DEVICE FOR MEASURING A PRESSURE DIFFERENTIAL

Abstract

A device for measuring a pressure differential comprises a tube, at least one pressure sensor and a processor. The tube comprises a closed insertion portion for insertion into a body, the insertion portion having an insertion end and an internal bore in communication with ambient pressure via an opening in the tube. The sensor is located in or on the insertion portion and comprises an internally facing region in communication with the bore and an externally facing region in communication with an exterior of the tube. The processor is configured to provide a stimulus, which may be an electrical stimulus, to the pressure sensor so that when the stimulus is provided, the pressure sensor provides a measurable response wherein the processor correlates the response with the pressure differential between the exterior of the tube and the bore. The measurable response may be indicative of a change in pressure differential between the exterior of the tube and the bore. There may be a plurality of pressure sensors, in which case at least two of the sensors may have different resonant frequencies at the same pressure differential. The insertion portion may comprise at least one aperture sealed by at least one pressure sensor. The pressure sensor may comprise an electromechanical or micro-electromechanical material and may comprises a piezoelectric and/or electrocapacitive sensor. The externally facing region of the pressure sensor may comprise a coating, which may be electrically insulative.

Claims

1. A device for measuring a pressure differential comprising: a tube comprising a closed insertion portion for insertion into a body, the insertion portion having an insertion end, and an internal bore which is in communication with ambient pressure via an opening in the tube; at least one pressure sensor located in or on the insertion portion, the pressure sensor comprising an internally facing region which is in communication with the internal bore and an externally facing region which is in communication with an exterior of the tube; and a processor configured to provide a stimulus to the pressure sensor so that when the stimulus is provided, the pressure sensor provides a measurable response, wherein the processor correlates the measurable response with the pressure differential between the exterior of the tube and the internal bore.

2. The device according to claim 1 in which the at least one pressure sensor is located proximal to the insertion end.

3. The device according to any previous claim comprising a plurality of pressure sensors.

4. The device according to claim 3 in which at least two of the pressure sensors have different resonant frequencies at the same pressure differential.

5. The device according to any previous claim, in which the externally facing region of the pressure sensor is substantially aligned with the exterior of the tube.

6. The device according to any previous claim in which the insertion portion comprises at least one sealed aperture, in which each aperture is sealed by at least one pressure sensor.

7. The device according to any previous claim in which the at least one pressure sensor comprises an electro-mechanical or micro-electromechanical (MEMs) material.

8. The device according to any previous claim in which the at least one pressure sensor comprises a piezoelectric pressure sensor.

9. The device according to any previous claim in which the at least one pressure sensor comprises an electro-capacitive pressure sensor.

10. The device according to any previous claim, in which the stimulus comprises an electrical stimulus.

11. The device according to any previous claim in which the externally facing region of the pressure sensor comprises a coating.

12. The device according to any previous claim in which the externally facing region comprises an electrically insulating coating.

13. The device according to any previous claim in which the measurable response is an electrical response.

14. The device according to any previous claim, in which the measurable response is indicative of an electrical impedance of the pressure sensor.

15. The device according to any of claims 1 to 13, in which the measurable response is indicative of a resonant frequency of the pressure sensor.

16. The device according to any previous claim in which the pressure sensor has a resonant frequency between 0.1 MHz to 100 MHz, between 1 MHz and 20 MHz, or between 1 MHz and 7 MHz.

17. The device according to any previous claim in which the measurable response is indicative of a change in pressure differential between the exterior of the tube and the internal bore.

18. The device according to any previous claim in which the tube further comprises a non-insertion portion, and the opening is located at the non-insertion portion.

19. The device according to any previous claim further comprising a user-holdable portion which is in connection with the tube, optionally the user-holdable portion is in connection with the non-insertion portion.

20. The device according to claim 19 in which the internal bore is in communication with ambient pressure via the user-holdable portion.

21. The device according to claim 19 or 20 in which, the user-holdable portion comprises one or more of a hand-grip, a finger-grip and/or a thumb-grip.

