ps~ent and G.L.
Pressman and P.M. Newgard ~rticle. Using the sbove-ment~oned rider selecting means, rider 22E, for example,may be selected as the ~centered" srterial rider, in ~hich csse the remainder of the riders, here riders 22A-22D and 22F through 22J comprl~e side plates which ~erve to flatten the underlylng ~kin snd srtery. Depending upon the pO8~ tioning of the transducer on the sub~ect, a different trsnsducer element may be positioned oYer the center of the artery and thereby be ~elected 3S the "centered" arterlsl rider.
Superficial arteries, such as the radial artery, are ~upported fros below by bone which, in Fig.
2 1B illu~trated by ground symbol 28 under ~he srtery~
The w~ll of srtery 24 behs~es ~ubstantlally like a ~embrsne ~n that it transmlts tension force6 but ~ot bendlnR ~o~snts. The ~rtery wnll re6ponds to the loading force of the transducer, snd during blood :: .
7 ~3~53 pressure measurements 8Ct6 as lf it i8 resting on the firm ba~e 28. The effecti~e ~iffness of an srtery wall i~ ~mall And differs between 6ub~ects. In prior art mech~nlcsl models of the physical ~ystem, the effecti~e stiffDess of the ~rtery wall i6 taken ~s zero, ln which case the actual hold-down pressure employed iB not consldered to affect ~ccur~cy of the blood pressure readingB BO lon~ as the transducer i6 pressed agalnst the 6kin surface with sufficient force to cause compre~sion but not occlusion of the underlying ~rtery.
Appllcant ha~ found thst not only are blood pre6sure readings dependent upon hold-down pressure within the ran~e of hold-down pressures that the sr~ery is flsttened but not occluded, but thst most accurate blvod pres~ure resdings ~re obtained where a hold~down preasure is selected thst iB subætsntially midway between the pressure where flattening of the ~rtery begln6 and the minimum pressure required for occluding the ~ame. Novel steps in~olved ln computing the correct hold-down pressure are described ln detail hereinbelow following completion of the description of the mechanic~l model of Fi8. 2 snd the oversll system 6hown in FiB. 3.
With the illu~trsted system, the trRnsducer csse 10 ~nd mounting str~p 14 together with sir pressure applied to ~ ~ellows, 54, supply the required compres6ion force and hold the riders 22A-22J in such a manner ~hat ~rterisl pressure chsnges are transferred to ehe riders ~hich oYerlie tbe ~rtery 24.
Di2grAmmat~c~lly thls 18 illustrated by showing the lndi~idual riders 22~-22J backed by rlder sprlng me~ber~ 30~-30J, re~pec~i~ely, a rigld ~pring bscking plste 32, aDd ~ hold-down force generator 36 between the b~cking plate 32 and the ~ounting strap system 38.
If, without force generator 36, the ooupling 4~53 between the mountlng s~Lrap 8y8tem 38 and sprlng backing pl~te 32 ~ere infinltely ~tiff ~o restrain the riders 22A-22J rigldly with respect to the bone structure 2B, the riders would be maintained in a fi~ed positlon relati~e to the artery. In practice, bowe~er, 6uch a ~y~tem i8 not practic~l, and hold-down fo~ce ~enerator 36, comprising a pneumatic or other 6uitable loadin~
system, is lncluded to keep const3nt the force applied by the mountin~ ~trap system 38 to riders 22A through 22J. In the mechanical model the 6prlng con~tant) k (pressure ~er unit of deflection) of the force generator 36 i8 nearly zero. Suitable pneumatic loading systems are shown and described in the abo~e-reerenced U.S.
Patent number~ 3,219,035, 4,269,193 and the Pressman-Newgard IEEE article.
In order to insure that the riders 22A through22J flatten the artery and pro~ide a true blood pressure measurement, they must be rigidly mounted to the backing plate 32. Hence, the rider ~prings 30A through 30J of the model ideally are infinitely ri~ld (~pring constant k .O~ ). It i8 found that as long as the sy~tem operates in such 8 mBnner that it can be ~odeled by rider spring6 30A ~hrou~h 30J having a spring constant on the order of about ten t~mes the value f~r ~he artery-~kin syBtem~ 80 that the deflection of rider~ 22A
throu~h 22J i6 small; 8 true blood pressure messurement may be obtained when the correct hold-down pressure is employed.
The octual physical structure of a prsctical transducer of a type which may be employed for transtucer ~rray 22 in the pre~ent ~ystem i 6hown ln the aboYe-mentioned J.S. Eckerle patent No. 4,269,193.
There, ~ transducer array i8 shown in which the ~ndl~idusl tran~ducer~ (riders) are formed in a thin ~onocrystslline silicon ~ubstrate which 18 made using ~: , ..
9 ~ 3 integrated circuit technlques. In Fig. 3, to which reference now i~ made, a slmplified ~ho~ing of trsnsducer 22 iB shown co~prislng a chlp 40 which lncludes an array of individual tr~nsducers, not shown.
Electrical conductors 42 connect the indivldual tr~n~ducers to the wlring 18 for connect~on thereof to a multiple~er 43.
A6 seen in Fig. 3, ca~e lO compri~es a gener~lly cylindrlcal, hollow, contsiner having rigid back snd aide walls 44 and 46, respecti~ely. The silicon tr~nsducer chip 40 i6 moun~ed within the face 48 of the ca~e ~de~ignated as the front or operati~e face) ln ~ cyli~drical cup-like tran6ducer housing 50. The operati~e foce 48 lncludes the silicon transducer chip 40 along with its included lndlvidual transducers and arterial riders. Chip 40 may be affixed to a conventional ceramic dual in-line package that i~
plugged into an associated dual in-line socket, neither of whlch are shown in the drawings. A silicone rubber filler 52 iB proYided lnside the housing 50 and sround the dual in-line package and 60cket to provide a good sesl, pre~ent electrical leakcge between the transducer circuits and housing 50, and provide a flat fiurface to press ag~inst the sub~ect. The front face 48 of the transducer includes the lower face6 of hou~lng 50 and filler 52.
