CA1259235A - Gravity-independent infusion system - Google Patents

Abstract

ABSTRACT OF THE INVENTION

An infusion system utilizing a pressurized source of infusate comprises a capillary tube flow restrictor having sufficient length and suitably large ratio of length to internal diameter to provide reliably repeatable fluid resistance and minimum damage to infusate. The internal diameter of the flow restrictor may be made to decrease with increasing temperature so that the flow rate of the infusate is substantially independent of temperature.
The flow restrictor may include means for indicating pressures and flow of infusate.

Description

~ ` ~.2S9~3~i 4 1. Field of the Invention 6 This invention relates to infusion apparatus and more 6 particularly to apparatus for infusing, intg a living patient, a 7 desired amount of infusate substantially independently of 8 gravitational forces or temperature chanyes.
9 2. Prior Art Problem Intravenous (IV) infusion of fluids is one of the most widespread and lifesaving procedures in medicine. However, the ~2 use of present IV infusion systems is playued by one or more of 18 three limitations: first, standard IV sets that depend upon 4 gravity for powering fluid flow are critically dependent upon elevation level of the infusate and are too tall for many 16 necessary sites such as ambulances and battlefields ; second, IV
17 sets that utilize pumps or drop-counting electronics tend to be both expensive and operationally fragile; and third, IV sets 9 which employ a tubing clamp to control the flow from a pressurized source of infusate are difficult to adjust and may ~1 not reliably maintain a desired rate of flow. And, superimposed æ upon these limitations is the tendency of temperature changes to 28 upset flow settings.
~4 SUMMARY OF THE INVENTION
26 It is accordingly the principal object of this invention to 27 provide an infusion system, of the class described, which 28 provides reliably predictable flow of infusate independently of ~9 elevation of infusate and without damage to infusate. It is a ~0 further object of this invention to provide a flow of infus~te ~1 which is substantially independent of temperature. According to li ' ~

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~his inventlon, the de~ired objects are attained by an in~usion syste~ utllizing a pressurized source of lnfusate of Xnown and controllable pressure which drives a known flow through a fixed selectable fluld resistance compr:L ing a capillary flow restrlctor havlng a sufficient l~ngth and suitably large ratio of length to internal diameter to provide reliably repeatable fluid resistance with mlnimum damage ~o infusate. Flow through the fluid resistance may be made substantially independent o~
temperature by causing the internal diameter of the ~low restrictor to decrease as temperature increases.
The invention may be æummarized, accordln~ to a first aspect, as an infusion system comprising a source of liquid infusate contalned in an infusate bag and pre~suri7ed by means external to said bag to a substantially constant known driving pressure, of at least 200 Torr, said pressure drlving a flow rate of said liquid infusate through a cutof~ valve in series with a flow restrictor of seleatable, substantially constant, known hydraulic reæistance, sald flow restrictor consisting o~
at least one small-bore flow passage having a leng~h and a diameter, the ratio of said length to said diameter being at least 10, and sald length belng at least 3 mlllimeters, and said restrictor providing substantially all o~ the resl~tance to flow of said liquid infusate when sald cutoff valve is open and said infusate is flowlng, whereby said flow rake is ~hen substantially constant.
Accordlng to a second aspeat, the invention provides a flow restrlctor, ~or use in an infuslon system comprising a source of liquid infusate pressurized to a substantially constant known driving pressure, from 200 up to about 500 Torr, said pressure clriving a ~low rate o~ said liguid infusate through said restrictor, said res~rictor aonsiæting of at least ~LZS9Z3~

