GB2469820A

GB2469820A - Fluid Pressure Sensing Device

Info

Publication number: GB2469820A
Application number: GB0907271A
Authority: GB
Inventors: William Richard Johns; Stephen Warwick James Brown; Richard Phillips; Dale Rogers
Original assignee: Haemair Ltd
Current assignee: Haemair Ltd
Priority date: 2009-04-28
Filing date: 2009-04-28
Publication date: 2010-11-03
Also published as: WO2010125379A1; GB0907271D0

Abstract

A fluid pressure sensing device is disclosed for sensing the pressure of a fluid within a tube. The device comprises a clamp for clamping the tube and sensing means for sensing changes in the cross-sectional area of the tube. By referencing the changes in the cross-sectional area of the tube to predetermined calibration data, which relate known changes in tube cross-sectional area to known fluid pressures, it is possible to continuously monitor fluid pressure within the tube.

Description

Fluid Pressure Sensing Device The present invention relates to a fluid pressure sensing device and particularly, but not exclusively to a pressure sensing device for measuring blood pressure.

A number of medical devices take blood from a person's body and pass it through equipment before returning it to the body. An example is a mass exchanger that is used as a respiratory aid. There are also devices, such as that disclosed in international patent application PCTIGB2008I00051 1, in which blood properties are assessed by passing it through tubes and measuring pressure drops. In all these devices, it is desirable to have accurate measures of the pressure of the flowing blood, primarily to assess pressure drops across the apparatus. The pressure drop measurement can provide an early warning of any build up of blockages and can provide an indication of the progress of blood clotting, for example to optimize the administration of anticoagulants. Furthermore, it is desirable to have a continuous reading that tracks the change in pressure as the blood responds to changes in flow rate or rheological properties.

The requirement is quite distinct from and unrelated to conventional blood pressure monitoring. Conventionally, "blood pressure" is measured using a sphygmomanometer or similar device. These devices measure a range of pressure in the brachial artery as an indication of the health of the heart and of the circulatory system. The pressure in the artery pulses between the systolic pressure at which the blood flow in the artery is cut off, and the diastolic pressure at which there is a free flow of blood. This pressure is determined by monitoring a so-called "Korotkoff Sound". In contrast, the present invention is concerned with fluid flows through devices and not with the pressure range of a pulsed flow in an artery.

When blood flows through a device, it is important that the device does not contribute to any clotting of the blood. Furthermore, it is important that the sensor should not introduce a zone in which the blood flow ceases. Any pressure tapping (namely a hole in a tube for example) or probe (which extends through the hole) will introduce a disturbance in the blood flow and creates a potential nucleus for blood clots to form.

Furthermore, the tapping or probe cannot be used for blood from a second patient because it can lead to cross-contamination.

There are similar incentives in a number of other applications. For example, in the pharmaceutical industry, the biotechnology industry and the food industry, fluids are frequently passed through heat exchangers, mass exchangers and other devices. As with blood flow, it is desirable to measure the pressure of these fluids to monitor pressure drops through equipment and thus to ensure that the operation is within a defined range of parameters. These fluids must not be contaminated and the equipment must be thoroughly cleaned between runs. This cleaning is particularly critical in multi-product plants where different fluids, and possibly containing different micro-organisms, are passed through the equipment on successive runs.

We have now devised a fluid pressure sensing device which meets the above objectives.

In accordance with the present invention as seen from a first aspect, there is provided a fluid pressure sensing device for sensing the pressure of a fluid within a tube, the device comprising sensing means for sensing changes in the cross-sectional area of the tube and for providing an indication of the pressure of the fluid within the tube in dependence upon the sensed changes in cross-sectional area of the tube.

In use, the pressure of the fluid, for example blood, can thus be continuously and reliably inferred from changes in the cross-sectional area of the tube. Furthermore, since no probes or projections extend into the tube, then the propensity of blood to clot is minimised.

The sensing device of the present invention further enables blood pressure to be determined by monitoring blood that is flowing in tubes of other medical instruments.

This obviates the need to join tubes and thus eliminates any discontinuities where the tubes are joined that would otherwise disturb the blood flow.

Preferably, the sensing means further comprises a clamp for clamping against the tube.

The tube is preferably clamped by the clamp along a length of the outer surface of the tube.

Preferably, the clamp comprises an aperture or opening to expose a portion of the outer surface of the clamped length of tube.

Preferably, the sensing means senses changes in the cross-sectional area of the exposed portion of the tube.

