CA1096652A - Non-invasive pressure sensor for communicating pressure inside a body to the exterior thereof - Google Patents

Abstract

ABSTRACT

A pressure sensor for indicating pressure in the animal or human body, such as intracranial pressure, including a housing, a bellows contained within the housing, 3 body pressure sensing tambour for placing the bellows in communication with pressure in the body so that the pressure will cause the bellows to move as a function of the pressure, output means, such as a radioactive source and associated shielding, contained within the housing and associated with the bellows for providing an output which is a function of the movement of the bellows, and a receiver, such as a radiation detector, located external to the body to receive the output and provide data indicative of the pressure in the body. The pressure sensor includes means associated with the bellows to enable in vivo calibration of the pressure sensor after implantation by establishing a preselected output condition during calibration. An ambient pressure sensing tambour is associated with the bellows for compensating for ambient pressure variations. The bellows is resilient, made of a material and shape which has essentially 100% memory of position and has a wall thickness of less than 1 mil in order to provide an output which accurately reflects the pressure being monitored. The bellows has a spring rate which is substantially greater than the spring rate of the pressure sensing and ambient pressure compensating tambours thereby making the pressure sensor essentially insensitive to temperature variations.

Description

This inverltion is related to applicant's Canadian ~,at-ent dpplicaLions Nos. 232,094, fi1ed July ~3, 1975 ~nd 28~,593 filed September 27, 1977O
The need for a non-invasive technique for measuring the pressure in body cavities of animals or humans is recognized as highly desi~able for continuous or in~ermittent moni-toring of bod~r conditions. Such cavities as the cranium, vena cava, bladder and others provide valuable and sometimes critical in-formation for maintaining the well being or survival of an animal or human. For example, it is known tha-t intracranial pressure provides a valuable indication of well being for a variety of clinical conditions, including shock trauma and hydrocephalus.
~ ccordingly, there is a recognized need for a pressure sensor for continuous or intermittent monitoring of body condi-tions. In particular, there is a need for a pressure sensor hav-ing compensation for ambient pressure variations and low sensi-tivity to temperature changes. Moreover, there is a need for a sensor which can be calibrated ln iVO and which provides an output which accurately re~lects the pressure in the animal or human body.
The pressure sensor of the present invention is fully implantable and contains output means, such as a radioactive source and associated shielding, so that the pressure can be read out non-invasively. In its preferred form, the pressure sensor includes a housing containing a bellows in communication with a body pressure sensing tambour placed in the body and ex- ~ -posed to the pressure to be sensed. An ambient pressure sensing tambour is also associated with the bellows for compensating for ambient pressure variations. The housing is located ex-ternal to the cavity being sensed and preferably situa-ted just under the skin. The housing also con-tains the output means, which is associated with the ''

- 2 -- ' ~6~i2 bellows.
The pressure acting upon the body pressure sensing tambour causes the bellows to expand and contract. The movement of the bellows causes the output means to provide an output which is a function of the pressure such as by causing radiation shielding to shield a radioactive source as a function of the pressure sensed. The output is sensed from outside the body by a receiver such as a conventional nuclear counter or crystal detector instrument, in case of a radiation output.
The pressure sensor also includes means associated with the bellows to enable ln vivo calibration of the pressure sensor after implantation by establishing a preselected output condition which is initially established during ln vitro calibration. More specifically, and using a radioactive source and associated radiation shielding as illustrative, a stop is provided so that there is a preselected radiation output which can be used for,calibration. -The bellows is resilient, made of a material and shape which has 100~ memory of position, and has a wall thickness of less than 1 mil in order to provide an output which accurately reflects the pressure being monitored. The bellows has a spring constant substantially greater than the spring constant of the body and ambient pressure sensing tambours, which offer effectively no resistance to pressure changes, thereby making the pressure sensor essentially insensitive to temperature variations.
The pressure sensor is fully implantable and does not require any energy source other than the radioactive '' material, for e~:ample, contained in the device. Another major advantage of the sensor is the elimination of leads or other penetrations through the skin to provide power or transmit a ' , signal. With a long-lived radioisotope, such as promethium-