22. The device according to any of claims 19 to 21 in which the user-holdable portion comprises a tube penetration depth indicator for indicating a tube penetration depth into the body.

23. The device according to claim 22 in which the tube penetration depth indicator comprises one or more legs relatively moveable with respect to the tube.

24. The device according to any of claims 19 to 23 in which the user-holdable portion comprises a penetration restraint mechanism operable to limit the tube penetration depth.

25. The device according to claim 24 in which the penetration restraint mechanism comprises a locking means.

26. The device according to any of claims 19 to 25 in which the user-holdable portion comprises or further comprises a fastening member for fastening the device to the body.

27. The device according to claim 26 in which the fastening member comprises one or more flanges.

28. The device according to any of claims 19 to 27 in which the processor is housed within the user-holdable portion.

29. The device according to any previous claim in which the tube is a needle.

30. The device according to any previous claim in which the tube has cross-sectional diameter of between 0.1 mm and 5 mm, preferably between 0.5 mm and 2.1 mm.

31. The device according to any previous claim in which the tube has a length of between 10 mm and 150 mm, preferably between 50 mm and 80 mm.

32. The device according to any previous claim in which the pressure sensor has a cross-sectional dimension between 0.1 mm and 25 mm, preferably between 0.2 mm and 10 mm, more preferably between 1 mm and 5 mm.

33. The device according to any previous claim in which the pressure sensor has a thickness of less than 1 mm, preferably less than 0.5 mm, more preferably less than 0.27 mm.

34. A kit of parts comprising: the device according to any previous claim; and an insertion site guide for indicating a tube insertion site.

35. The kit of parts according to claim 34 in which the insertion site guide comprises a wearable sleeve or sheet.

36. A method of measuring a pressure differential using the device according to claim 1, the method comprising the steps of: providing a stimulus to the pressure sensor so that the pressure sensor provides a measurable response; measuring the measurable response, wherein the measurable response is indicative of the pressure differential between the exterior of the tube and the internal bore, the internal bore being in communication with ambient pressure via the opening in the tube; and correlating the measurable response with the pressure differential between the exterior of the tube and the internal bore.

37. The method according to claim 36 further comprising the step of calibrating the device, the step of calibrating the device comprising the sub-steps of: providing a stimulus to the pressure sensor whilst maintaining the externally facing region of the pressure sensor at ambient pressure so that the pressure sensor provides a measurable calibration response indicative of the pressure differential between the exterior of the tube and the internal bore; measuring the measurable calibration response; and correlating the measured calibration response with a pressure differential of zero.

38. The method according to claim 36 or 37 in which the stimulus is an alternating and/or pulsed electrical stimulus.

39. The method according to claims 36 to 38 in which the frequency of the stimulus is varied.

40. The method according to any of claims 36 to 39 in which the frequency of the stimulus is increased or decreased from a first frequency to a second frequency.

41. The method according to any of claims 36 to 40 in which the stimulus has a frequency between 0.1 MHz and 100 MHz, between 1 MHz and 20 MHz, or between 1 MHz and 7 MHz.

42. The method according to any of claims 36 to 41 in which the measurable response is an electrical response.

43. The method according to any of claims 36 to 42 in which the measurable response is indicative of an electrical impedance of the pressure sensor.

44. The method according to any of claims 36 to 43 in which the measurable response is indicative of a resonant frequency of the pressure sensor.

45. The method according to any of claims 36 to 44 in which the measurable response is measured at a sampling rate of between 0.1 samples per minute and 25 samples per second, preferably between 1 sample per minute and 10 samples per second.

