GB1589171A

GB1589171A - Pressure sensor apparatus for indicating pressure in the body

Info

Publication number: GB1589171A
Application number: GB4018777A
Authority: GB
Original assignee: Hittman Corp
Current assignee: Hittman Corp
Priority date: 1976-09-27
Filing date: 1977-09-27
Publication date: 1981-05-07
Also published as: AU2900677A; DE2743401B2; DE2743401A1; AU510265B2; FR2365326A1; DE2743401C3

Description

(54) PRESSURE SENSOR APPARATUS FOR INDICATING PRESSURE IN THE BODY (71) We, HITTMAN CORPORATION, a corporation organised and existing under the laws of the State of Delaware, United States of America, of 9190 Red Branch Road, Columbia, Maryland 21045, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to pressure sensor apparatus for indicating pressure in the body.

Reference is directed to our UK Patent No. 1 516 729, which is also directed to such pressure sensor apparatus.

The need for a non-invasive technique for measuring the pressure in body cavities of animals or humans is recognised as highly desirable for continuous or intermittent monitoring of body conditions. Such cavities as the cranium, vena cava, bladder, and others provide valuable and sornetimes critical information for maintaining the well being or survival of an animal or human.

For example, it is known that intracranial pressure provides a valuable indication of well being for a variety of clinical conditions, including shock trauma and hydrocephalus.

According to the present invention there is provided a pressure sensor apparatus for indicating pressure in the body, the apparatus comprising detection means for detecting the pressure in the body, output means associated with the detection means for providing an output which is a function of said pressure to a reservoir means located external to the body to provide data indicative of said pressure, and calibration means for calibrating the apparatus in vivo by reproducing an output during in vivo calibration which is established during in vitro calibra tion.

A pressure sensor apparatus that embodies the present invention and is described in detail hereinbelow is fully implantable and contains output means, such as a radioactive source and associated shielding, so that the pressure can be read out noninvasively. The preferred pressure sensor apparatus includes a housing containing a bellows in communication with a body pressure sensing tambour placed in the body and exposed to the pressure to be sensed. An ambient pressure sensing tambour is also associated with the bellow for compensating for ambient pressure variations. The housing is, in use, located external to a body cavity being sensed and preferably situated just under the skin. The housing also contains the output means, which is associated with the 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 preferred pressure sensor apparatus includes means associated with the bellows to enable the in vivo calibration of the pressure sensor after implantation by reproducing the output condition which is initially established during in vitro calibration. More specifically, and using a radioactive source and associated radiation shielding as illus trative, a stop is provided so that there is a fixed radiactive source to radiation shielding relationship which results in a repeatable radiation output during in vivo calibration.

The bellows of the preferred apparatus is resilient, made of a material and of a shape which has 100% memory of elastic deformation and has a wall thickness of less than 0.001 inch in order to provide an output which accurately reflects the pressure being monitored. The bellows has a spring con stant 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 apparatus essentially insensitive to temperature variations.

The preferred pressure sensor apparatus is fully implantable and does not require any energy source other than the radiactive material, for example, contained in the apparatus. Another major advantage of the preferred apparatus 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 145, carbon-14, nickel-63, strontium-90, or americium-241, the preferred pressure sensor apparatus can be fully implanted and left in place for long periods of time.

The preferred pressure sensor apparatus functions accurately to within several millimeters of water pressure and in unaffected by variations in ambient pressure. Also, it is generally insensitive to ambient temperature and can, as mentioned, be calibrated in vivo. Furthermore, the materials used to construct the preferred sensor apparatus are biologically inert and do not pose any health hazard to the animal or human body or make the patient more susceptible to mechanical trauma. The preferred sensor apparatus is of relatively small size and so does not produce unsightly bulging when implanted subdermially. Further the apparatus has low sensitivity to temperature change.