The transdu~er hou~ing 50 i~ fixed to the inside of the tranaducer CBBe 10 by me~ns of a cup-like sllicone rubber bellows 54 whlch i8 sealed ~round the lower outalde lip of the cup-shaped transducer housing 50, e~tends upwardly inside the outer wall of the tr~n~ducer case 10~ and is ~ealed to a ring 56, whlch in turn iB fl~ed and se~led to the inside back of the trsnsducer cs~e lO. A chamber i8 formed $nside the bello~ ~hich iB connected to on ~ir pressure source 58 \ I
lo ~30A~53 through tube 20. A pres6ure controller 58A may be lncluded ln the pre~sure ~ource. Slnce the flexible bellows 54 1~ ~ealed both to the ~ran~ducer hou~ing 50 and the inside of the transducer case 10, flir under presxure from source 58 pneumstically losd~ the operatlYe face 48. With ~he trsnsducer ~trapped to the subject'~ wri6t, the hold-down force Fl exerted by the transducer array a8s~nst the skin of the 6ubject is ad~ustsble by control of the ~ir pres6ure. tIn the diagrammatic ~echanical model 6hown ln Fig. 2 the hold-down force Fl i8 generated by hold-down force generator 36.) As ~oted above, electrical signals related to pressure 6ensed by the indi~idual transducers 22A-~2J of transducer 22 are supplied a~ inputs to an analog multlplexer 43. From the multipleser, the signals are digitized by ~n analog-to-digital (A-D) converter 60, and the digitized slgnals sre 6upplied to B digital computer 62 hs~ing memory 62A and a clock 62B. Other information, ~uch a6 the sub~ects name, sex, weight, hei~ht, s~e, arm/wrist dimensions, and the like, may be fiupplied to the computer throu~h 8 keyboard 64. Output from the computer i~ supplied to data dlsplay and recorder means 66 which may include a recorder, cathode ray tube monltor, a sQlid state di6play, or the like.
If desired, at least a portion of the ~isual display may be included in transducer case 10. In fact, all of the components ~hown ln Figure 3 may be lncluded in the case lO witbout depar~ing from the prlnciples of thi6 lnventlon. Obviously, the computer output may be supplied to a printer, un oudible slarm, or the ~ike, a~
desired. ~1BO~ an output from the computer iB ~upplied o~er llne 68 to the pressure controller for control of the transducer hold-down pressure.
In Fi8. 4, to whicb reference now 16 msde, the 1 1 ~30~53 signsl waYeform of the output from one of the pre sure sensitive elemen~ 22A through 22J which overlles ~rtery 24 i8 ehown. Other element~ of the ~ransducer arrsy which oYerlie the ar~ery ~ill have wAveforms of almilar shape. ~ith a correct hold down pressure and correct selection of the ~centered" arterial rider (i.e. the rlder ~ubstantislly centered o~er the artery~ the ~aveform i~ representstiYe of the sub~ect'6 blood pressure ~ithiD the underlying artery. Systolic, diastollc and pulse ampl~tude pressures are indicated on the wa~eform, wherein pulse amplitude ls the difference between the ~ystolic and diastolic pressures for a giYen heartbeat.
OVERALL SYSTEM OPERATION
Fi~s. 5A and 5B, together ahow a flow ch~rt of an algorithm for gener~l, overall, operation of the blood pressure monitorlng system. Some of the operstions indicated there~n are under control of computer 62 responsive to programming instructions contained in memory unit 62A. Obviously, one or more progr~mming ~tep~ may be in~olved in the actual implementation of the lndicated operations. Since the progrsmming of ~uch 6tepB iB ~ell within the skill of the s~ersBe programmer, a complete program listin~ is not required ~nd i8 not included herein.
Preparation for monitoring is begun at START
~tep 100 ~t which time ~ystem power 1B turned on or a re~et operation læ per~ormed, by mesn6 not shown, and counter~, regi~tere, tlmera, and the like ln computer 62 ore lDitialized. At ~tep 102 information concerning the ~ub~ect, ~uch A~ the sub~ect'6 name, ~e~, weight, height, ~ge, arm ond/or ~rist dimensiona, ~nd the like, is entered into the compute~ memory thrsu~h U8e of the keyboard 64. Ne~t, at 6tep 104, ~ nominsl hold-down pres~ure (~-D.P.) i8 applied wherein ~ir under pressure ~3C)~53 from source 58 ls 6upplied to the transducer. For example, 8 hold-down pressure of ~ay 120 mmHg may be supplied to the transducer~ which pressure ser~es to extend the bellows 54 whereby operati~e face 48 extends out~rdly a 6hort d~stance from the bottom of the case lO. The tr~nsducer i8 attached to the ~ub~ect at ~tep 106 at a location wherein ~ centraily locsted transducer element, ~uch ~s element 22E of transducer srray 22 o~erlies the center of artery 24. Of course, the exact posltion of the transducer srrsy relati~e to the underly~ng artery generally 1~ not ~isually apparent to the subject, or operator, and repo.sitioning Qf the transducer may be required to properly position tne same .
~ith the transducer attached to the subject, step 108 ls enterea (select centered transducer element) at which point the transducer element which overlies.the center of the artery is identified. The location of the selected trsnsducer element is displsyed at step llO. If desired, the exact transducer element 22A
through 22J selected at step 108 may be di~plsyed at 6tep llO. Alternati~ely, a l~near array of, say, three lights may be provided wherein energization of the center light indicates that a centrally located transducer element wa6 Belected st Btep 108.
Illumlnation of elther of the end li~hts would lndicate thnt NoYement of the transducer to the right or left is required for proper posltioning of the transducer array relati~e to the underlyln~ artery. ~8 noted above, Yelection step 108 may lnclude means for selecting the one pressure sensiti~e element ~hat has a local mlnimum of at least one of the diastollc and ~ystolic pre~sures that ~B ~i~h~n substantially one artery diameter of the o~e pressure sensitl~e element which generates the wa~e~orm of ~aximum pulse amplitude, as disclosed in '~ : . '- ': '' ' U.S. P~tent Number 402S9,193. Processes whlch ~ay be ~mployed 1~ ~election ~ep 108, including identifying 6y~tolic and diastol~c pressure~, pul6e ~mplitude, ~s~im~, loc&l minima, ~nd the like, from the transducer outputs sre resdily ~mplemented u6ing diglt~l computer 62.
After the trsnsdueer ele~ent directly o~er ~h~
artery 16 ~elected and it~ location dlsplayed, decision step 112 i8 entered to determine whether sr not the selected element 18 nesr the center of the tran6ducer ~rr~y. If it i8 not, the transducer i6 repositioned on the subJect at 6tep 114, and ~tep 108 is reenterPd. The proce6~ is repeated until the transducer is properly loc~ted on the subject.
When tec~6ion step 112 i6 affirmative, the hold-do~n pre~sure to u6e i6 computed ~t 6tep 11~. An a$firmatlve decl~ion at 6tep 112 may require the operator to actuate an "Adjust Pressure" but~on, or the like, to enter ~tep 116 from 6tep 112. No~el algorithms which may ~e used in computing the correct hold-down pre6sure (step 116) are shown in Flg. 7 snd described below. For present purpoæes it will be under~tood that ~ correct hold-down pressure for accur~te blood pre~ure monitoring i6 computed at step 116, follo~in~ ~hlch, at step 118, the computed hold-down pressure i6 set by control of pre6sure con~roller 58A by the computer 62.