one small-bore flow passage having a length and a dlameter, the ratio of said length to said diameter being at least 10, and said length being at least 3 millimeters, sald restrictor having hydraulic resistance adequate to provlde a pressure differential of flowing llquid in:Eusate o~ the order of 200 to 500 Torr, and said restrictor further comprislng connection means for connecting said restrictor into said lnPusion sy~tem, said connection means being adequate to with~and drlving pressure up to at least 500 Torr.
According to a third ispect, ~he inventlon provldes a flow restrictor, for use in an infusion system comprlsing a source of liquld infusate pressurized to a substantially constant known drlvlng pressure driving a flow rate of liquld infusate through said restrictor, compri6ing a small-bore ~low passage, de~ined within a leng~h of capillary tubing and having a length and a dlameter, the ratio of said length to sald dlameter beln~ at least 10, and said length belng at leas~ 3 mlllimeters, said flow restrictor being further capable o~
indlcating pressuxe and flow of sald llquld in~usate and further comprising: a tubular duct sealably surrounding sald caplllary tubing and extending upstrea~ and downstream thereo~
holes in the wall of said duct at locations ju~t upstream and downstream of ald capillary tubing; thin-wall elastomeric tublng surrouncling said duct and sealed thereto at dlstanaes upstream and downstream of sald holes to provlde sealed annuli inflatable by pressurized lnfusate flowing outward through sald holes; and a transparent cylindrical ~acket surrounding sad elastomeric tubing and radially spaced therefrom at a small distance such that said elastomerlc tublng touches sald ~acket when said annu:li are infla~ed.

2a ~25i923S
6~21-369 Other ob~ect~ of thls lnventlon, as well as means for attaining them, are set ~orth in the accompanyin~ Speci~lcation and Drawings, wherein 5 BRIEF DESCRIPTION OF THE DRAWINGS
Flgure 1 is a schematlc block diagram of an infuslon system according to this inventlon;
Figure 2 ls a longitudlnal cross-section of a simple flow restrictor which may be used with th~s system;
Figure 3 is a longltudinal cro~s-section of a temperature compen~atlng flow rest:rictor which controls a flow rate which may be substantially independent o~ temperature.
Figure 4 is a longitudinal cro~s-section of a flow restrlctor which incorporates means for indicat1ng infu~ion pressure and flow.
DESCRIPTION OF THE I~VENTION
Reference iB made to Figure 1, which is a schematic block dlagram of an infusion 6ystem accordlng to this invention. The system comprlses a source of pressurized gas which may typically be a s~andard small carbon dioxide car~ridge 10 which can be opened by actuator 11; thi~
combination is frequently provided to 2b ~1 lZ59Z35 1 inflate life vests and the like. Carbon dioxide gas at a pressure

2 of 800-1100 psi flows through tubing 12 to adjustable pressure regulator 13 where the pressure is reduced to a driving pressure 4 in the range of 200-500 Torr (4-10 psi). Pressure gauge 15 verifies the pressure of this gas which is then carried by tubing 6 14 to an inflatable gas bladcler 16 which presses upon a septum 7 18 which in turn presses upon an infusate bag l9 which contains 8 the liquid to be infused - physiologic saline, blood plasma, 9 citrated whole blood, or the like. Gas bladder 16, septum 18 and infusate bag 19 are snugly housed in rigid case 20, so that the 11 liquid pressure in infusate bag 19 equals the driving pressure in 12 tubing 14. The infusate liquid flows through flexible tubing 21 18 through a tubing clamp 22 which can be manipulated to allow or ~4 cut off flow, and then flows through a flow restrictor 23 which 1~ provides a high and constant resistance to flow. The liauid then 16 flows through flexible tube 24 and hypodermic needle 25 to the ~q patient, indicated by 26. To monitor the progress of the 18 infusion, and the amount of infusate remaining, septum 18 is 9 provided with pointer 27 and s~cale 28.
Since the gas driving pressure is considerably greater than 21 changes in hydrostatic pressure of the order of 0.5 psi, due to 22 changes of the order of one foot in the relative altitudes of the 23 infusate bag and the patient, flow rates in this infusion system 24 are substantially independent of gravity forces.
If flow restrictor 23 exhibits a constant known resistance to 26 flow of infusate, the rate of flow may be ascertained by noting 27 the setting of pressure regulator 13 or the indication of ~8 pressure gauge 15, in the context of that resistance. Therefore, 29 the regulator and pressure gauge may conveniently be calibrated ao in terms of flow, rather than merely in terms of pressure.
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~2 1 ~259Z35 1 ¦ Reference is now made to Figure 2, which is a longitudinal ¦~cross-section of a simple flow restrictor, acc~dLng ~o this B ¦ invention, which can e~hibit the re~uis~te flow characteristics 4 ¦ Flow restrictor 23 comprises a small-bore flow passage 30 defined ¦ by a length 31 of hypodermic needle tubing snugly held in a ¦ longer length 32 of elastomeric tubing. Flexible tubes 21 and 24 ¦ are slipped over the ends of tubing 32. ~hese components are 8 ¦ joined together by tight wralppings 34, and the joint between 9 ¦ tubing 32 and flexible tubes 21 and 24 may be reinforced by some 10 ¦ applied adhesive 33. Typically, flow passage 30 may be one to 11 ¦ several centimeters in length, with a bore of the order of a few 12 ¦ tenths of a millimeter. It is to be understood that, in the ~ Figures, it is necessary to exaggerate dimensions, such as tubing 14 bore and thickness, in the interest of ease of comprehension.
15 ¦ In order that flow restrictor 23 may exhibit a constant ¦ and linear known resistance, it is required that the flow be ~7 ¦ laminar and that the length-diameter ratio of flow passage 30 be 18 ¦ sufficient. And, to protect the formed elements of blood used as 19 ¦ an infusate, it is necessary that fluid shear forces not be ~0 ¦ excessive. These requirements can be met if the length-diameter ~1 ¦ ratio of flow passage 30 is at least 10, if the flow Reynolds æ ¦ number is well below 2000 and, if the infusate is blood, if shear 23 ¦ rate is less than 10 sec . The Reynolds Number criterion is 24 ¦ easily met for all structures of interest. The shear rate 2~ criterion is met if the length, in centimeters, is at least 0.0012 times the product of the driving pressure, in Torr, times ~7 the cube root of the flow rate, in milliliters per hour.
Experience shows that these criteria ma~ be met, for infusion 29 flow rates of interest, by flow passage diameters of a few tenths ~0 of a millimeter and lengths of three millimeters or more for ~1 aqueous infusates or a few centimeters for blood, with driving .