46. The method according to any of claims 36 to 45 in which the pressure differential between the exterior of the tube and the internal bore is between −200 mmHg and +200 mmHg, preferably between −120 mmHg and +120 mmHg, more preferably between 0 mmHg and +60 mmHg.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0100] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0101] FIG. 1 is a schematic side view of a device according to a first embodiment of the invention;

[0102] FIG. 2 is a schematic plan view of a device according to a first embodiment of the invention;

[0103] FIG. 2A is a perspective view of the device according to a first embodiment of the invention;

[0104] FIG. 3A is a schematic side view of the sensor;

[0105] FIG. 3B is a schematic cross-sectional view of the needle/tube at the line X-X;

[0106] FIG. 4 is a schematic underside view at the line Y;

[0107] FIG. 5 is a perspective view of a device of the first embodiment;

[0108] FIG. 6 is a perspective view of a second embodiment of the invention;

[0109] FIGS. 7A, 7B and 7C are graphs showing the change in impedance of a pressure sensor (of size 9 mm×9 mm) at different hydrostatic pressures; and

[0110] FIGS. 8A, 8B, 8C and 8D are graphs showing the change in impedance of a pressure sensor (of size 4 mm×4 mm) at different hydrostatic pressures.

[0111] FIG. 9 is a graphical illustration of a pulsatile pressure wave; and

[0112] FIG. 10 is a graphical illustration of the fourth exemplary method.

DETAILED DESCRIPTION

[0113] FIGS. 1, 2 and 2A illustrate a device (shown generally at 10) for measuring a pressure differential in a body according to a first embodiment of the invention. The device 10 comprises a tube 12 and a handgrip (or handle) 14. The tube 12 is hollow, comprising an internal bore 16, a closed first end (or tip) 18, and an open second end 20. The bore 16 is delimited by walls 21 and the closed first end 18. In the first embodiment the tube 12 is a needle. The closed first end 18 is sharp to allow the needle 12 to be inserted into a patient body by penetrating body tissue. In other embodiments, the closed first end is blunt. The internal bore 16 is maintained at ambient atmospheric pressure. In the first embodiment, the open second end 20 is in open communication with the atmosphere, and therefore the internal bore 16 maintained at ambient atmospheric pressure. The second end 20 of the needle 12 extends into the handgrip 14. In some embodiments, the handgrip 14 comprises breather holes (or slits) to allow the open end 20 to be exposed to the atmosphere. The breather holes ensure that the internal bore 16 remains in communication with atmospheric pressure. In the first embodiment, the internal pressure (P.sub.bore) in the bore 16 is maintained at atmospheric pressure due to the open second end 20. The pressure in the bore 16 of the needle 12 is used as a known reference pressure.

[0114] The needle 12 further comprises a pressure sensor 22, shown in more detail in FIGS. 3A, 3B and 4. The sensor 22 is set into a slot or other aperture 23 formed in the walls 21 of the needle 12. In some embodiments, the needle 12 comprises a plurality (or array) of discrete pressure sensors set into a single slot. In other embodiments, the needle 12 comprises a plurality of slots with one or more sensors set into each slot. In some embodiments, there is a plurality of sensors arranged along the needle length (e.g. in a “ladder” arrangement), each sensor providing a discrete sensing element. In some embodiments each sensor is optimised to detect a specific pressure range.

[0115] The slot 23 is conveniently formed using known machining (or electro-machining) methods. The sensor 22 is shaped to be substantially flush with the walls 21 so as not to obstruct or impede the movement of the needle during insertion. That is, the walls and sensor provide a substantially smooth outer surface during needle insertion. In the first embodiment, the sensor 22 is a sector of a hollow cylinder in shape. The sector may be up to a maximum of a hemi-cylinder in some embodiments. Other two- or three-dimensional geometries of the sensor, such as square, rectangular, ellipse, circular, oval, or annular may be employed in other embodiments. In some embodiments, the sensor has a geometry optimised to detect specific forcing modes, for example a “hoop” stress component. In other embodiments, the sensor has a longitudinal component to detect a pressure pulse passing along a vessel.