Data obtained and u6ed to compute the correct hold-down pre6sure (step 116) may be u~ed in the calculatlon of an ~ccur~cy index ~t Etep 119, which lndex ~l~ply co~prises ~ measure of the complisnce o~
the underlyi~g crtery 24. Generally, ~he more compli~Dt, or le86 ~tlff, the underlying artery, the better will be the ~ccurecy of the blood pres6ure ~eHsurement~. Means for computing the compliance and i 14 ~3~S3 accuraoy index are deRcribed ln deta~l hereinbelow. For pre6ent purpo~es, lt ~111 be understood that the accuracy indeY m~y be cslculated ~t s~ep 119 (Fi~. 5B) ~nd the ~lue thereof stored for di6plsy at B later step 5 with blood pressure messurements.
With the transducer properly positioned on the sub~ect and the correct hold-do~n pressure supplled thereto~ the syste~ ie ln condition for obtaining accurate blood pres~ure readlngs. At step 120 an ~ndication that the system i6 operati~e i6 provided, as by di6pl~y of the ~ords "Readings Val~dn. ObYiously, other d~spl~ys, such 8S a 8reen indicator light, ~ay be employed for indic~ting the opersting state of the ~y6te~.
From the output from the 6elected tran~ducer element, systolic and diastolic pres6ure ~alues toge~her with pulse amplitude values are readily determined in step 122. Also, pulse rate i6 readily calculated by determinlng the time between 6ucces6ive diastolic or 6y6tolic pressures. At 6tep 124, ~slue6 calculsted and determined ln step6 119 and 122 are displayed and/or recorded nlong with the actual waveform.
ObYiously, the Yalues whlch are calculated and di6played depend upon the use to be made of the blood pre6sure monitor, a dl~play of all of the ~alues not being requlred in ~any lnst~Dce6. For example, the blood pressure waveform could ~e recorded without cslculaeion ~nt di~play o~ any of the ~lues identlfied ln ~teps 119 and 122.
After the value6 ldenti$ied in step 124, such a~ systollc Dnt/or dla~tolic pres6ure, 3re displayed, step 126 i8 entered ~erein the 6ystem ~alt~ for the next heart~eat cycle. Dia~tolic or systolic pressure point~ 0ay be used to ldenti~y reference point6 ln the 35 hesrtbe~t cycle6. Deci6ion ~tep 128 then ls entered at ~ 4~53 ~hich time 6 timer ln computer 62 18 tested to determine ~hether or not it ha~ reached ~ predetermined time "M", where M 1B a tlme period of, 8~y, 30 minu~e6. If the el~psed tl~e exceeds the predetermined time period, M, the deci6ion i6 affirmatiYe, the timer 1~ re~et at step 130, hold-down pressure i~ reduced to appro~im~tely 120 ~mH8 at step 131, and step 116 i6 reentered for recomputstion of ~he correct hold-down pressure and resetting thereof, if required. Periotic checking and resettlng of hold-down pressure helps to ~ssure long-term accuracy of the blood-pressure readings.
If the predetermined time period has not been exceeded, 6tep 132 is entered for qelection of the centr~l transducer element, which step is the ~ame as step 108 described abo~e. Decision step 134 then ~s entered in which the selected transducer element deter~ined ln ~tep 108 1~ compAred to that determined in step 132. If there ls no change in the 6elected transducer element, step 120 is reentered indicsting thst the readln~s are ~alid. However, if there has bPen a change in the ~elected transducer element, ~uch that decl~lon step 134 ls ~ff~rmati~e, then step 136 is entered wherein the newly-determined eelected transducer is displayed. Thi~ ~tep corresponds to step llO wherein either the actusl ~electad trsn~ducer element iB
identified, or arrow6 or llghts indlcate the direction tha~ the tran~ducer needs to be mo~ed to recenter the ~ame over the underlying artery. A ~arnlng is l~sued at ~tep 138 indlcatlng to the ~ub~ect or operator that movement of the trsn~ducer relatiYe to the artery hs~
taken place. If deslred~ the transducer then m~y be reposltioned and the process researted. If the tran~tucer i~ not repositloned, end ~he process 18 no~
termlnsted and restarted, step 120 i~ reentered for contlnuatlon of the monltoring process, but now u ing . .
16 iL;304~5~
the output from the newly selected tran~ducer element.
DET~RMINATION OF OLD-DOWN PRESSUP~E
1) Di~stolic pressure vs ~old-Down Pres~ure Method Reference now 18 made to Fig. 6 wherein plots of diastolic pres6ure and pulse smplitude ver~us hold-down pres6ure are shown which wlll facllitste an under6tanding of novel meAn~ for determinin~ correct hold-down pressure for accurs~e blood pressure measurement6. The method for obt~ining the d~ta points in this plot wlll be described below. A third-order polynomisl i~ fitted using, for example, least squares techniques to the Fig. 6 series of diastolic pressure points to pro~ide a curve 140 which has the typical shspe 6hown reg~rdless of physical characteristics of the sub~ect.
A thlrd-order polynomisl fitted to the measured data may be written aR follows:
Pm - aO + 8lPh ~ a2Ph ~ a3ph (1) whereln:
Pm ' measured diastolic pressure, Ph - hold-down pressure, and aO, al, e2~ and a3 are coefficients of the polynomisl.
For hold-down pressures between zero and Pl, the uDderlyin~ artery remains unflsttened, and the measured pre88Ure iB pri~arily dependent upon the hold-down pressure snd secondsrily upon the intraarterial pressure, Pa. The graph of the polynomial ls a relatively ætrai8ht line o~er this range. Up to pre~ure Pl the effecti~e spring const~nt of the artery, uslng the mechanical model of the system shown in FiR.
2, i6 large.
Between hold-down pressures Pl and P2 the hold-do~n pres6ure ls 8rest enough to part~lly flstten ~ . ~
. . . - .
~04~53 the underlyiDg artery, but no~ 8reat enough to occlude lt. Experiment hss shown thBt ~DoBt accurate blood pre3sure mea6urement~ are sbtained when a hold-down pre6sure that i~ Rub~tsnti~lly mitway between presRures 5 Pl and P2 ls employed. Between pressures Pl and P2 the effectiYe ~pr$n~ coDstant of the artery using the olechan~cal model of Fig. 2, i6 relDtively small.