~ ~Z~5~3~235 pressure of 500 Torr or less, the recommended pressure limit for 2 many infusate bags. In practice, flow passage length may be 8 varied to compensate for variation of internal diameter among 4 different lots of hypodermic needle tubing, a length reduction of 4N percent compensating for a diameter reduction o~ N percent.
6 Longer flow passage lengths facilitate such trimming.
q Typical infusates, being largely water, exhibit a fluid 8 viscosity which decreases as temperature increases, roughly at 9 the rate of 2% viscosity change per degree Celsius. While this viscosity change, and the consequent change in flow rate, can be compensated by suitable scaling or interpretation of the pressure 2 settings or measurements, it would be convenient to use a flow ~8 restrictor having a fluid resistance which varies with 14 temperature in a compensating manner.
1~ Reference is now made to Figure 3, which is a longitudinal 16 cross-section of a flow restrictor which can exhibit flow 17 resistance which varies with temperature in such a way that flow is substantially independent of changes in temperature. Flow 9 passage 30`is defined by the inner diameter of a thick-walled elastomeric tube ~0, the outer diameter of which fits snugly in a al surrounding metallic tube 41. Flexible tubes 21 and 24 are 22 slipped over the ends of tube 40 and, again, the components are 23 joined together by tight wrappings 34, and the joint between 24 flexible tubes 21 and 24 and metallic tube 41 may be reinforced 2B with some applied adhesive 33.
26 Typical metals have thermal coefficients of linear expansion 27 of the order of 10 per degree C, while those of typical ~8 elastomers are about ten times larger. Therefore, as temperature 2g increases, elastomeric tube 40 tends to swell more than can be accommodated by the expansion of surrounding metal tube 41. Since ~1 elastomers are not very compressible, the swelling is a2 5 ~L25923S