-3-~$~

145, carbon-14, nickel-63, strontium-90, or americium-2~1, the inventive pressure sensor can be fully implanted and left in place for the life of the patient.
The pressure sensor furlctions accurately to within several millimeters of water pressure and is unaffected by variations in ambient pressure. Also, it is generally insensitive to ambient temperature and can be calibrated in vivo. Furthermore, the materials used to construct the sensor are biologically inert and do not pose any health ha7ard to the animal or human body or make the patient more susceptible to mechanical trauma. The sensor unit is of relatively small size and so does not produce unsightly bulging when implanted subdermially.
Other features and advantages of the invention will be apparent from the following description of the preferred embodiments of the invention as shown in -the accompanying drawings.
Figure 1 is a perspective view of a preferred embodiment of the non-invasive pressure sensor of the invention in an installed position for monitoring the pressure in an intracranial cavity and communicating the monitored pressure to the exterior of the body, Figure 2 is a plan view of the apparatus of Figure l;
Figure 3 is a sectional view taken substantially along :
line 3-3 of Figure ~in the direction of the arrows;
Figure 4 is an enlarged sectional view of a portion of the apparatus of Figure 3;
Figure 5 is an enlarged perspective view of the mounting arrange:ments for the apparatus of Figure l;
Figure 6 is a perspective view of a portion of the skull of a patie:nt prior to installation of the apparatus of the invention; and

-4-Figure 7 is a sectional view taken substantially along lines 7-7 of Figure ~ in the direction of the arrows.
Referring now to the drawings and to Figures 1 and 2 in particular, there is shown the pressure sensor apparatus of the invention with a housing des:ignated generally by the numeral 10, and a body pressure sensing means designated generally by the numeral 11 and connected to the housing 10 for sensing the pressure in a body portion such as a cavity. Ambient pressure sensing means, designated generally by the numeral 12, is also 50nnected to the housing 10 and is responsive to ambient pressure to compensate for changes in ambient pressure during the operati~n o~ the apparatus. Although the pressure sensor apparatus of the invention is shown in an installed position on the head of a human body for non-invasively monitoring intracranial cavity pressure and communicating it to the exterior, it should be understood that this is only a preferred example of the invention and that it is equally adaptable for monitoring pressure in other areas of the body, both animal and human.
Therefore, while the description to follow will be directed to the use of the invention for monitoring intracranial cavity pressure, it should be understood that the invention is equally applicable to monitoring pressure in other body portions and cavities.
Referring now to Figure 3, and as specifically ~ ;
illustrative of the invention, the housing 10, which is preferably formed of titanium, is of tubular shape having a side wall 13 defining an interior 1~. A first support member 16, also preferably formed of titanium, has a central bore 17 positioned within one end of the housing 10 in sealing relationship with the housing side wall 13 by means of an epoxy resin or the like. Preferably, an annular shoulder 13a is formed in the housing side wall 13 for positioning the support member

-5-. ~ .
-~ Q ~ 216 in a precise location within the housing 10 as will be explained hereinafter. The first support member central bore 17 is provided with a portion 17a of substantially enlarged diameter defining a recess 20 which communicates with the interior 14 of the housing 10.
The body pressure sensing means 11 includes a fluid conduit 22 of deformable metallic material, preferably titanium, which has been heat treated for deformability, one end 22a of ~hich is arranged to be connected to the body pressure sensing device or tambour 23 having an interior 24 which is arranged to be positioned within a body cavity such as the intracranial cavity of Figure 1.
The tambour 23 is formed of a suitable elastomeric - ~ ` material such as medical grade Silastic~rubber and is of a substantially flat configuration including a neck portion 25 in the wall of which iq molded a helical spring 26, preferably of stainless steel, for imparting rigidity to the neck portion 25. The neck portion 25 includes a central bore 25a which communicates with the interior 24 of the tambour 23 and which is arranged to receive the end 22a of the conduit 22 as shown in Figure 2. Preferably, a U-shaped clip 27 of tantalum or the like is disposed within the interior 24 of the tambour 23 for maintaining the side walls of the tambour in spaced-apart relationship and to serve as a locating means for the tambour with the use of X-rays. The tambour 23, the fluid conduit 22 and the communicating portions of the housing interior 14, including the recess 20, are filled with a pressure transmitting fluid through which the pressure ~-sensed by the t~mbour 23 in the body cavity is transmitted to the housiny interior 14.
The conduit 22 is press-fitted through the bore 17 with the end 22b of the conduit 22 extending through the bore 17