Preferably, the clamp comprises a clamping member for clamping the tube to a clamp body. The aperture or opening is preferably formed in the clamp body. Alternatively, however, the aperture or opening may be formed in the clamping member.

Preferably, the sensing means comprises an optical sensing device which detects changes in cross-sectional area of the tube by monitoring the deflection of a light beam that is incident upon the exposed portion of the tube.

Alternatively, the sensing means preferably comprises a strain gauge, comprising an arm that is rigidly secured at a proximal end thereof with respect to the clamp.

Preferably, the distal end of the arm comprises a pad that is arranged to contact the exposed portion of the outer surface of the tube.

The proximal end of the arm is preferably rigidly secured between the clamp body and a base member. The pad of the strain gauge is preferably arranged to extend within the aperture or opening. The arm is preferably arranged to flex as the cross-sectional area of the tube changes so as to cause the pad to move within the aperture or opening.

The clamp body and base member extend substantially parallel to each other and are arranged in spaced relation to each other. Preferably the spacing between the clamp body and the base member can be adjusted using adjustment means to vary the extent to which the pad of the strain gauge extends within the aperture or opening in the clamp body.

Preferably, the clamping member comprises a groove for receiving the tube and which constrains the tube to expand and contract within the aperture or opening. The groove is preferably dimensioned to provide a close fit with the tube and extends across the width of the clamping member, such that when the tube is clamped, any movement of the tube outside the groove has minimal effect on the shape of the tube within the groove. Alternatively, or in addition thereto, the clamp body preferably comprises a groove for receiving the tube and which constrains the tube to expand and contract within the aperture or opening.

The sensing device preferably further comprises processing means for processing signals output from the sensing means. Preferably, the signals output from the sensing means are indicative of changes in cross-sectional area of the tube and thus the fluid pressure within the tube.

In accordance with the present invention as seen from a second aspect there is provided a fluid pressure measurement device for measuring the pressure of a fluid within a tube, the device comprising the fluid pressure sensing device of the first aspect and a tube for receiving the fluid.

Preferably, the measurement device further comprises memory means for storing calibration data, which relates sensed changes in the cross-sectional area of tube to fluid pressure within the tube.

In accordance with the present invention as seen from a third aspect, there is provided a method of determining fluid pressure within a tube, the method comprising the steps of: -providing a tube containing a fluid; -sensing changes in the cross-sectional area of the tube; and, -referencing the sensed changes in cross-sectional area to calibration data which relate sensed changes in the cross-sectional area of the tube to fluid pressure within the tube.

In accordance with the present invention as seen from a fourth aspect, there is provided a method of determining the propensity of a fluid to change apparent viscosity, the method comprising the use of the device of the first or second aspect.

Preferably, the method comprises passing the fluid through at least two tubes, the at least two tubes comprising different cross-sectional areas.

Embodiments of the present invention will now be described by way of example only and with reference to the accompanying drawings in which: Figure 1 is perspective view of the fluid pressure sensing device according to the present invention; Figure 2 is a sectional view of the fluid sensing device of figure 1, taken along line A-A, with a tube suitably clamped; Figure 3 is a front view of the fluid pressure sensing device of figure 1, with a tube suitably clamped; Figure 4 is a plan view of the fluid pressure sensing device of figure 1, with a tube suitably clamped; Figure 5 is a rear view of the fluid pressure sensing device of figure 1, with a tube suitably clamped; and, Figure 6 is a graphical representation of the calibration curve for a particular tube incorporated in the pressure sensing device of figure 1, illustrating how pressure varies with sensed changes in tube diameter.

Referring to figures 1 to 5 of the drawings, there is shown a fluid pressure sensing device 10 according to the present invention. As the fluid is passed through a tube, the cross-sectional area of the tube will change slightly in dependence of the pressure of the fluid. The device of the present invention senses deformations in the tube wall and thus changes in the cross-sectional area of the tube, and references the changes to predetermined calibration data to determine the pressure of the fluid. The tube is illustrated in the drawings as being substantially circular, however, it is to be appreciated that tubes having other cross-sectional shapes could also be used.

The device 10 of the present invention comprises an elongated clamp body 11 comprising first aperture 12 and a second aperture or opening 13 disposed substantially upon a longitudinal axis of the plate 11. The first aperture 12 comprises an internally screw threaded wall (not shown) and is arranged to receive a threaded bar or bolt 14 or similar. The bolt 14 is screwed upwardly through the first aperture 12 so that the threaded section 14a extends beyond the plate 11 on the upper side thereof, and until the head 14b of the bolt 14 abuts the underside of the plate 11.