The invention will now be further described, by way of illustrative and non-limiting example, with reference to the accompanying drawings, in which: Figure 1 is a perspective view of an implantable non-invasive pressure sensor apparatus constituting a preferred embodiment of the invention, the apparatus being shown 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 1; Figure 3 is a sectional view taken substantially along line 3-3 of Figure 3 in the direction of the arrows; Figure 4 is an enlarged sectional view of a portion of the apparatus as shown in Figure 3; Figure 5 is an enlarged perspective view of mounting arrangements for the apparatus of Figure 1; Figure 6 is a perspective view of a portion of the skull of a patient prior to installation of the apparatus; and Figure 7 is a sectional view taken sub stantially along line 7-7 of Figure 5 in the direction of the arrows.

Referring now to the drawings and to Figures 1 and 2 in particular, there is shown a pressure sensor apparatus embodying the invention with a housing designated 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 connected to the housing 10 and is responsive to ambient pressure to compensate for changes in ambient pressure during the operation of the apparatus. Although the pressure sensor apparatus 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 use of apparatus embodying 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 apparatus ambodying the invention is equally applicable to monitoring pressure in other body portions and cavities.

Referring now to Figure 3, the housing 10, which is preferably formed of titanium, is of tubular shape having a side wall 13 defining an interior 14. 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 16 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 which is arranged to be connected to a 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 (registered trade mark) rubber and is of a substantially flat configuration including a neck portion 25 in the wall of which is moulded a helical spring 26, pre ferably 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 spacedapart relationship and to serve as a locating means for the tambour with the use of Xrays. 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 tambour 23 in the body cavity is transmitted to the housing interior 14.

The conduit 22 is inserted through the bore 17 with the other end 22b of the conduit 22 extending through the bore 17 into the recess 20 as shown best in Figures 3 and 4. Sealing engagement between the conduit 22 and bore 17 is obtained by means epoxy resin or the like. Thus, the conduit end 22b communicates with the recess 20 and with the housing interior 14.

An output means of the pressure sensor apparatus preferably comprises a source 29 of radioactive material, normally in the form of 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 neckeddown 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 neckeddown 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.

The bellows 32 is made of a resilient material, preferably a metal such as goldplated nickel, and has essentially 100% memory of position. The bellows 32 has a wall thickness 32c which is less than 0.001 inch and preferably less than about 0.0005 inch. The most preferred wall thickness range is between about 0.00025 and 0.00033 inch. The bellows 32 has a length, typically about 1 inch, which varies according to the pressure which the pressure sensor apparatus is intended to monitor. The spring rate of the bellows 32 is in the range of about 0.130 to 0.240 pound per inch with the preferred spring rate being about 0.185 pound per inch. By providing the bellows 32 with a wall thickness of less than 0.001 inch and a corresponding spring rate, the bellows will typically travel (contract or expand) a distance of about 0.050 to 0.060 inch when the body pressure sensing tambour 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 the bellows 32 is caused by the pressure being monitored. Since the bellows (spring) is operated or stressed within its elastic range, it will return to its equilibrium position when the stress (pressure) is removed. In other words, the bellows 32 has 100% memory of elastic deformation. Accordingly, 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 adjoins a fluid conduit 44 via an angled surface 51 a, the conduit 44 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 (registered trade mark) rubber, and includes an annular peripheral portion 47 and a recessed central portion 48 defining an interior 49 extending through the central passage 44a.

The second support member 34 which receives the end of fluid conduit 44 is formed of a suitable elastomeric material such as medical grade silastic rubber. A suitable adhesive, such as medical grade Silastic (registered trade mark) adhesive, seals the fluid conduit 44 to the second support member 34 and the tambour 46.

A rigid metal tube 51, preferably formed of titanium, is also sealed in the conduit 44, such as by a Silastic (registered trade mark) adhesive, and extends through the conduit central passage 44a and the central bore 38 of the second support member 34 into the bellows interior 33 to communicate the interior 49 of the tambour 46 with the bellows 32. An 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 tambour 46, the fluid conduit 44, 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 this preferred em embodiment, all of the exposed metallic sur faces of the sensing apparatus are coated with a suitable biocompatible material, such as a medical grade Silastic (registered trade mark). As shown in the drawings, this Silastic (registered trade mark) coating 50 extends from the fluid conduit connection 44 to the tambour 46 to the joint between the metal conduit 22 and the neck portion 25 of the 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 of the bellows 32. Similarly, the side member 56 is adhesively secured by means of an epoxy resin to the 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 portion 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.