-6--,: - . . . . ~ .
- . - .-: : -ii5~

into the recess 20 as shown best in Figures 3 and 4. Sealing engagement between the conduit 22 and bore 17 is obtaine~ by means of epoxy resin or the like. Thus, the conduit end 22b communicates with the recess 20 and with the housing interior 14.
The output means of the pressure sensor apparatus preferably comprises a source 29 of radioactive material, normally in the form of a shaped article, disposed within the housing interior 14 together with associated radiation shielding means designated generally by the numeral 31. Means are provided in the housing interior 14 for resiliently urging the radioactive material and radiation shielding into a shielding relationship.
More specifically, resilient means such as a bellows 32 having an interior 33 is disposed within the housing interior 14, one end 32a of which is mounted on a necked-down portion 34a of a second support member 34, preferably formed of titanium, suitably mounted in the other end of the housing 10 in sealing engagement with the housing side wall 13 by means of an epoxy resin or the like. The other end 32b of the bellows 32 is closed as will be explained hereinafter.
The second support member 34 is provided with a central bore 38 and the necked-down portion 34a is arranged to support the bellows end 32a in a sealing relationship therewith by means of an epoxy resin 41 or the like.
Bellows 32 is made of a resilient material, preferably a metal such as gold-plated nickel and has essentially 100~
memory of position. The bellows has a wall thickness 32c which is less than 1 mil and preferably less than about 0.5 mil. The most preferred wall thickness range is between about 0.25 and 0.33 mil. The spring rate of the bellows is a function of the wall thickness and the material from which the bellows is made.

For a gold-plated nickel bellows having a wall thickness of about 0.25 to 0.33 mils, the spring rate of the bellows is in the ran~e of about 0.130 to 0.240 pound per inch with the preferred spring rate being about 0.185 pound per inch. The bellows has a length, typically about 1/2 inch, which varies according to the pressure which the pressure sensor apparatus is intended to monitor. By providing the bellows with a wall thickness of less than l mil and a corresponding spring rate, the bellows will typically travel (contract or expand) a distance of about 50 to 60 mils when the body pressure sensing tambour lQ 23 is exposed to pressure variations such as those normally found in monitoring intracranial pressure. In this manner, a relatively large movement (contraction or expansion) of bellows 32 is caused by the pressure being monitored, and therefore the pressure sensor apparatus is capable of more accurately monitoring pressure changes in the animal or human body than conventional pressure sensors.
The central bore 38 of the second support member 34 includes a portion of enlarged diameter forming a recess 42 and which is provided with an annular, inwardly directed flange 43.
The recess 42 receives the nipple g4 having a central passage 44a of an ambient pressure sensor or tambour 46 forming the ambient pressure compensating means 12. The tambour 46 is formed of a flexible material, preferably an elastomeric material such as medical grade Silastic rubber, and includes an annular peripheral portion 47 and a recessed central portion 48 defining an interior 49 extending through the nipple central passage 44a.
A suitable adhesive such as a medical grade Silastic adhesive, seals the nipple 44 in the recess 42 and the annular flange 43 compr~esses the outer surface of the nipple 44 to form a mechanical compression seal to securely retain the nipple 44 in the recess 42.
A rigid metal tube 51, preferably formed of titanium, ~ - , . . . . .. . .