[0116] The sensor 22 comprises an active surface 24 and a non-active surface 26. The sensor has an externally facing region and an inwardly facing region. The active surface 24 of the sensor 22 comprises an externally facing active surface 24a and an internally facing active surface 24b. The externally facing active surface 24a faces outwardly, and is in communication with a pressure exterior to the bore 16 of the needle 12. That is, the externally facing active surface 24a is in communication with an external load pressure (P.sub.ext). When the needle 12 is inserted into a body, the external active surface 24a is in communication with the body interior (e.g. body tissue). When the needle 12 is inserted into a body, the external pressure corresponds to the pressure in the body interior. The body pressure may be a compartment pressure. In some embodiments, the external active surface 24a comprises a coating 25. The coating 25 physically isolates the active surface 24 from the body, for example, isolating the chemical components of the sensor from the body interior. This is of particular benefit if the sensor 22 comprises a potentially harmful chemical, such as a lead-containing material. In some embodiments, the coating 25 (or an additional coating layer) electrically isolates the active surface 24 from the body. In some embodiments, the coating is a polymer coating. In some embodiments, the polymer is poly(p-xylylene).

[0117] The internally facing active surface 24b faces inwardly, and is in communication with the internal pressure (P.sub.bore) within the bore 16. A pressure differential (P.sub.ext−P.sub.bore) exists between the external pressure (P.sub.ext) and the internal pressure in the bore 16. The principle sensing action of the pressure sensor (P.sub.bore) arises due to flexure in the pressure sensor as the pressure differential (P.sub.ext−P.sub.bore) varies. This flexure can provide a measurable response to be generated.

[0118] The non-active surface 26 is attached to the walls 21 of the needle 12. In the first embodiment, a non-conductive adhesive is used. The active surface 24 and the walls 21 are electrically isolated.

[0119] In some embodiments, the active surface 24 of the sensor 22 comprises an electro-mechanical material or material that embeds one or more micro-electromechanical system (MEMS) devices. In the first embodiment, the active surface 24 comprises a piezoelectric material or transducer. In some embodiments, the piezoelectrical material has a low Q-factor. In other embodiments, for example, where a plurality of sensors is deployed each sensor may have a high Q-factor.

[0120] In some embodiments, the sensor 22 comprises a ceramic. In some embodiments the piezoelectric material is an intermetallic inorganic polycrystalline ceramic compound. In some embodiments the piezoelectric material is lead-containing, such as lead zirconate titanate. In other embodiments, the piezoelectric material does not containing lead, and, for example, may be formed from barium titanate, o bismuth sodium titanate, bismuth potassium titanate, sodium niobate, potassium niobate, and/or potassium sodium niobate. In further embodiments, the sensor 22 comprises poly(vinylidene difluoride) (PVDF). PVDF exhibits long-term biocompatibility.

[0121] A piezoelectric material produces an electrical signal in response to a change in external physical or mechanical stimulus. The electrical signal may be an electro-motive force (or a voltage) which changes in response to a change in the external stimulus, such as a change in applied mechanical force. The external stimulus may, for example, be an applied stress caused from a change in pressure differential across the sensor. A change in applied stress may cause the impedance of the piezoelectric material to change. However, the measurable change in voltage (or impedance) will decay very rapidly if the change in external physical or mechanical stimulus ceases.

[0122] A piezoelectric sensor also responds to an applied electrical stimulus. For example, an applied electrical stimulus (of appropriate frequency) may cause a piezoelectric sensor to resonate at a characteristic resonant frequency. The resonant frequency is dependent upon the pressure differential across the sensor. The resonant frequency may be determined using known processing techniques.

[0123] In some embodiments, it is preferable for the sensor 22 to be formed from a piezoelectric material that has a narrow resonant peak (i.e. a high Q factor). This helps to improve the sensitivity of the device 10. That is, a change in pressure differential can be detected as a well-defined change in resonant frequency. In other embodiments, a low Q-factor is used to give a broad range of resonant frequency with good sensitivity. It may be preferable to use a Q-factor having an intermediate value. This compromise provides a broad based of resonance, whilst giving a higher resonance response.

[0124] The impedance of a piezoelectric sensor is also dependent upon the pressure differential. Measuring the electrical impedance at a particular frequency may be correlated with a pressure differential across the sensor.