At hold-down pressures greater than P2, the underlying artery i6 completely occluded, and the effectiYe spring con~tant of the underly~n~ artery is a~in relatl~ely l~rge. Consequently, the me~sured pres~ure i6 again ~ubstantially independent of the intraarterlsl pre~ure, Pa, and the curYe ls sub~tantially 8 strai8ht line above pressure P2. As ~een in Fig. 6, the ~lope of curve 140 i~ lowest between pressure~ Pl and P2 where the underlying ~rtery is flatte~ed but not occluded. A~ noted above, ~ubstantislly the center, or midpoint, of this region of lowest slope, bet~een Pl and P2, i6 the corr ct hold-down pressure for obtainin~ accura~e blood pressuremeasurement6. This midpoint also substantially coincldes wlth the point ~here the slope of the grsph of the polynomial (equation 1) is minimum. Therefore, the correct hold-do~n pressure value iB readily determlned 25 by locating the mini~um slope point using coefficients of the graph 140 of the polynomial fitted to the di~stolic pre~sure point~. In particulsr:
p3 , ~2 ~2) where: P3, the polnt of mlnimum ~lope, is the correct hold down pressure, and ~ 2 ana a3 are the roefficlents of the second and third degree terms of the third-order polynomial.
- ~
18 ~ 53 For curYe 140 of Fl~. 6, a2 ~~ 0.084760 and a3 . 0.00014431 whereby, ~rom equat~on (1) a correct hold-down prçssure of approxlmately 196 mm~g i~
indlcated.
52) Systolic Pressure vs Hold-Down Pressure Method Instead of using a plot of diastolic pressure ~s hold-down pressure to determ~ne the correct hold-down pressure, a plot of systolic pressure versus hold-down pressure points msy be employed. The method is the same a~ that described sbo~e except that a third order polynomlal i6 fitted to the series of systolic pres~ure points, and equatlon (2) $s appliea to the re~ultant polynom i81 to pro~ide an lndioation of the correct hold-down pres6ure.
153) Pul~e Amplltude Y8 Hold-Down Pressure Method A. A third method of determining the correct hold-down pressure to u~e in monitoring blood pressure lnYol~es the use of the plot of pulse smplitude vs hold-down pressure point3 shown in Fig. 6. As with the plot of dlastolic pre6sure ~s hold-down pressure, a third order polynomial is fitted to the series of pulce amplitude polnt~ which results in a generally ln~erted U-shaped cur~e 1~2. It has been found that value~ of hold-down pressure corre~ponding roughly to pressures Pl ~nd P2 shown in Fi8. 6 msy be iound by taking the pre~sures where the pul~e amplitude i~ 0.6 times the maxlmum pulse amplltude on the graph of the polynomi~l, 142 . In Fi 6, these 0.6 maximum hold-down pressure points ore ldentified as pll ~nd P2~. The correct hold-down pressure, P3', i8 the Yalue substantlally midwaybet~een these points. Uslng thl~ method, the correct hold-to~ press~re IB
- ~ .
: ' .
- . :
- - ' .
.~ ' ~ .'' :', ' .
. ~ . ~'-:
~30~5;3 2 (3) For cur~e 14~ of F~8. 6 ~ correct hold-down pres~ure of ~pproYimately 195 mmHg B indicated by Eq. (3). For the ~ub~ect fro~ ~hich the diastolic snd pulse amplitude 5 plot8 of Fig. 6 were obt~lned, the correct hold-down pressure determined u8ing equa~ion~ (2) ~nd (3) differ by only one mmHg. If "correct" hold-down pressure ~alue~ celculated u~ing the three abo~e-described methods agree within approsimstely 10 mmHg, then ~ub~tantially correct blood pre~sure measurements ~re obtaiDed usin~ 6ny one of 6cid calculated values.
B. In a variation of ~he pulse amplitude VB H-D.P.
method, the pressure, P3', may be found directly from polynomial coefficients. For example, if a 6econd-order polynomisl is fltted to the pulse amplitude ~s N-D.P.
data, ~ith coefficients aO, al, and a2, then P3' is gi~en by -al (4) 2~2 This corre~poud~ to the m~ximum of the polynomial.
Si~ r expressions may be used for third ~nd hi~her-order polynomials.
ARTERY ,COMPLIANCE - ACCURACY INDE:R CALCULATIONS
The accuracy of tonometric methods of the pre~ent type for measuring blood pressure i8 dependent upon the compllance, or ~tlf~ness, of the underlying artery; the ~ccuracy of measurement decreasing with incressed stiffness of the artery. A messure of the stlffnecs, or co~pliance, of the srtery may be obtained usln~ lnfor~Dtion contained in the diastollc (or .. ..... .. , , , _ _ __ _ 2~ 13~53 systollc) pre66ure curve 140 and the pulse amplitude curYe 142 of Fi~. 6. In particulsr, a meaeure of complisnce of the artery i8 provided by the ratlo of the slope,S2, of the tia~tolic tor Rystolic) pres~ure curve at the correct h~ld-down pres6ure P3, and the slope, Sl, thereof bet~een zero snd ~old-d~wn pressure Pl.
From the abo~e, lt will be aeen that a measure of compliance, C, of the underlying srtery ia C ~ S2 (5) ~s noted ~bo~e, pressure~ Pl' and P2', equal to 0.6 of the maximum pulse amplitude value of ~raph 142, zubstantially correspond to hold-down preæsures Pl snd P2. Slope Sl ia determined by fittlng a straight l~ne 144 to the messured diastolic ~ersus hold-down pressure points for hold-down preRsures less than Pl.
As noted sbo~e, flattening of the underlyi~g artery does not beg~n until hold-down preasure Pl iB reached. Slope S2 at the correct hold-down pressure P3 is re~dily obtainsble using coefficients of the polynomial 20 (equation 1) identlfylng curYe 140. In particular, S2 ~ Bl ~ (6) If the mea6ure of compliance, C, i6 Bmall compared to unlty (aay C<0.3) the srtery ~tiff~ess is relatl~ely low, ~nd lntr~arterial blood pressure wlll be measured relatiqely ~ccurately by the present tonometric method. Lar8er v~lues of C, appro~ching unity, inticate that t~e artery atiffnes6 ls relati~ely large, which may le~d to relati~ely lnaccurate measurements. The compllance psrame~er obtained from equation (5) i8 . . ' ' ' ' .