is also sealed in nipple 44, such as by a Silastic adhesive, and extends through nipple central passage 44a and the central bore 3~ of the second support member 34 into the bellows interior 33 to communicate the interior 49 of the tambour 46 with the bellows 32. The other end 51b of the tube 51 forms a stop for the bellows end 32b.
In order to transmit the sensed ambient pressure to the interior of the bellows 32, the interior of tambour 46, the bellows interior 33 and the tube 51 are filled with a pressure transmitting fluid, isolated by means of the bellows 32 from the pressure transmitting fluid in the body pressure sensing means 11. In the preferre~ embodiment, all of the exposed metallic surfaces of the sensing apparatus of the invention are coated with a suitable biocompatible material, such as a medical grade Silastic adhesive. As shown in the drawings, this Silastic adhesive 50 extends from the nipple 44 to tambour 46 to the joint between the metal conduit 22 and the neck portion 25 of tambour 23.
In the illustrated embodiment, the radiation shielding means 31 includes a first portion 53 of radiation shielding material such as tantalum having a cup-shaped configuration.
The first portion 53 preferably includes an end plate 54, typically in the form of a disc, and an annular side member 56 both mounted on the other end 32b of the bellows 32 in closing relationship therewith as shown best in Figure 4.
The radiation shielding means first portion 53 is mounted on an inwardly directed channel portion 57 adjacent the last accordion pleat in the bellows 32, and a tubular sleeve portion 58 extends axially outward therefrom which together define an enclosure 59. The end plate 54 is adhesively secured in the end of the enclosure by a suitable adhesive such as an epoxy resin closing the end 32b o~ the bellows 32.

9~ ~'"

Similarly, side member 56 is adhesively secured by means of an epoxy resin to sleeve portion 58.
The radiation shielding means also includes a second portion 61 in the form of a tubular sleeve of radiation shielding material, also preferably made of tantalum, which is press-fitted or the like within the recess 20 of the first support member 16. It can be seen that the second p~rtion 61 extends throughout the depth of the recess 20 and has a forward end edge portion 61a terminating flush with the end of the first support member 16 abutting the housing side wall shoulder 13a. Thus, the second portion end 61a is precisely positioned axially in the housing interior 14 adjacent the end edge portion 56a of the first portion side member 56.
Radioactive source 29 is mounted on the end 32b of the bellows 32 and is accommodated for guiding movement within the radiation shield means second portion 61 disposed in the recess 20. As will be appreciated, however, the radiation shielding means rather than the radioactive source 29 may be mounted on bellows 32, such an arrangement merely representing an obvious reversal of cooperating parts. The radioactive source 29, which is preferably of cylindrical shape, has an outer diameter conforming generally to the inner diameter of the bellows sleeve portion 58 and is adhesively secured within the enclosure 59 defined by the tubular sleeve portion 58 by means of a suitable adhesive such as an epoxy resin. The end cap 6~ has a meniscus 62a formed by the adhesive material.
The blellows 32 resiliently urges the radioactive source 29, together with the sleeve portion 58, in the direction of the arrow I into the recess 20 with the edge portion 56a of the radiation shielding means first portion side member 56 in adjacent cooperating relationship with the edge portion 61a of the tubu:Lar sleeve forming the radiation shield means :.
-1 0- , .. - .... : : . :
- , ' . , ' ' -: ~

second portion 61 to establish a shielding relationship with the radioactive source 29. The end cap 62 is therefore disposed adjacent the outlet end 22b of the pressure transmitting fluid conduit 22 as will be discussed in more detail hereinafter.
The outer diameter o~ the bellows sleeve portion 58 is selected to produce a loose-fitting relationship with the inner surface of the sleeve ~Eorming the radiation shielding means second portion 61 so that fluid introduced into the recess 20 from the end 22b of conduit 22 may flow freely therebetween and through a gap between the first and second end edge portions 56a and 61a, respectively, to fill the interior 14 of the housing 10 on the outside of the bellows 32. It should be understood that in the assembled apparatus of the invention before installation in the body there is virtually no pressure differential in the housing 10 between the pressure-transmitting fluids on opposite side of the bellows 32. In this condition, there is a gap as will be discussed in more detail hereinafter between the adjacent end edge portions 56a and 61a of the first and second portions 53 and 61, respectively. ~hen the apparatus is installed in the body, the normal fluid pressure in the body cavity slightly increases the pressure on the tambour 23 introducing additional pressure transmitting fluid into the housing interior 14 on the outside of bellows 32, moving the bellows in the direction of the arrow D, and thereby increasing slightly the gap between the end edge portions 56a and 61a.
In the operation of the invention after installation, an increase in body pressure is sensed in the body cavity by the body pressure sensing device or tambour 23, the sensed pressure is transmitted by the pressure transmitting fluid flowing into the support member recess 20 through the end 22b of conduit 22 around the end cap 62 through the gap between .