The radioactive source 29 is mounted on the end 32b of the bellows 32 and is accommodated for guided 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 the 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 62 has a meniscus 62a formed by the adhesive material.

The bellows 32 resiliently urges the radioactive source 29, together with the sleeve portion 58, in the direction of an 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 tubular sleeve forming the radiation shield means 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 of the bellows sleeve portion 58 is selected to produce a loosefitting relationship with the inner surface of the sleeve forming the radiation shielding means second portion 61 so that fluid introduced into the recess 20 from the end 22b of the 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 before installation in the body there is virtually no pressure differential in the housing 10 between the pressuretransmitting 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. When 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 an arrow D, and thereby increasing slightly the gap between the end edge portions 56a and 61a.

In the operation of the apparatus 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 urging force exerted by the bellows. During this movement, the radiation shielding means first 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 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 the body pressure sensing means 11 whereby the sensing apparatus responds to body cavity pressure changes only.

The pressure sensor apparatus 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, the bellows 32 has a spring constant which is orders of magnitude greater than the spring constant of the 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 in the body or ambient temperature changes will act to distend the body pressure sensing tambour 23 and ambient pressure sensing tambour 46, respectively, instead of causing a movement (contraction or expansion) of the 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. Moreover, all of the components of the pressure sensor apparatus are implanted in lose proximity to each other and therefore are essentially at a constant temperature (i.e., body temperature). Since none of the components of the pressure sensor are temperature sensitive and since all of the components are essentially at the same temperature, the sensor is temperature insensitive for this additional reason.

A unique feature of the present apparatus is the provision of means to enable in- vivo calibration of the pressure sensor apparatus after implantation by reproducing an 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 fixed radioactive source to radiation shielding relationship which results in a repeatable radiation output during calibration. This fixed radioactive source to radiation shielding relationship is set at a point where the bellows 32 is essentially in its equilibrium condition (i.e., where there is essentially a zero pressure differential across the bellows). This stop is provided in the embodiment illustrated by the end 22b of the 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 transmitting fluid in the interior of tambour 46, the bellows interior 33 and the tube 51 to move the bellows 32 and radioactive source 29 in the direction of arrow I so that the end cap 62 abuts against the end 22b of the conduit 22. In this extreme stop position, there is a fixed and repeatable amount of radiation emitted from the pressure sensor apparatus, representing in effect a zero (or known) pressure differential across the bellows 32.

Because the housing 10 in which the output means of the pressure sensor apparatus is housed is implanted under the scalp, for example, 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 with the bellows 32 and radioactive source 29 at the extreme stop position is measured for a range of simulated scalp thickness, for example, 3 millimeters, 6 millimeters, and 9 millimeters of simul;ated scalp thickness. This measurement is also made over a range of pressures which corresponds to the pressures which would normally be encountered in the particular body cavity being monitored.

Then, a family of curves is produced which correlate the radiation output with the pressure 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 the 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 or this data is used to properly establish the relationship of radioactive counts and pressure differential in a direct readout instrument.

To ensure a long life for the pressure sensing apparatus commensurate with body compatibility, 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, mineral oil, or synthetic cerebrospinal fluid between which there is virtually no chemical or physical reaction thereby ensuring 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, outstanding results are accomplished when the elastomeric materials are selected from butyl, neoprene, Buna N and Viton A rubbers.

("Viton" is a registered trade mark.) It should be understood, however, that other elastomeric materials and fluids perform satisfactorily but with less desirable results.

One major concern in selecting a fluid is the osmotic pressure effects produced after 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 apparatus 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 the 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 the tambour 23 is normally positioned subdurally. However, it should be appreciated that it is also possible to position 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 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 tab 76 on the housing 10 as shown best in Figure 5.