`: , :
~L3~ ;3 calculated at ~tep 119 after the polynomials for the ~y~tolic, di~stolic ~nd pulse a~plitude qersus hold-down pressure curYes are determined, and the calculated compliance parameter ~alue i~ 6tored for ti~play ~t step 124. The ~alue it~elf may be diæplayed, or indicstion6 that arCur~cy i8 "goodn, "fsir~, ~poorn, or the like, may be displ~yed, dependent ~pon the calculsted value.
Reference now i~ made to tbe flow ehart of Fig. 7 wherein deteils of ~tep 116 of Fi8. 5A for computing hold-down pressure are shown. A~ no~ed sbove, when deci~ion step 112 of Fig. SA i6 affirm~tive, indicsting thst the selected trsnsducer element i~ near the center of the tran~ducer array, the correct hold-dow~ pressure iB computed at step 116. As seen in Fi~.
7, thi~ ~tep (step 116) ~ncludes waiting for the next hesrtbeat at Btep 150, following which the centered transducer element 15 ~elected at gtep 152. At 3tep 154 syætolic snd dis6tolic pre sures are determined from the blood pressure measurement6 obtaiDed from the selected trsnsducer element, or rlder, snd the systolic and diastolic pressure ~slue~, along with the hold-down pressure employed, are stored in computer memory 62A.
It will be recalled that at step 104 (Flg. 5A) a nominal hold-do~n pressure of approsimately 120 mm~g was Z5 applied. Therefore, the first sy~tolic snd diastolic pressure values are obtained using the nominal, 120 mmHg, hold-down pre~sure.
~ t step 156 the hold-down pres~ure is lncreased by ~n incrementsl amount of, 6ay, 5 mmHg.
Decision ~tep 158 i~ then entered to determine whether or ~ot ~he hold-down pressure i~ greater than, say, 300 mmHg. If the dn~wer is negati~e, ~tep lS0 i6 reentered whereupo~ another ~et of sy~tolyr snd diastolic pressure6 are obtained end ~tored for thls lsrger hold-down pressure.
.
, .. . . .
, ~()4~53 After an entire set of ~y6tolic and dlastolicpre~ure ~alue& have been obtained for a range of hold-down pre~sure6 betwee~ 120 and 300 mmH~, decis~on ~tep 158 is affir~tiYe and ~tep 160 i~ entered wherein a set of pulse ~mplitude v~lues are calculated by subtraction of d$~stolic pre~sure from the a~sociated ~ystolic pres~ure Yalue. The pulse amplitude ~alues Are stored in memory along with the ~e~ of systolic aDd diastolic pressure ~alues.
At ~tep 16Z, a polyDomial fit (typically a third-order fit~ is computed for each of the syætolic, diAstolic and pul~e amplitude ~ersus hold-down pressure ~ets of dst~ obtained at ~teps 154 and 160. The shape of curve 140 ln Fig. 6 is representative of the shspe obtsi~ed for both disstolic and systolic pressure ~s hold down pre6sure plots, and cur~e 142 in Fig. 6 has a ~hape that 1~ representati~e of pulse amplitude versus hold-down pre6sure plots. The constants and coefficlent6 of the polynomials obtained at ~tep 162 are stored for u~e in step 164 where a correct hold-down pressure i8 calculated for each polynomial.
As described above, the correct hold-down pressure using the 8y6tolic ~nd diastolic versus hold-down pres~ure points i6 obtained using equatlon (2) which locates the point of minimum ~lope of the plot of the thlrd-order polynomial fltted to the points. In p~rticular, the hold-down pressure is taken ~6 the negatl~e of the coefficient o the ~econt degree term di~lded by three time~ the eoeffic$ent of the third degree term of the polynomial.
Uslng the set the pul~e amplltude ~ hold-down pressure point~, the correct hold-down pres~ure i8 deter~inet b~ first calculatlng a pul~e amplitude Yalue substanelally equal to slxty percent of maximum pulse empl~tude on the graph of the polynomial fitted to the - .
.
: ; :
.
, : :
.
23 ~.304~i3 polnt~. The two hold-do~n pre~sure ~alue~ along the graph ~t ~hich the pul6e ~mplltude ~alues ~re sub6tantlally equal to ~aid BiXty percent of ~axi~um pul6e amplitude are identified, and the mean ~lue of the~e two hold-down pre66ure ~lues i6 taken as the correct hold-down pre~sure. AlternatiYely, the maximum of the polynomial may be determlned directly from the polynom~al coefficients, which maximum value i6 tQken 8S
the correct hold-down precsure.
At decision ~tep 166 the three (or four) hold-down pressure ~alues calcul~ted in 6tep 164 ~re comp~red to determine if the Yalues substantially Q~ree, e.g., if they agree within, ~Ry, 10 mmHg of each other. If the correct hold-down values do subs~al~tlslly agree, the deci~ioD ~s affirmati~e and step 118 (F$g. 5A) is entered where the hold-down pressure i5 set within the range of computed ~alue~. If they do not ~ubstantially agree, the hold-down pressure 18 reduced to ~ low ~alue, say 120 ~mHg, at step 168 snd step 150 is reentered for redetermination of a correct hold-down pressure ~alue.
Although the operation of the blood-pressure monltoring system is belie~ed to be apparent from ~he sbove-de~cription, ~ brief descrlption thereof now will be pro~lded. After turning on or resettlng the 6ystem (6tep 100) infor~ation regarding the sub~ect $s eDtered into computer memory 62A through keybo~rd 64 (~tep 102).
A hold-down pressure of apyroximately 120 mmHg is supplied to the transducer ~hrough tube 20 from pre~sure ~ource SB (s~ep 104) sfter ~hich the transducer i~
~ttached to the ~ubJect (step 106). Outputs from the tr~nsducer elements 22A through 22J are digltized at anslog to digital con~erter 60 ~nd are supplied to digital co~puter 62 for proce6~ing. Us$ng outpu~6 from eac~ of the trsnsducer ele~ent~ 22A through 22J, the transducer element thst i8 sub6tant~ally centered oYer ' :
the underlylDg artery 1B selected as thst element from whl h blood pree~ure mes~urement6 are to be obtalned (step 108). A method of ~electing the centered transducer element employing analog circuitry ifi shown ln U. S. Pateht 4~26~,193, which method iR readlly implemented digit~lly u6ing digital computer 62. The locat~on of the selec~ed transducer element i~ diRplayed (~tep 11~) and if the selected element 18 not nesr the center of the ~rray, the array may be repositloned (steps 112 and 114) and the process of selectin~ the centered element i6 repested.