the edge portions 56a, 61a to move the bellows 32 together with the radiation shielding means first portion 53 and the radioactive source 29 in the direction indicated by the arrow D in opposition to the urg:ing force exerted by the bellows. During this movement, the radiation shielding means 'irst and second portions 53, 61 move apart increasing the gap proportionally with the increase in cavity pressure thereby modifying the shielding relationship between the shielding means 31 and radioactive source 29 to expose more of the radioactive source in accordance with the magnitude of the cavity pressure. The radioactive output of the exposed portion of the radioactive source 29 may then be sensed by a receiver means (not shown) such as a conventional nuclear counter or crystal detector disposed externally of the housing 10 and the body.
The provision of the ambient pressure sensing means 12 permits the pressure sensor apparatus of the invention to be responsive to pressure changes in the body cavity regardless of ambient pressure changes. More specifically, ambient pressure changes are imposed equally on both the ambient pressure sensing means 12 and body pressure sensing means 11 whereby the sensing apparatus of the invention responds to body cavity pressure changes only.
The pressure sensor apparatus of the present invention is essentially insensitive to ambient temperature variations as well as temperature variations which may occur inside the animal or human body whose pressure is being monitored.
More specifically, bellows 32 has a spring constant which is orders of magnitude greater than the spring constant of pressure sensing tambours 23 and 46 which offer effectively no resistance to pressure changes. Accordingly, any volumetric changes in the pressure transmitting fluid due to temperature variations .

- : . . : ~ , :

s~

in the body or ambient temperature changes will act to distend the body pressure sensing tambour 23 and ambient pressure sPnsing tambour 46, respectively, instead of causing a movement (contraction or expansion) of bellows 32. Therefore, temperature variations whether external or internal to the cavity being monitored do not affect or change the output of the pressure sensor apparatus and therefore do not cause erroneous pressure measurements.
A unique feature of the present invention is the provision of means to enable in vivo calibration of the pressure sensor apparatus after implantation by a preselected output condition which is initially established during in vitro calibration. More specifically, in accordance with the present design, and using a radioactive source and associated radiation shielding as illustrative, a stop is provided so that there is a preselected output during calibration. This stop is provided in the embodiment illustrated by the end 22b of fluid conduit 22. After the pressure sensor apparatus is implanted, the surgeon can calibrate the instrument by pressing on the ambient pressure compensating tambour 46 which will cause the pressure transmittin~ fluid in the interior of tambour 46, the bellows interior 33 and the tube 51 to move bellows 32 and radioactive source 29 in the direction of arrow I so that the end cap 62 abuts against the end 22b of conduit 22. In this extreme stop position, there is a fixed and repeatable amount of radiation emitted from the pressure sensor apparatus.
Because the housing 10 in which the output means of the pressure sensor apparatus is housed is implanted under the scalp, for ~e~ample, the scalp or other body tissue surrounding the housing will attenuate the radiation output signal as a function of the scalp thickness. Accordingly, in the laboratory before implantation, the radiation output . . .