The tab 76 includes an intermediate portion 76a of arcuate cross-sectional shape for accommodating the tubular housing 10 in underlying engagement therewith. The tab portion 76a is secured to the outer surface of the housing 10 by suitable means such as a body compatible adhesive, welding or the like. The tab 76 also includes oppositely disposed end portions 77 and 78 extending laterally outwardly of the housing 10 and secured within the intermediate portion 76a.

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 76a and housing 10 being accommodated within the groove 73. Also accommodated within the groove 73 is a spacer 74 which surrounds the end of housing 10 adjacent pressure sensing tambour 46. The thickness of the spacer 74 is the same as the thickness of the tab intermediate portion 76a so that the housing 10 will lie relatively flat in the groove 73. The spacer 74 may be made from any suitable material such as a plastics or a radiation shielding material such as tantalum.

The pressure sensor apparatus also includes a collimator 84 for collimating the output of the pressure sensor apparatus.

The collimator 84 comprises radiation shielding in the housing. Thus, not only does the one tab 76 prevent downwardly directed radiation into the body, but the radioactive output of the source 29 is confined in a non-attenuating manner to the upward direction to permit easy detection byl an externally positioned detection device.

The output of the radioactive source 29 need only be of a extremely low order of magnitude, typically less than 0.1 microcurie, a magnitude far less than that at which the adjacent body tissue may be adversely affected. However, 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. Preferred radioisotopes for use in the present apparatus are promethium-145, carbon-14, nickel-63, strontium-90 and americium-241, and, to obtain the proper radioactive output from the source 19, it should be in the form of a shaped article of highly homogeneous composition.

The radioactive source 29 typically comprises promethium-145 chloride (PmCl3), for example, uniformly distributed and absorbed onto an inert carrier such as diatomaceous earth and 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, it will be appreciated that 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 by the radiation shielding as the pressure being monitored increases. In this type of arrangement, the in vivo calibration is performed in the same manner as described herein except that the output will be a repeatable and/or predetermined maximum output rather than a repeatable and/or 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 ex ample, the output means could comprise a resonant L-C circuit having a variable capa citor or inductor in which the 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.

WHAT WE CLAIM IS: 1. A pressure sensor apparatus for indicating pressure in the body, the apparatus comprising detection means for detecting the pressure in the body, output means associated with the detection means for providing an output which is a function of said pressure to a reservoir means located external to the body to provide data indicative of said pressure, and calibration means for calibrating the apparatus in vivo by reproducing an output during in vivo calibration which is established during in vitro calibration.

2. A pressure sensor apparatus according to claim 1, comprising a housing and a bellows contained within the housing, wherein the detection means comprises means for placing the bellows in communication with the pressure in the body so that said pressure will cause the bellows to move as a function of said pressure, and wherein the output means is contained within the housing and is associated with the bellows such that said output thereof is a function of the movement of the bellows.

3. A pressure sensor apparatus according to claim 2, wherein the output means comprises at least two components, a first of the two components being operatively connected to the bellows and movable therewith when the bellows expands and contracts, and the second of the two components being operatively associated with the first component but not movable with the bellows so that when the first component moves the output of the output means varies as a function of said pressure.

4. A pressure sensor apparatus according to claim 3, wherein the detection means further comprises flexible means associated with the housing and arranged to move as a function of the pressure in the body and a pressure transmitting fluid contained within the housing and communicating with the flexible means so that said pressure acting upon the flexible means will cause said bellows to contract and expand as a function of said pressure, the apparatus further comprising ambient pressure sensing means associated with the bellows for compensating for ambient pressure variations acting upon the flexible means.