With the transducer properly positioned on the subject, the correct hold-down pressure to empl~y for obtaiDl~g accur~te blood pressure measurements is determi~ed (step 116). Four dlfferent methods of computing the correct hold-down pressure are disclosed, one or more of which may be employed in a monitoring ~ystem. The first, second, and third methods use ~y6tolic pressure, diastolic pressure, and pulse amplitude ~ersus hold-down pressure mea6urements, respectl~ely, which sre obt~ined o~er hold-down presQureR which rsnge from a pressure where the underlging artery i6 unflattened to a pressure where it iB occluded. The fourth method is a ~sristion of the third, using a formula of polynomial coefficie~ts rather thsn the aforementioned 60% polnt6.
After 8 heartbeat ~step 150, Fig. 7) the tran~ducer e~e~ent centered over ~he underlying artery le selected (step 152) ~nd systolic and dia~tolic preasure~ are determined from the blood pres~ure waYe~orm (Flg. 4) from the eelected transducer element.
These maximum and minimum pre6sure points sre readily determlned by digital computer 62 UB~ n8 known progr~mming methots. The sy6tolic and diastolic ~ressures sre 6tored in computer memory 62A together '' .. . ' . ' ' -- ,' :
~ 4S3 wlth the associated hold-down pre~sure. The hold-down pressure then i~ lncrea6ed ~n incremental amount (s~y by 5 mmHg) ~ ætep 156, and, if the hold-down pre sure i8 le6s thsn about 300 mmHg, the ~y6tollc ~nd di~stolic pressure ~alues for the new hold-down pres~ure are obtained aDd stoTed in computer memory 62A. If the hold-down pressure iB ~reater than 300 mmHg (~tep 158),whlch i6 ~ pressure greater than that required for occlusion of the underlyin~ srtery, the ~cquisition o systolic and di~tollc pre6sures ~er~us hold-down pressure ~alues is 6topped, and pulse amplitudes for the collected dat~ ~re calculsted and stored. As ~een in Fig 4, pulse ampl~tude simply comprises the difference ln systolic and diastolic pressures for a given cycle of the blood pressure wa~eform.
Third order polynomials using a le~st ~quares method, for example, are fitted to the ~ystolic, disstolic and pulse amplltude ~er~u~ hold-down pressure ~slue6 (6tep 162). From the~e cur~es, correct hold-down 2f) pressure6 are c~lculated (step 164). From the ~ystolic and di~Etolic pre6sure cur~es, the point of mlDimum ~lope i~ determined to provide an indication of correct hold-down pressure. This minimum slope point is located substsnti~lly midwsy between hold-down pressures at which flattenlng of the underlying artery begins, and at which the artery is occluded. From the pulse amplitude curve, hold-down pres~ure Yalue~ at which the pulse amplltude ls si~ty percent of maximum are determlned9 which ~Alues sub~tantislly equal hold-down pres~ure~ at 30 which flattenln~ of the underlylng artery be8in~ and ~t which the artery ~ occluded. Hold-down pre~sure at the midpoint between the two si~ty percent point6 i8 tAken a~ the po~ne at which ~he hold-dovn pre~6ure i8 correct.
Also, correct hold-do~n pressure i6 determlned from coefflcients of the polynomial fitted to the pul~e .
~ , 26 ~30~3 ~plitude ~alue6 using, for esample, equ~tlon ~4).
If the plural~ty of "correct" hold-down pre6aure8 calculated at step 164 a8ree to vithln, 6~y, 10 mm~g, then the hold-do~n pre~Bure ~8 Bet at a ~alue ~ithln the calculated ran3e of ~alues (6tep 118) and an lndlcatton that blood pre~fiure resdin8s now sre ~alid 1B
pro~lded at ~tep 120. The blood pre6sure va~eform from the aelected tran~ducer element ~ay be di6played or recorded (step 124), ~nd the systolic and diastolic pressures, as ~ell BB pulse amplitute and pulse rate m~y be obtained $rom the blood pre6sure waveform (~tep 122) ~nd dl~plsyed at step 124.
~ t the ne2t heartbeat (step 126), the el~psed tlme of operstlon is eompared to a predetermined time perlod, M, ~uch as 30 ~inu~e6, and if M minutes have not elapsed, the process of selecting the central transducer element iB performed (atep 132). If there i~ a change in the selected trsnsducer element from that which was 18 t-determined, the newly selected ele~ent is displayed (~tep 136) ~Dd e ~arnln~ lssued (step 138). The transducer oay be repositloned ln response to the w~r~n~, or the monitoring proce6s may be con~lnued without reposltlonlng of the transducer, but u61ng the newly-~elected tr~Dsducer ele2ent. If, at atep 134, there hss been no change ln the selected transducer element, the oonltorl~ process contlnue~ without l~susnce of ~ ~arnlDg.
If, ~t declslon step 128, the respon6e 1B
~f~lrmstiYe lndlcatlng thst M ~inutes haYe elapsed, ~he tlmer 1B reBet ~t step 130, tbe hald-down preseure i~
decrecsed to ~ub~taDtially 120 ~8 ~t step 131~ and correct kold-do~n pres~ure to be u~cd 1~ recomputet by reentry Into tep ~16.
~t ~tep 119 ~n accuracy lndex may be calculated, vhich then i8 d~played at 6tep 124. The accuracy indes 2~ 53 18 tsken ~ the ratl~ of the ~lope of the ~y6tolic (or ti~ollc~ pressure ~erau~ hold-down curve ~here the ~lope ~B ~lnioum, to the alope of 8 6traight llne fitted to the cur~e befor~ fl~te~in~ of the artery be~in6.
The ~inl~um slope of the cur~e 16 readi?y deter~ined from coefficients of the polyno~l~l fitted tc the curve, ond the slope of the strai~ht llne i~ determlned by fitting a atr~ight llne to the lnitlal portlon of the cur~e. The hold-down pres~ure Pl' at ~hlch flsttening of the underlying artery begins i8 determlned from the lo~er BiYty percent o~ ~a~imum pulse smplitude point of the pulse amplltude YersUs hold-down pressure curYe 142.