from the pressure sensor apparatus wi-th bellows 32 and radio-active source 29 at the e~treme stop position is measured for a range of simulated scalp thickness, for example 3 mil]imeter, 6 millimeter, and 9 millimeter of simulated scalp thickness. This measurement is also made over a range of prcssures which corres-ponds to the pressures which would normallybe encountered in the particular body cavity being monitored. Then, a family of curves is produced which correlate the radiation outpu-t with the pres-sure being monitored for each scalp thickness. After the sensor is implanted, the surgeon performs the in vivo calibration as described above by pressing on the ambient pressure compensating tambour 46 and forcing bellows 32 and radioactive source 29 to the extreme stop position. The radiation count obtained will fall on or near one of the family of curves. This curve is then used in monitoring the pressure.
To insure a long life for the pressure sensing appara-tus of the invention commensurate with body compatability, it has been found that specific non-reactive fluids and elastomeric materials eliminate such reactions. More specifically, the best results that have been obtained are when the elastomeric material of the various components are formed, in one example, from a Silastic type of silicone rubber and the pressure transmitting fluids are either castor oil or mineral oil between which there is virtually no chemical or physical reaction thereby insuring proper functioning of the pressure sensor apparatus throughout its life. It has also been found that, when the pressure transmitting fluid is a silicone oil, the outstanding results of the invention are accomplished when the elastomeric materials are selected from the group consisting of butyl, neoprene, Buna N (a -trade-mark) and Viton A (a trademark) rubbers. It should be understoodhowever, that . . .

other elastomeric ma~erials and fluids perform satisfactorily but with less desirable results.
One major concern in selecting a fluid is the osmotic pressure effects produced during implantation. In order to eliminate these effects, it is preferred that a simulated cerebrospinal fluid be used as the pressure transmitting medium, and it may be used with all materials of construction as it will be compatible with body fluids and will not leak through the elastomeric materials as a consequence of osmotic pressure.
In the use of the invention to monitor the fluid pressure within an intracranial cavity and with reference to Figures 5-7, the common practice is to provide a burr hole or aperture 70 within the bony structure of skull 71 overlying the intracranial cavity through which the metallic fluid conduit 22 is inserted, the body pressure sensing device 23 being suitably disposed within the intracranial cavity. A
body pressure sensing means such as tambour 23 is normally positioned subdurally. However, it should be appreciated that the invention also contemplates positioning the body pressure sensing means 11 epidurally in which case the shape and size of the tambour will be appropriately changed. The housing 10, together with the ambient pressure compensating device 12, is mounted on the outer surface of the skull 71 under the scalp 72.
The apparatus of the invention includes means for permanently mounting the housing 10 and ambient pressure sensing means 12 subcutaneously on the outer surface of the skull 71 in an inconspicuous, securely retained position.
More specifically, an elongated concave groove 73 is formed within the outer surface of the skull 71 adjacent the burr hole 70, and mounting means are provided for securing the . : ' :

housing 10 in a seated relationship within the groove 73.
The mounting means includes at least one, preferably two, tabs 74 arranged in longitudinally spaced relationship on the housing 10 as shown best in Figure 5. Each of the tabs 74 includes an intermediate portion 75 of arcuate cross-sectional shape for accommodating -the tubular housing 10 in underlying engagement therewith. The tab portion 76 is secured to the outer surface of the housing 10 by suitable means such as a body compatible adhesive, welding or the like. The tabs 74 also include oppositely disposed end portions 77 and 7~
extending laterally outward of the housing 10 secured within the intermediate portion 76.
Openings 79, 81 are provided in the tab end portions 77, 78 respectively for accommodating screw means such as screws 82 extending therethrough in threaded engagement with the underlying bone of the skull 71, and with the end portions 77, 78 in overlying engagement with the outer surface of the skull 71, the tab intermediate portion 76 and housing 10 being accommodated within the groove 73.
In the preferred embodiment, the tabs 74 are preferably formed of a radiation shielding material such as tantalum.
One of the tabs 74 is positioned on the housing 10 with its intermediate portion 76 extending throughout the path of movement of the radioactive source 61 within the housing.
Thus, not only does the one tab 74 prevent downwardly directed radiation into the body, but the radioactive output of source 29 is confined in a non-attenuating manner to the upward direction to permit easy detection by an externally positioned detection device.
The output of the radioactive source 29 need only be of an extremely low order of magnitude, typically less than 0.1 microcurie, a magnitude far less than that at which .