[0125] Contact wires 28 form an electrical contact between the sensor 22 and processor located in the handgrip 14. In the first embodiment, the processor correlates electrical signals (or responses) from the sensor 22 with a pressure differential (P.sub.ext−P.sub.bore). In the first embodiment, the contact wires 28 are disposed within the bore 16. However, in other embodiments, the contact wires are disposed on the outside of the bore. The contact wires 28 are electrically isolated from the walls 21.

[0126] The contact wires 28 relay electrical signals between the processor (not shown) and the sensor 22. For example, the contact wires 28 allow electrical power or an electrical stimulus (including electrical pulses or other waveforms) to be applied to the sensor 22. The contact wires 28 also allow electrical signals generated by the sensor 22 to be measured by the processor.

[0127] In other embodiments, the processor is disposed within the bore 16. The invention is not limited by location of the processor. In some embodiments the processor provides one or more of signal amplification, signal conditioning, and/or analogue to digital encoding. In some embodiments the processor is wirelessly connected to the sensor 22. In further embodiments, the processor is disposed in a central processing unit, suitable for data storage and data analysis of one or more devices 10. For example, the central processing unit may be configured to store a timed sequence of regular measurements to track trends in pressure over time. The central processing unit may provide power, enable control of the device 10, and display pressure readings. In some embodiments, the processor provides an alert if the measured pressure, such as a compartment pressure or intracranial pressure, exceeds a pre-determined threshold value set by the user. In some embodiments, the central processing unit is configured for bidirectional communication with the pressure sensor. The central processing unit may provide additional facilities for comparative display of a plurality of pressure values obtained from a plurality of pressure measurements. The display may show raw or processed data. The central processing unit may comprise a user interface which provides individual control to one or more devices connected to the central processing unit. Conveniently, the central processing unit may be configured to be a fixed installation, for example at a bedside.

[0128] The handgrip 14 may further comprise a thumb grip 30, a display 32, and measurement controls 34 (FIG. 5). The thumb grip 30 provides additional support and comfort to the user. Embodiments comprising a handgrip 14 may be portable devices. The display 32 provides a read-out of the measured pressure differential and other parameters, which may be measured simultaneously, such as arterial pulse rate. In some embodiments the display includes an alert system dependent upon the measured pressure differential. In some embodiments the display flashes to indicate the pulse rate. The measurement controls 34 allow the user to initiate the device, zero the device, and/or operate the device in the desired mode, for example, providing a switch between stimulated and passive operational modes as described below. In some embodiments, the measurement controls 34 allow the sampling rate to be adjusted by the user.

[0129] In a second embodiment, the device 210 (shown in FIG. 6) further comprises a means for indicating and controlling the needle penetration depth on insertion into a body. The same reference numerals as used in previous Figures have been used in FIG. 6 to refer to features that are identical. The second embodiment is formed in a similar manner to the first embodiment. However, the handgrip 214 further comprises a pedestal 236. The pedestal 236 is a substantially circular band, having the needle 12 disposed at its centre. The diameter of the pedestal 236 is optionally the same width as the handgrip 214. The invention is not limited by the geometry of the pedestal 236. The pedestal 236 further comprises legs 238 configured to slide in complementary grooves in the handgrip 214 when the needle 12 is inserted into a body. The legs 238 comprise a scale 242 and a pointer 244. When the needle is inserted into a body, the movement of the legs 242 in the grooves allows the depth of the penetration to be determined using the scale 242 and pointer 244. This configuration beneficially provides a precise indication of needle penetration and allows a user, such as a clinician, to repeatedly insert the needle 12 to the same desired depth for long-term or repeated pressure measurements.

[0130] The handgrip 214 further comprises a locking means, such as a friction clip or lock tab 246, to restrain the movement of the legs, thereby securing the legs 238 at a fixed, desired position. This beneficially prevents unwanted further movement or penetration of the needle when the required depth is attained. That is, the friction clip 246 limits the needle penetration depth into a body.

[0131] The device 210 further comprises a fixation means or an anchor 248 in the form of wing-tabs. The anchor 248 provides a surface allowing the device 214 to be fixed or attached to the body, for example, by adhesive tape or a strap.