1 accommodated by a decrease in the inner diameter of elastomeric 2 tube 40, the decrease being greater for larger ratios of outer to B inner diameter of tube 40. For most combinations of metal and 4 elastomer, a ratio of 4 to 10 can yield a fluid resistance which compensates reasonably well for viscosity change over the 8 temperature range of lnterest; a ratio of 6, in a typical case7 7 could compensate within 1.5% Erom 18 to 42 degrees C.
8 Reference is now made tlo Figure 4, which is a longitudinal 9 cross-section of a flow restrictor which incorporates means for indicating infusion pressure and flow. The fluid resistance 11 element is similar to that shown in Figure 2, comprising a 12 length 31 of capillary tubing defining the flow passage 30 and 18 snugly held by a length 32 of elastomeric tubing. This assembly is snugly mounted within a long metallic tube or duct 50 which is 6 provided with holes 51 just outside the ends of tubing 32.
16 Surrounding duct 50 is a long thin-wall elastomeric tube 52 which is joined to duct 50 with tight wrappings 34 to form a pair of ~8 inflatable annuli 55. This assembly is then placed in a 9 transparent cylindrical spaced-apart jacket 53 which is provided with small vent holes 54 . Flexible tubes 21 and 2~ may be 2~ slipped over the ends of duct 50 and fastened thereto with tight æ wrappings, as in the case of the restrictor assemblies shown in 23 Figures 2 and 3. And the joint between thin-wall tube 52 and 24 duct 50 may be reinforced by some applied adhesive 33.
2~ The inner surface of transparent jacket 53 is rough or 26 frosted, so that it appears white unless touched by the outer 27 surface of an inflatable annulus 55, in which case it takes on 28 the color of that surface. To exploit this fact, the upstream ~9 annulus 55, nearest flexible tube 21, is colored green on the outside, and the downstream annulus, nearest flexible tube 24, is ~1 colored red. Therefore, if flow pressure is applied, it will æ 6 . ~L25~235 1 inflate the upstream annulus 55, and the upstream part of jacket 2 53 will exhibit a green band, signifying flow. But if flow is 8 blocked downstream, as b~ kinking of flexible tube 24 or 4 occlusion of the needle, pressure will rise in the downstream annulus 55 and the downstream part of jacket 53 will exhibit a 6 red band, signifying blockage.
7 Given the foregoing teaching, those skilled in the art to 8 which this invention pertains may readily devise further or extended embodiments. For one example, the restrictor assembly 0 shown in Figure 2, comprising a metallic tube encased in an 11 elastomeric tube, may be replaced by a thick-walled small-bore 12 plastic cylinder. For another example, the gas-powered pressure 18 source shown and described with respect to Figure 1 may be 4 replaced by a spring-driven mechanism for squeezing infusate bag 1~ 19. Also, it is not necessary to provide a pressure gauge 15 if lB the setting of regulator 13 is readable and reliable. In case of a regulator malfunction which might apply excessive pressure, I
8 prefer to add a pressure-relief safety valve to tubing 14, 9 connected to a shrill whistle to warn the operator of such an 2~ event. And, while I prefer to make the inner surface of the 21 transparent jacket rough, and to color the outside of the 22 inflatable annuli, in the embodiment of Fig. 4, neither of these ~3 provisions is nècessary. various other features and advantages ~4 not specifically enumerated will occur to those versed in the art! as likewise many variations of ~he embodiments which have 26 been illustrated, all of which may be achieved without departing 27 from the spirit and scope of the invention as defined by the following claims:

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Claims (19)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. An infusion system comprising a source of liquid infusate contained in an infusate bag and pressurized by means external to said bag to a substantially constant known driving pressure, of at least 200 Torr, said pressure driving a flow rate of said liquid infusate through a cutoff valve in series with a flow restrictor of selectable, substantially constant, known hydraulic resistance, said flow restrictor consisting of at least one small-bore flow passage having a length and a diameter, the ratio of said length to said diameter being at least 10, and said length being at least 3 millimeters, and said restrictor providing substantially all of the resistance to flow of said liquid infusate when said cutoff valve is open and said infusate is flowing, whereby said flow rate is then substantially constant.

2. An infusion system according to claim 1, for use with blood-containing infusates, in which said length, in centimeters, is at least 0.0012 times the product of the driving pressure, in Torr, times the cube root of the flow rate, in milliliters per hour.

3. An infusion system according to claim 1 in which said small-bore flow passage of said flow restrictor is defined by the length and inside diameter of a length of capillary tubing.

4. An infusion system according to claim 3 in which said length of capillary tubing is composed of rigid material.

5. An infusion system according to claim 4 in which said length of rigid capillary tubing is snugly held in a length of elastomeric tubing adapted to be received within ends of flexible tubes of said infusion system, to connect to said system.