s~

the adjacent body ~issue may be adversely affected. ~owever, it should be characterized by an extremely precise and uniform output rate which accurately reflects the changes in fluid pressure within the body cavity throughout its range of operation. The preferred radioisotopes used in the present invention are promethium-145, carbon-14, nickel-63, strontium-90 and americium-241, and, to obt:ain the proper radioactive output from the source 29, it should be in the form of a shaped article of highly homogeneous composition.
The radioactive source 29 typically comprises promethium-145 chloride (PmC13), for example, uniformly distributed and absorbed onto an inert carrier such as diatomaceous earch the uniformly distributed throughout a suitable binder such as an epoxy resin. Sources 29 of this composition are extremely uniform regarding the concentration or distribution of the radioisotope.
Although the invention has been described in terms of a single preferred embodiment, nevertheless, changes and modifications may be made within the scope of the invention.
For example, the pressure sensor apparatus as illustrated provides an output which is a direct function of the pressure being monitored since the output increases with increasing body cavity pressure. However, as will be appreciated by one of ordinary skill in the art, the sensor can also be constructed so that the output is an indirect function of the pressure by mounting the radioactive source and the associated radiation shielding so that the radioactive source is increasingly shielded b~ the radiation shielding as the pressure being monitored increases. In this type of arrangement, the ln vivo calibration is performed in the same manner as described herein except that the output will be a predetermined maximum output rather than a predetermined minimum output. Also, while a radioactive source and associated radiation shielding have been illustrated as the output means it will be appreciated that other output means can be employed without altering the basic characteristics of the invention. For example, the output means could comprise a resonant L-C circuit having a variable capacitor or inductor in which bellows 32 is mechanically connected to the variable component to vary the value of capacitance or inductance and hence the resonant frequency of the L-C circuit in response to the pressure changes in the body cavity being monitored. The output could then be detected by a variable-frequency oscillator, for example, or other means well known in the art. Accordingly, the invention should not be limited by the specific embodiment illustrated but only as defined in the appended claims.

.

Claims (18)

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

1. A pressure sensor apparatus for indicating pressure in an animal or human body comprising a housing, a bellows contained within said housing, said bellows having a wall thickness of less than 1 mil and a spring rate of 0.130 to 0.240 pounds per inch, means for placing said bellows in communication with said pressure in the body so that said pressure will cause said bellows to move as a function of said pressure, and output means contained within said housing and associated with said bellows for communicating the move-ment of said bellows to a receiver means located external to said body to provide data indicative of said pressure.

2. The pressure sensor apparatus of claim 1 in which said bellows has a wall thickness of less than about 0.5 mil.

3. The pressure sensor apparatus of claim 1 in which said bellows has a wall thickness of about 0.25 to 0.33 mil.

4. The pressure sensor apparatus of claim 1 in which said bellows has a spring rate of about 0.185 pound per inch.

5. The pressure sensor apparatus of claims 1, 2 or 3 in which said bellows is a gold-plated nickel bellows.

6. The pressure sensor apparatus of claim 1 in which said bellows is resilient and made of a metal having essentially 100% memory of position, and in which said output means con-tained within said housing and associated with said bellows produces an output, which is a function of the movement of said bellows, to said receiver means located external to said body to provide data indicative of said pressure.

7. The pressure sensor apparatus of claim 1 in which said means for placing said bellows in communication with said pressure in the body is such that said pressure will cause said bellows to contract and expand, and said output means contained within said housing produces an output which is a function of said pressure, said output means having at least two components, the first of said two components being operatively connected to said bellows and movable therewith when said bellows expands and contracts, the second of said two components being operatively associated with said first component but not movable with said bellows so that, when said first component moves, the output of said output means varies as a function of said pressure, said receiver means comprising sensor means which is located external to said body and which is responsive to said output to provide data indicative of said pressure, the output of said output means being adapted so as to be sensed by said sensor means.