The ~nvent~on ha~in8 been described in detail 1D accordance ~ith re~uirements of the P~tent St~tutes, varlou6 other ch3nge6 und modific~tions will 6uggest themsel~es to those skilled this ast. For example, not all ~our of the abo~e-described methods of determlning correct hold-dowD pres~ure Deed be employed 6ince the four methods 8enera~1y result in hold-down pres~re6 that Are 6ubstantially the same. If only one of the methods i6 employed, then &teps 166 snd 168 of Fig. 7 would be ellminated from the process. Also, once the transducer i6 properly positloned ~nd the correct hold-do~n pre~sure determlned aDd applled to the transducer, theD the waYe~orm fro~ the ~elected transtucer element ~ESy be di6pl~yed, recorded, or the like, ond/or any de~lret value derl~ed there~rom may be ti6pl~yed, recorded, or the l~ke. ~ny de6ired use ~y ~e ~ade of the bloDt ~r2~sure ~cYe~orm, the in~entlon not being l~mited ts an~ partlcular uae. ~1 BO ~ ~n a10~ c~rcult ~esns ~ be employed for proces~lng the blood pres~ure w~Yeform ln ~lsce of the ~llustrated dlgital proce~lng meaD~. Flhall~, tronsducera of ~arioua conatructions (Includ~n~ ~lngle-element tran6ducer6, capaci~lYe, fiber-optlc, plezoresl~tl~e ~nd other types of force or 2&
3~5i3 pre~sure tren6ducers) may be used to obtEIln pre~sure waveforms from the sub~ect. It ls lntended that the sbove and other such chsnges and modiication~ shall ~all w~thin the ~plrlt and scope of the invention defined ln the appended claims.
:
Claims (24)
1. A machine implemented method for determining hold-down pressure for use in a blood pressure measuring system comprising steps of, (1) applying a hold-down pressure to an external pressure transducer that includes a pressure sensitive element and obtaining a continuous measurement of blood pressure in an underlying artery, (2) obtaining from the blood pressure measurement of step 1 a set of at least one of the diastolic pressure, systolic pressure, and pulse amplitude values versus hold-down pressure over a range of hold-down pressures, (3) fitting a polynomial to at least one set of values obtained in step 2, and (4) using the polynomial obtained in step 3, obtaining an indication of the correct hold-down pressure required for accurate blood pressure measurements.
2. A method as defined in Claim 1 wherein a set of diastolic pressure versus hold-down pressure values are obtained in step 2, and in step 3 a polynomial of at least third-order is fitted to the set of diastolic pressure versus hold-down pressure values.
3. A method as defined in Claim 2 wherein step 4 includes locating the point of minimum slope of the graph of the polynomial fitted in step 3 using coefficients of the polynomial, the hold-down pressure at the point of minimum slope providing an indication of the correct hold-down pressure.
4. A method as defined in Claim 3 including (5) setting the transducer hold-down pressure to substantially the indicated hold-down pressure obtained at step 4.
5. A method as defined in Claim 1 wherein a set of systolic pressure versus hold-down pressure values are obtained in step 2, and in step 3 a polynomial of at least third-order is fitted to the set of systolic pressure versus hold-down pressure values.
6. A method as defined in Claim 5 wherein step 4 includes locating the point of minimum slope of the graph of the polynomial fitted in step 3 using coefficients of the polynomial, the hold-down pressure at the point of minimum slope providing an indication of the correct hold-down pressure.
7. A method as defined in Claim 6 including (5) setting the transducer hold-down pressure to substantially the indicated hold-down pressure obtained at step 4.
8. A method as defined in Claim 1 wherein a set of pulse amplitude versus hold-down pressure values are obtained in step 2, and in step 3 a polynomial of at least second-order is fitted to the set of pulse amplitude versus hold-down pressure values.
9. A method as defined in Claim 8 wherein step 4 includes locating a pair of hold-down pressure points at which pulse amplitude is substantially sixty percent of maximum pulse amplitude of the graph of the polynomial, the hold-down pressure at substantially the midpoint of said pair of hold-down pressure points providing an indication of the correct hold-down pressure.
10. A method as defined in Claim 9 including (5) setting the hold-down pressure to substantially the indicated hold-down pressure obtained at step 4.
11. A method as defined in Claim 8 wherein step 4 includes locating the point of maximum pulse amplitude on the graph of the polynomial fitted in step 3 using coefficients of the polynomial, the hold-down pressure at the point of maximum pulse amplitude providing an indication of the correct hold-down pressure.
12. A method as defined in Claim 11 including (5) setting the transducer hold-down pressure to substantially the indicated hold-down pressure obtained at step 4.
13. A blood pressure monitoring system for the continuous external measurement of blood pressure in an underlying artery comprising a pressure transducer that includes a pressure sensitive element having at least one dimension smaller than the lumen of the underlying artery in which blood pressure is measured and substantially centrally positioned over the underlying artery, said pressure sensitive element producing a continuous waveform having an amplitude which is a function of blood pressure in the underlying artery, means for applying a variable hold-down pressure to the transducer, means responsive to the waveform for obtaining a set of at least one of the diastolic pressure, systolic pressure, and pulse amplitude values over a range of hold-down pressures, means for fitting a polynomial to a set of at least one of the diastolic pressure, systolic pressure, and pulse amplitude versus hold-down pressure values to obtain coefficients of the polynomial, and means employing the polynomial for obtaining an indication of the correct hold-down pressure required for making accurate blood pressure measurements.
14. A blood pressure monitoring system as defined in Claim 13 wherein said means responsive to the waveform comprises means for obtaining a set of diastolic pressure versus hold-down pressure points, said fitting means includes means for fitting a polynomial of at least third-order to the set of diastolic versus hold-down pressure points where the polynomial is of the following type:
Pm = a0 + a1Ph + a2Ph2 + a3Ph3 wherein:
Pm = measured diastolic pressure, Ph = hold-down pressure, and a0, a1, a2, and a3 are coefficients of the polynomial, and, said means for obtaining an indication of correct hold-down pressure comprises means for locating the hold-down pressure, P3, at the point of minimum slope of the graph of the polynomial in accordance with the following equation;
.
Pm = a0 + a1Ph + a2Ph2 + a3Ph3 wherein:
Pm = measured diastolic pressure, Ph = hold-down pressure, and a0, a1, a2, and a3 are coefficients of the polynomial, and, said means for obtaining an indication of correct hold-down pressure comprises means for locating the hold-down pressure, P3, at the point of minimum slope of the graph of the polynomial in accordance with the following equation;
.