5. A pressure sensor apparatus according to claim 4, wherein the spring rate of

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. in the housing. Thus, not only does the one tab 76 prevent downwardly directed radiation into the body, but the radioactive output of the source 29 is confined in a non-attenuating manner to the upward direction to permit easy detection byl an externally positioned detection device. The output of the radioactive source 29 need only be of a extremely low order of magnitude, typically less than 0.1 microcurie, a magnitude far less than that at which the adjacent body tissue may be adversely affected. However, 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. Preferred radioisotopes for use in the present apparatus are promethium-145, carbon-14, nickel-63, strontium-90 and americium-241, and, to obtain the proper radioactive output from the source 19, it should be in the form of a shaped article of highly homogeneous composition. The radioactive source 29 typically comprises promethium-145 chloride (PmCl3), for example, uniformly distributed and absorbed onto an inert carrier such as diatomaceous earth and 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, it will be appreciated that 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 by the radiation shielding as the pressure being monitored increases. In this type of arrangement, the in vivo calibration is performed in the same manner as described herein except that the output will be a repeatable and/or predetermined maximum output rather than a repeatable and/or 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 ex ample, the output means could comprise a resonant L-C circuit having a variable capa citor or inductor in which the 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. WHAT WE CLAIM IS:

1. A pressure sensor apparatus for indicating pressure in the body, the apparatus comprising detection means for detecting the pressure in the body, output means associated with the detection means for providing an output which is a function of said pressure to a reservoir means located external to the body to provide data indicative of said pressure, and calibration means for calibrating the apparatus in vivo by reproducing an output during in vivo calibration which is established during in vitro calibration.

5. A pressure sensor apparatus according to claim 4, wherein the spring rate of

the bellows is substantially greater than that of the flexible means.

6. A pressure sensor apparatus according to claim 4 or claim 5, in which the flexible means comprises a flexible tambour made of a material which offers effectively no resistance to pressure changes in said body.

7. A pressure sensor apparatus according to claim 5 or claim 6, in which the ambient pressure sensing means comprises second flexible means associated with the housing and arranged to move as a function of the ambient pressure and a second pressure transmitting fluid contained within the housing and communicating with the second flexible means so that the ambient pressure acting upon the second flexible means will be communicated to the bellows.

8. A pressure sensor apparatus according to claim 7, in which the spring rate of the bellows is substantially greater than that of the second flexible means.

9. A pressure sensor apparatus according to claim 7 or claim 8, in which the second flexible means comprises a flexible tambour made of such a material and of such a shape that it offers effectively no resistance to pressure changes.

10. A pressure sensor apparatus according to claim 9, in which the calibration means comprises stop means associated with said first component so that the first component can be moved to a predetermined fixed position relative to the second component by pressing on the flexible tambour comprising the second flexible means.

11. A pressure sensor apparatus according to any one of claims 3 to 9, in which the calibration means comprises stop means associated with said first component so that the first component can be moved to a predetermined fixed position relative to the second component.

12. A pressure sensor apparatus according to any one of claims 4 to 11, which is designed to act as an intracranial pressure sensor, wherein the housing is capable of being positioned between the skull and scalp, the bellows divides the housing into first and second chambers, the housing has inlet means connected to the first chamber and outlet means connected to the second chamber, the detection means is connected to the inlet means and the flexible means thereof is capable of being positioned inside the skull, and the ambient pressure sensing means is connected to the outlet means and is capable of being exposed to pressure between the scalp and skull to compensate for changes in ambient pressure externally of the scalp.

13. A pressure sensor apparatus according to any one of claims 2 to 12, wherein the bellows has a wall thickness of less than 1 mil.

14. A pressure sensor apparatus according to claim 13, in which the bellows has a wall thickness of less than 0.5 mil.

15. A pressure sensor apparatus according to claim 14, in which the bellows has a wall thickness of from 0.25 to 0.33 mil.

16. A pressure sensor apparatus according to any one of claims 2 to 15, in which the bellows has a spring rate of from 0.130 to 0.240 pound per inch.

17. A pressure sensor apparatus according to any one of claims 2 to 16, in which the bellows is made of metal.

18. A pressure sensor apparatus according to claim 17, in which the bellows is a gold-plated nickel bellows.

19. A pressure sensor apparatus according to any one of claims 2 to 18, in which the bellows has essentially 100% memory of position so that it is capable of recovering its size and shape after deformation.

20. A pressure sensor apparatus for indicating pressure in the body, the apparatus being substantially as herein described with reference to the accompanying drawings.