6. An infusion system according to claim 1 in which said flow restrictor exhibits hydraulic resistance increasing with temperature, aid small-bore flow passage being defined by the length and inside diameter of a length of thick-wall capillary tubing of elastomeric material having a first temperature coefficient of expansion, and the outside diameter of said capillary tubing being constrained by a surrounding length of tubing of rigid material having a second, and lesser, temperature coefficient of expansion.

7. An infusion system according to claim 6 in which the ratio of said outside diameter to said inside diameter of said thick-wall capillary tubing is in the range of 4 to 10.

8. A flow restrictor, for use in an infusion system comprising a source of liquid infusate pressurized to a substantially constant known driving pressure, from 200 up to about 500 Torr, said pressure driving a flow rate of said liquid infusate through said restrictor, said restrictor consisting of at least one small-bore flow passage having a length and a diameter, the ratio of said length to said diameter being at least 10, and said length being at least 3 millimeters, said restrictor having hydraulic resistance adequate to provide a pressure differential of flowing liquid infusate of the order of 200 to 500 Torr, and said restrictor further comprising connection means for connecting said restrictor into said infusion system, said connection means being adequate to withstand driving pressure up to at least 500 Torr.

9. A flow restrictor according to claim 8, for use with blood-containing infusates, in which said length, in centimeters, is at least 0.0012 times the product of the driving pressure, in Torr, times the cube root of the flow rate, in milliliters per hour.

10. a flow restrictor according to claim 8 in which said small-bore flow passage is defined by the length and inside diameter of a length of capillary tubing.

11. A flow restrictor according to claim 10 and further capable of indicating pressures and flow of infusate, said flow restrictor further comprising, a tubular duct sealably surrounding said length of capillary tubing and extending upstream and downstream thereof; holes in the wall of said duct at locations just upstream and downstream of said capillary tubing; thin-wall elastomeric tubing surrounding said duct and sealed thereto at distances upstream and downstream of said holes to provide sealed annuli inflatable by pressurized infusate flowing outward through said holes; and a transparent cylindrical jacket surrounding said elastomeric tubing and radially spaced therefrom at a small distance such that the elastomeric tubing touches the jacket when the annuli are inflated.

12. A flow restrictor according to claim 11 in which the inside surface of said transparent cylindrical jacket bears an optically rough surface at locations adjacent said sealed annuli.

13. A flow restrictor according to claim 11 in which the outside surfaces of said sealed annuli are colored.

14. An infusion system according to claim 1 in which said known driving pressure may be as high as 500 Torr.

15. An infusion system according to claim 1 in which said means, by which said source of liquid infusate is pressurized, comprises an inexpansible case which contains said infusate bag and which also contains an inflatable gas bladder inflated by a source of gas at a substantially constant known gas pressure.

16. An infusion system according to claim 15 in which a septum is interposed between said infusate bag and said inflatable gas bladder, and said septum bears a pointer which moves with respect to a scale fixed to said inexpansible case, as said liquid infusate flows from said infusate bag.

17. A flow restrictor, for use in an infusion system comprising a source of liquid infusate pressurized to a substantially constant known driving pressure driving a flow rate of liquid infusate through said restrictor, comprising a small-bore flow passage, defined within a length of capillary tubing and having a length and a diameter, the ratio of said length to said diameter being at least 10, and said length being at least 3 millimeters, said flow restrictor being further capable of indicating pressure and flow of said liquid infusate and further comprising: a tubular duct sealably surrounding said capillary tubing and extendlng upstream and downstream thereof; holes in the wall of said duct at locations just upstream and downstream of said capillary tubing; thin-wall elastomeric tubing surrounding said duct and sealed thereto at distances upstream and downstream of said holes to provide sealed annuli inflatable by pressurized infusate flowing outward through said holes; and a transparent cylindrical jacket surrounding said elastomeric tubing and radially spaced therefrom at a small distance such that said elastomeric tubing touches said jacket when said annuli are inflated.

18. A flow restrictor according to claim 17 in which the inside surface of said transparent cylindrical jacket bears an optically rough surface at locations adjacent said sealed annuli.

19. A flow restrictor according to claim 17 in which the outside surfaces of said sealed annuli are colored.