15. A blood pressure monitoring system as defined in Claim 13 wherein said means responsive to the waveform comprises means for obtaining a set of systolic pressure versus hold-down pressure points, said fitting means includes means for fitting a polynomial of at least third-order to the set of systolic versus hold-down pressure points where the polynomial is of the following type:
Pm = a0 + a1Ph + a2Ph2 + a3Ph3 wherein:
Pm = measured systolic pressure, Ph = bold-down pressure, and a0, a1, a2, and a3 are coefficients of the polynomial, and, said means for obtaining an indication of correct hold-down pressure comprises means for locating the hold-down pressure, P3, at the point of minimum slope of the graph of the polynomial in accordance with the following equation;
P3 =
Pm = a0 + a1Ph + a2Ph2 + a3Ph3 wherein:
Pm = measured systolic pressure, Ph = bold-down pressure, and a0, a1, a2, and a3 are coefficients of the polynomial, and, said means for obtaining an indication of correct hold-down pressure comprises means for locating the hold-down pressure, P3, at the point of minimum slope of the graph of the polynomial in accordance with the following equation;
P3 =
16. A blood pressure monitoring system as defined in Claim 13 wherein said means responsive to the waveform comprises means for obtaining a set of pulse amplitude versus hold-down pressure points, said fitting means comprises means for fitting a polynomial of at least a second-order to the set of pulse amplitude versus hold-down pressure points, said means for obtaining an indication of the correct hold-down pressure comprises means for locating a pair of hold-down pressure points on a graph of the polynomial at which the pulse amplitude equals substantially sixty percent of maximum pulse amplitude of the graph, the hold-down pressure at substantially the mean of said pair of hold-down pressure points comprising the correct hold-down pressure.
17. A blood pressure monitoring system as defined in Claim 13 wherein said means responsive to the waveform comprises means for obtaining a set of pulse amplitude versus hold-down pressure points, said fitting means comprises means for fitting a polynomial of at least second order to the set of pulse amplitude versus hold-down pressure points, and said means for obtaining an indication of the correct hold-down pressure comprises means employing coefficients of the polynomial for identifying the hold-down pressure at which pulse amplitude is maximum.
18. A machine-implemented method of obtaining a measure of the compliance of an artery comprising the steps of, (1) applying a hold-down pressure to an external pressure transducer that includes a pressure sensitive element and obtaining a continuous measurement of blood pressure in an underlying artery, (2) obtaining from the blood pressure measurement of step is set of at least one of the diastolic and systolic pressure versus hold-down pressure values over a range of hold-down pressures, (3) fitting a polynomial of at least third-order to a set of values obtained in step 2, (4) determining the slope of a straight line fitted to a sub-set of values obtained in step 2 over a range of hold-down pressures below which flattening of the underlying artery occurs, and (5) determining the minimum slope of the graph of the polynomial fitted to said set of values, the ratio of slopes determined in steps 5 and 4 providing a measure of compliance of the underlying artery.
19. A method as defined in Claim 18 wherein step 5 includes using coefficients of the polynomial in determining the minimum slope of the graph of the polynomial fitted to said set of pressure values.
20. A method as defined in Claim 19 including (6) obtaining from the blood pressure measurement of step 1 a set of pulse amplitude versus hold-down pressure values over said range of hold-down pressures recited in step 2, (7) fitting a polynomial of at least second-order to said set of pulse amplitude versus hold-down pressure values, (8) identifying the hold-down pressure on a graph of the pulse amplitude versus hold-down pressure polynomial at which the pulse amplitude substantially equals sixty percent of maximum pulse amplitude of the graph of the pulse amplitude versus hold-down pressure polynomial, and (9) employing the lower hold-down pressure identified in step 8 as the upper end of the range of hold-down pressures used in step 4 for determining the slope of said straight line.
21. A blood pressure monitoring system for continuous external measurement of blood pressure in an underlying artery and for obtaining a measure of compliance of said underlying artery comprising a pressure transducer that includes a pressure sensitive element having at least one dimension smaller than the lumen of the underlying artery and substantially centrally positioned over the artery, said pressure sensitive element producing a continuous waveform having an amplitude which is a function of blood pressure in the underlying artery, means for applying a variable hold-down pressure to the transducer, means responsive to the waveform for obtaining a set of diastolic pressure values, a set of systolic pressure values, and a set of pulse amplitude values over a range of hold-down pressures, means for fitting a polynomial of at least third-order to a set of at least one of the diastolic pressure and systolic pressure versus hold-down pressure values and for fitting a polynomial of at least second-order to said pulse amplitude versus hold-down pressure values, means for identifying the lowest hold-down pressure at which the pulse amplitude substantially equals sixty percent of the maximum pulse amplitude on the graph of the pulse amplitude versus hold-down pressure polynomial, means for determining the slope of a straight line fitted to a sub-set of at least one of the diastolic pressure and systolic pressure values over a range of hold-down pressures below said hold-down pressure identified as the lowest hold-down pressure at which the pulse amplitude substantially equals sixty percent of the maximum, and means for determining the minimum slope of the graph of at least one of the polynomials of diastolic pressure and systolic pressure versus hold-down pressure values, the ratio of said minimum slope to the slope of said straight line providing a measure of compliance of the underlying artery.
22. A system as defined in Claim 21 including means for setting the hold-down pressure of the transducer at substantially the hold-down pressure at which the slope of at least one of the graphs of the diastolic pressure and systolic pressure versus hold-down pressure polynomial is minimum.
23. A system as defined in Claim 21 including means for setting the hold-down pressure of the transducer at substantially the hold-down pressure midway between the pair of hold-down pressures at which the pulse amplitude polynomial substantially equals sixty percent of the maximum pulse amplitude on the graph of the pulse amplitude versus hold-down pressure polynomial.
24. A system as defined in Claim 21 including means for setting the hold-down pressure of the transducer at substantially the hold-down pressure at which the pulse amplitude polynomial is maximum.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000553722A CA1304453C (en) | 1987-12-08 | 1987-12-08 | Blood pressure monitoring method and apparatus |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000553722A CA1304453C (en) | 1987-12-08 | 1987-12-08 | Blood pressure monitoring method and apparatus |
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| Publication Number | Publication Date |
|---|---|
| CA1304453C true CA1304453C (en) | 1992-06-30 |
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ID=4137020
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| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000553722A Expired - Lifetime CA1304453C (en) | 1987-12-08 | 1987-12-08 | Blood pressure monitoring method and apparatus |
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Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN110017938A (en) * | 2019-03-20 | 2019-07-16 | 常州天利智能控制股份有限公司 | A kind of bellows type pressure sensor and the automatic controller with it |
| CN113974579A (en) * | 2021-10-28 | 2022-01-28 | 天津大学 | Three-part pulse condition acquisition device for finger-like treatment |
-
1987
- 1987-12-08 CA CA000553722A patent/CA1304453C/en not_active Expired - Lifetime
Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN110017938A (en) * | 2019-03-20 | 2019-07-16 | 常州天利智能控制股份有限公司 | A kind of bellows type pressure sensor and the automatic controller with it |
| CN113974579A (en) * | 2021-10-28 | 2022-01-28 | 天津大学 | Three-part pulse condition acquisition device for finger-like treatment |
| CN113974579B (en) * | 2021-10-28 | 2023-12-05 | 天津大学 | Three pulse condition acquisition device with simulated finger |
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