8. The pressure sensor apparatus of claim 7 in which said bellows is resilient and is made of a metal.

9. The pressure sensor apparatus of claim 1, further comprising flexible means associated with said housing and adapted to move as a function of the pressure in said body, and a pressure transmitting fluid contained within said housing and communicating with said flexible means so that said pressure acting upon said flexible means will cause said bellows to contract and expand as a function of said pressure, said output means contained within said housing having first and second components cooperating to produce an output, said first component being operatively associated with said bellows and movable therewith, and said second component being operatively associated with said first component but not movable with said bellows so that said output produced by said output means is a function of the contraction and expansion of said bellows, said apparatus further comprising ambient pressure sensing means associated with said bellows for compensating for ambient pressure variations acting upon said flexible means, whereby said out-put is sensed by said receiver means, said receiver means comprising a sensor means located external to said body to provide data indicative of said pressure.

10. The pressure sensor apparatus of claim 9 in which said flexible means associated with said housing comprises a flexible tambour made of a material and shape which offers effectively no resistance to pressure changes in said body.

11. The pressure sensor apparatus of claim 9 in which said ambient pressure sensing means comprises second flexible means associated with said housing and adapted to move as a function of the ambient pressure, and a second pressure transmitting fluid contained within said housing and communi-cating with said second flexible means so that said ambient pressure acting upon said second flexible means will be communicated to said bellows.

12. The pressure sensor apparatus of claim 11 in which said second flexible means comprises a flexible tambour made of a material which offers effectively no resistance to pressure changes.

13. The pressure sensor apparatus of claim 1, in which said bellows is resilient and made of gold-plated nickel, said bellows further having a wall thickness of about 0.25 to 0.33 mil.

14. The pressure sensor apparatus of claim 1, in which said bellows has essentially 100% memory of position so that it is capable of recovering its size and shape after deforma-tion, said apparatus further comprising flexible means associated with said housing and adapted to move as a function of the pressure in said human body, and a pressure transmitting fluid contained within said housing and communicating with said flexible means so that said pressure acting upon said flexible means will cause said bellows to contract and expand as a function of said pressure, said output means contained within said housing having first and second components cooperating to produce an output, said first component being operatively associated with said bellows and said second component being operatively associated with said first component but not movable with said bellows so that said out-put produced by said output means is a function of the contraction and expansion of said bellows, said apparatus further comprising ambient pressure sensing means associated with said bellows for compensating for ambient pressure variations acting upon said flexible means, whereby said output is sensed by said receiver means, said receiver means comprising a sensor means located external to said body to provide data indicative of said pressure.

15. The pressure sensor apparatus of claim 14 in which said flexible means associated with said housing comprises a flexible tambour made of a material which offers effectively no resistance to pressure changes in said body.

16. The pressure sensor apparatus of claim 14 in which said ambient pressure compensation means comprises second flexible means associated with said housing and adapted to move as a function of the ambient pressure and a second pres-sure transmitting fluid contained within said housing and communicating with said second flexible means so that said ambient pressure will be communicated to said bellows.

17. The pressure sensor apparatus of claim 16 in which said second flexible means comprises a flexible tambour which offers effectively no resistance to pressure changes.

18. The pressure sensor apparatus of claim 1 in which the apparatus particularly indicates intracranial pressure, said apparatus further comprising first flexible means associated with said housing and adapted to move as a function of the pressure in said body, a first pressure transmitting fluid contained within said housing on one side of said bellows and communicating with said first flexible means so that said pressure acting upon said first flexible means will cause said bellows to contract and expand as a function of said pressure in said body, said output means contained within said housing producing an output which is a function of the contraction and expansion of said bellows, said apparatus further comprising second flexible means associated with said housing, a second pressure transmitting fluid contained within said housing on the other side of said bel-lows and communicating with said second flexible means, said flexible means having essentially no resistance to pressure changes, said bellows being resilient, said spring rate of said bellows being substantially greater than the spring rate of said flexible means, whereby said output is sensed by said receiver means, said receiver means comprising a sensor means located external to said body to provide data indicative of said pressure in said body, and whereby said apparatus is essentially insensitive to temperature variations.