[0132] In operation, the device 10 is first initiated by supplying a power supply to the processor whilst the device is outside of a patient body. The power supply provides an electrical source to electronic components in the device, such as the sensor 22 and the processor. The supplied power enables pressure measurement, data processing and display.

[0133] In the first embodiment, a pull-tab initially prevents contact between the power supply and the processor so that the electrical circuit is incomplete. The device 10 is initiated by closing an electrical contact between the power supply and the processor, thereby completing the electrical circuit. In the first embodiment, the device is initiated by removing the pull-tab to complete the electrical circuit. In another embodiment, the electrical contact is formed on removal of a suitable needle shield or sheath. Removal of the needle shield completes the electrical circuit and initialises the device. This beneficially necessitates that the device is initiated, and calibrated, prior to insertion into a body. Other embodiments employ other known methods of completing the electrical circuit, such as connecting an external power source, using a mechanical switch, button, or other known method of supplying power to electrical circuitry.

[0134] Devices 10, 210 used in medical applications may be single-use devices to avoid contamination and to reduce the risk and spread of infection. In single-use devices, the power may be supplied by a battery, which continuously supplies power until the battery is fully discharged. A pull-tab initialisation mechanism provides a one-time initiation method so that the battery may continuously supply power to the electrical circuitry until the battery has fully discharged. This is particularly beneficial for disposable or single-use devices, and helps to prevent re-use, which could lead to an increased risk of infection.

[0135] Some embodiments of the device may be used multiple times, with sterilisation techniques used between each use. It is beneficial for a multiple use device to have power switches and/or replaceable/rechargeable batteries to conserve battery power when not in use.

[0136] When the device is initiated, an auto-calibration (or self-calibration) step is performed, as detailed below. When the device of the first or second embodiment of the invention is initiated outside of a body, the external pressure (P.sub.ext) and the internal bore pressure (P.sub.bore) are both at atmospheric pressure. That is, the pressure differential (P.sub.ext−P.sub.bore) is 0. Therefore, there is no net external strain on the sensor. The bore, processor uses this initial condition (i.e. P.sub.ext=P.sub.bore) to determine a zero relative pressure state, thereby self-calibrating the device 10. This initial condition sets a calibration baseline. The device measures changes in pressure differential based on this calibrated baseline.

[0137] When the needle 12 is inserted into a body, the inner bore 16 remains in direct contact with atmospheric pressure. Therefore, the internal pressure (P.sub.bore) maintains at ambient atmospheric pressure. The internal bore pressure (P.sub.bore) is used as a known reference pressure value.

[0138] However, upon insertion into a body, the exterior of the needle 12 is subjected to a change in external pressure (P.sub.ext). The external pressure is representative of the pressure within the body. For example, if the needle is inserted into a muscular compartment, the external pressure acting on the needle would correspond to the compartment pressure. When the needle 12 is fully inserted into a body, the internal bore pressure (P.sub.bore) is isolated from the external pressure (P.sub.ext) by the sealed walls 21, closed end 18, and sensor 22. It is preferable for the walls 21 to have a degree of rigidity to withstand the changes in external pressure (P.sub.ext), so that the bore 16 is not deformed upon insertion.

[0139] Due to the constant internal pressure (P.sub.bore) within the bore 16, but change in external pressure (P.sub.ext), a change in pressure differential (P.sub.ext−P.sub.bore) is observed. Consequently, the forces acting on the sensor are unbalanced (or asymmetrical). The pressure differential (P.sub.ext−P.sub.bore) or change in pressure differential (Δ(P.sub.ext−P.sub.bore)) is correlated to a change in electrical and/or mechanical properties of the sensor 22, which provide a measurable response.