Arranged either side of the first aperture 12, along a line substantially transverse to the longitudinal axis of the plate 11 there is provided a pin 15, both of which extend from the surface of the body 11 substantially perpendicularly thereto and in the same direction as the bolt 14. Each pin 15 locates within a recess (not shown) formed within a clamping member 16.

The clamping member 16 comprises a solid body and comprises an aperture 17 through which the threaded section 14a of the bolt 14 can pass. The recesses (not shown) are arranged either side of the aperture 17 formed within the clamping member 16, such that as the clamping member 16 is lowered onto the bolt 14 by passing the threaded section 14a of the bolt 14 through the aperture 17, the clamping member 16 becomes captively aligned with respect to the clamp body 11 as the pins 15 locate within the recesses (not shown). The clamping member 16 is dimensioned to extend substantially parallel to the clamp body 11 and over the second aperture 13 formed within the support plate 11.

The clamp body 11 is mounted above a base 18 and is mounted to the base 18 at one end thereof by a pillar 19. The base 18 and pillar 19 are formed as one piece and create a substantially L-shape structure. The clamp body 11 is rigidly secured at one side to the end face of the pillar 19 that is arranged furthest from the base, using fasteners such as screws (not shown), such that the clamp body 11, pillar 19 and base 18 together form a substantially C-shaped structure.

The end face of the pillar 19 to which the clamp body 11 is secured comprises a channel 20 formed therein, within which is secured a proximal end of an arm 21 of a strain gauge 22. The arm 21 extends from the channel 20 into the space between the clamp body 11 and base 18 and is arranged to extend substantially parallel to the base 18 and clamp body 11. The distal end of the arm 21 terminates at a position substantially adjacent the second aperture or opening 13 and comprises a contact pad 23 rigidly secured thereto. The contact pad 23 extends substantially perpendicular to the arm 21 and is arranged to extend within the second aperture or opening 13 such that the distal end face 24 of the pad 23 extends substantially within the plane of the upper surface of the clamp body 11.

The underside of the clamping member 16 comprises a groove 25 formed therein that extends across the clamping member 16 in a direction substantially perpendicular to the longitudinal axis of the clamp body 11. The groove 25 comprises a base 25a that is substantially semicircular in section having a radius substantially corresponding with the outer radius of the tube 26, and a pair of parallel side walls 25b having a depth substantially corresponding with the outer radius of the tube 26. In this manner, the depth of the groove 25 corresponds substantially with the outer diameter of the tube 26.

The groove 25 extends substantially over the second aperture 13 and is arranged to receive a tube 26 through which the fluid (not shown) whose pressure is to be measured, is passed. The groove 25 is dimensioned to constrain any expansion and/or contraction of the tube within the second aperture 13.

In the embodiment illustrated in Figures 1 to 5 of the drawings, the depth of the groove within the clamping member 16 corresponds substantially with the diameter of the tube. However, in an alternative embodiment that is not illustrated, the tube may be clamped between the clamping member 16 and clamp body 11, within a pair of opposing grooves that are substantially semicircular in section and which extend across the clamping member 16 and the clamp body 11, respectively. In this case, it is envisaged that the depth of each groove will correspond substantially with half the tube diameter, such that when the clamping member 16 and clamp body 11 are brought together, the tube 26 becomes intimately clamped therebetween.

In use, the tube 26 is placed upon the clamp body 11 such that it extends over the second aperture or opening 13 and against the contact pad 23. The clamping member 16 is then lowered onto the clamp body 11 by passing the threaded section 14a of the bolt 14 through the aperture 17, so that the groove 25 locates with the tube 26 and the pins 15 on the clamp body 11 locate within the recesses (not shown) within the clamp body 16. A locking nut 27 or similar is then screwed upon the threaded section 14a of the bolt 14 until it abuts the clamp member 16 and thus clamps the tube to the clamp bodyll.

As fluid is passed through the tube 26, the cross-sectional area of the tube 26 will vary in dependence upon the pressure of the fluid. Since the tube 26 is clamped at its periphery at the side that is substantially opposite the side that is in contact with the pad 23, the tube 26 is constrained to expand and contract, and thus deform, solely within the second aperture or opening 13. This causes the pad 23 to move within the second aperture or opening 13, thereby causing the arm 21 of the strain gauge 22 to flex and bend.

The strain gauge 22 comprises an electrically resistive pathway (not shown) printed upon the surface of the arm 21. Electrical current is passed through the pathway via electrical connections 28a-d arranged upon the pillar 19. As the arm 21 flexes, the electrical resistance of the pathway (not shown) will vary and so by monitoring changes in electrical resistance, the strain applied to the arm 21 and thus changes in cross-sectional area of the tube 26 can be sensed.

In order to calibrate a particular tube 26, water for example, or in situations where it is not desirable to wet a tube, silicone oil for example, is passed into the tube 26 and the changes in electrical current that is output from the electrical connections 28a-d is monitored as the water or oil pressure is varied. A typical calibration curve relating strain measurements to fluid pressure is shown in figure 6.

Different tubes will expand and contract by different amounts in dependence upon various factors including the material from which the tube 26 is made, the thickness of the tube wall, tube diameter, and possibly ambient temperature and fluid temperature.

The strain gauge 22 can be arranged so that the reading is nulled when the tube 26 is undisturbed with zero pressure differential between the inside and the outside of the tube 26. Alternatively, the tube 26 can be slightly stressed so that there is a depression when there is zero pressure differential. The depression is created by a spring (not shown), which presses radially inwardly from the outside of the tube 26 at the point that the contact pad 23 is applied. As the internal pressure increases, the spring compresses and allows the tube 26 to return towards its original shape (e.g. circular cross-section), and even bulge through the second aperture or opening 13. The amount of depression is arranged to be very small, so that the change in the shape of the tube 26 is negligible. The advantage of this arrangement is that it allows flexible tubes to be employed that do not have the desired elastic properties. The desired elastic response to pressure difference is then principally determined by the spring, which may comprise a leaf spring, for example.

In order to suitably position the distal face 24 of the pad 23 with respect to the upper surface of the clamp body 11 and thus suitably null the strain gauge reading, an adjustment bolt 29 is employed. The bolt 29 is screwed through an aperture 30 formed within the base 18, from the underside thereof, so that the end face 29b of the threaded section 29a abuts the under side of the clamp body 11. The adjustment bolt 29 extends from the base 18 to the clamp body 11 at the side that is opposite the pillar 19, such that upon further screwing the adjustment bolt 29 into the base 18, the clamp body 11 is forced away from the base 18 so as to reduce the extent to which the distal face 24 of the pad 23 extends within the second aperture or opening 13. Similarly, by unscrewing the adjustment bolt 29 from the base 18, the separation of the clamp body 11 from the base 18 will reduce so as to increase the extent to which the distal face 24 of the pad 23 extends within the second aperture or opening 13.

Once a particular tube 26 has been calibrated, then it is suitable for measuring the pressure of different fluids, for example, blood and air. The calibration data is stored in memory means, for example a flash memory (not shown) or a computer hard disk (not shown), which may be arranged in communication with the sensing device 10. A processor (not shown) is then used to give a direct indication of the pressure of a fluid within the tube 26 by referencing the measured strain values with the calibration data.

According to a further embodiment of the present invention, the sensing device 10 enables the suitability of materials for medical devices (not shown) to be comparatively assessed. Blood is a non-Newtonian fluid with relatively complex equations relating flow rate and flow rate gradient to pressure drop. When blood leaves the blood vessels (whether through an accidental cut, or through deliberately taking a sample), it tends to clot. The rate of clotting depends both on the blood sample and on the material with which the blood comes into contact. The extent to which it approaches clotting determines the flow-rate pressure relationships. As a non-Newtonian fluid, the apparent viscosity of blood depends also on its flow rate and the diameter of the tube through which it flows.

By passing an identical volumetric flow rate of a given sample of blood through tubes of different diameter, it is possible to determine parameters in equations describing its non-Newtonian behavior. By recycling the blood through such tubes, we can also determine how such parameters vary with time. It is then possible to derive the time at which an initial clot appears. For standard tube surfaces (for example, for polyurethane surfaces), this time is a measure of the propensity of a given sample of blood to clot.

Similarly, for a standard sample of healthy blood, the time to form an incipient clot is a measure of the propensity of the inner surface of the tube material to stimulate clotting.

Thus, it is a measure of the biocompatibility of the material.

The time taken for the clotting condition to arise is related to the so-called "gel time", which is dependent on the properties of the inner surface of the tube and which is a measure of the propensity of blood to clot. The clotting process can thus be assessed by monitoring the changes in the apparent viscosity of blood, since the clotting process will manifest as a change in the blood pressure, which can be readily determined as described above. The gel-time is thus a measure of the extent to which a tube promotes blood clotting. The longer the gel time, the more suitable is the material for use in such devices.

By comparing the gel-time with tubes having a particular surface coating with tubes having a different surface coating, it is thus possible to assess the suitability of a material for use in arteries, for example. Similarly, by comparing the gel-time of blood from a patient with a known standard, it is possible to determine the propensity of the blood to clot, which is particularly useful in assessing how much anticoagulant to administer to the patient, for example.

Claims

Claims 1. A fluid pressure sensing device for sensing the pressure of a fluid within a tube, the device comprising sensing means for sensing changes in the cross-sectional area of the tube and for providing an indication of the pressure of the fluid within the tube in dependence upon the sensed changes in cross-sectional area of the tube.
2. A sensing device according to claim 1, comprising a clamp for clamping against the tube.
3. A sensing device according to claim 2, wherein the tube is clamped by the clamp along a length of the outer surface of the tube.
4. A sensing device according to claim 3, wherein the clamp comprises an aperture or opening to expose a portion of the outer surface of said length of tube within the clamp.
5. A sensing device according to claim 4, wherein the sensing means senses changes in the cross-sectional area of the exposed portion of the tube.
6. A sensing device according to claim 4 or 5, wherein the clamp comprises a clamping member for clamping the tube to a clamp body.
7. A sensing device according to claim 6, wherein the aperture or opening is formed within the clamp body.
8. A sensing device according to claim 6, wherein the aperture or opening is formed within the clamping member.
9. A sensing device according to claim 7 or 8, wherein the clamp body comprises a groove for receiving the tube and which constrains the tube to expand and contract within the aperture or opening.
10. A sensing device according to claim 7 or 8, wherein the clamping member comprises a groove for receiving the tube and which constrains the tube to expand and contract within the aperture or opening.
11. A sensing device according to any of claims 4 to 10, wherein the sensing means comprises an optical sensing device which detects changes in cross-sectional area of the tube by monitoring the deflection of a light beam that is incident upon the exposed portion of the tube.
12. A sensing device according to any of claims 4 to 10, wherein the sensing means comprises a strain gauge, comprising an arm that is rigidly secured at a proximal end thereof, with respect to the clamp.
13. A sensing device according to claim 12, wherein a distal end of the arm comprises a pad that is arranged to contact the exposed portion of the outer surface of the tube.
14. A sensing device according to claim 12 or 13 as appended to claim 6, wherein the proximal end of the arm is rigidly secured between the clamp body and a base member
15. A sensing device according to claim 13, wherein the pad of the strain gauge is arranged to move within the aperture or opening.
16. A sensing device according to claim 13, wherein the arm is arranged to flex as the cross-sectional area of the tube changes so as to cause the pad to move within the aperture or opening.
17. A sensing device according to claim 14, wherein the clamp body and base member extend substantially parallel to each other and are arranged in spaced relation to each other.
18. A sensing device according to claim 17, wherein the spacing between the clamp body and the base member can be adjusted using adjustment means.
19. A sensing device according to any preceding claim, further comprising processing means for processing signals output from the sensing means.
20. A sensing device according to claim 19, wherein the signals output from the sensing means are indicative of changes in cross-sectional area of the tube and thus the fluid pressure within the tube.
21. A fluid pressure measurement device for measuring the pressure of a fluid within a tube, the device comprising the fluid pressure sensing device of any preceding claim and a tube for receiving the fluid.
22. A measurement device according to claim 21, wherein the measurement device further comprises memory means for storing calibration data, which relates sensed changes in the cross-sectional area of the tube, to fluid pressure within the tube.
23. A method of determining fluid pressure within a tube, the method comprising the steps of: -providing a tube containing a fluid; -sensing changes in the cross-sectional area of the tube; and, -referencing the sensed changes in the cross-sectional area to calibration data which relates sensed changes in the cross-sectional area of the tube to fluid pressure within the tube.
24. A method of determining the propensity of a fluid to change apparent viscosity, the method comprising the use of the device of any of claims 1 to 22.
25. A method according to claim 24, the method comprising passing the fluid through at least two tubes, the at least two tubes comprising different diameters.
26. A fluid pressure sensing device substantially as herein described with reference to the accompanying drawings.
27. A fluid pressure measurement device substantially as herein described, with reference to the accompanying drawings.