[0140] In some embodiments, the pressure differential (P.sub.ext−P.sub.bore) is correlated to the electrical impedance of the sensor 22. In other embodiments, the pressure differential (P.sub.ext−P.sub.bore) is correlated to the resonant frequency of the sensor 22. In further embodiments, the change in pressure differential (Δ(P.sub.ext−P.sub.bore)) is correlated to an electrical property of the sensor (e.g. impedance) or a signal generated by the sensor. In some embodiments, the processor is configurable to simultaneously or selectively measure one or more of the electrical impedance, the resonant frequency and/or electrical signals generated from the sensor 22. Consequently, the pressure differential (P.sub.ext−P.sub.bore) or change in pressure differential can be measured.

[0141] The processor is configured to correlate one or more of a measured electrical impedance, resonant frequency, and/or signal generated by the sensor 22 with the external pressure (P.sub.ext). This may, for example, be carried out using calibration curves based on the auto-calibration step.

[0142] By using intrinsic properties of the material to determine the pressure differential (P.sub.ext−P.sub.bore) there is no need to use balanced hydraulics, or manometer lines for the pressure measurement. Further, variations in temperature have negligible effects on the accuracy of the pressure measurement. Ambient pressure changes affect both the internal pressure in the bore 16 (P.sub.bore) and pressure in the body equally, and therefore do not affect pressure measurements. Moreover, long-term drift in pressure measurements is negligible since the reference pressure is atmospheric pressure.

[0143] The auto-calibration step does not rely on removal of air bubbles from a syringe or on manometer lines. Further, the reference pressure (P.sub.bore) inside the bore 16 remains at a fixed pressure when the device is orientated at any angle. Therefore, it is not necessary to perform the calibration step at the same angle as the pressure measurement, which allows more reliable pressure measurements to be taken. For these reasons the device 10, 210 provides a simplified calibration process compared to known pressure sensors, and allows accurate and reliable pressure measurements to be taken whilst a patient is moving or active, for example, in the measurement of exertional compartment pressure.

[0144] A clinician may be interested in at least some of the following parameters: [0145] a. instantaneous compartment pressure (relative to atmospheric pressure); [0146] b. trends or variations of the compartment pressure over an extended period; [0147] c. fractional (percentage) over-pressure variation from detected pulsatile arterial blood flow (also known as delta pressure); [0148] d. arterial pulse rate; and [0149] e. intracranial pressure, particularly following surgery.

[0150] One or more of these, or other, clinically useful parameters may be measured using embodiments of the invention by adopting a variety of exemplary methods or modes. For example, embodiments of the present invention can be used to measure static and/or dynamic pressures.

[0151] In a first exemplary method (or first exemplary stimulated mode), the device 10, 210 allows an instantaneous (or static) pressure, such as an instantaneous compartment pressure, to be measured. The first exemplary method is also suitable for measuring trends in compartment pressure over extended periods. Dynamic pressures can also be measured. A second exemplary method (described below) is further suitable for determining pulse rates, and amplitudes of the cardiac variation.

[0152] On initialisation, the processor applies a stimulus, such as an electrical or ultrasound stimulus, to the sensor 22. The electrical stimulus is applied at a known, pre-determined frequency and causes the sensor 22 to vibrate. In some exemplary methods, the electrical stimulus is an AC voltage. However, pulsed stimuli or other waveforms are applied in other exemplary methods. At a characteristic frequency, the sensor 22 will resonate at a resonant frequency.

[0153] The electrical impedance of the sensor is measured using known processing techniques. The electrical impedance of the sensor is indicative of (and dependent upon) the pressure differential (P.sub.ext−P.sub.bore). Therefore, it is possible to correlate the change in electrical impedance to the pressure differential (P.sub.ext−P.sub.bore) or external load pressure (P.sub.ext), for example, by using a calibration curve.

[0154] The maximum sensitivity of the impedance measurement occurs when measured at a specific advantageous frequency. In some exemplary methods, this advantageous frequency corresponds to the resonant frequency of the sensor.

[0155] In some exemplary methods, the measured electrical impedance is indicative of a mean static pressure. The measured electrical impedance may be averaged over several seconds. In some exemplary methods, the measured electrical impedance of the pressure sensor is mapped against calibrated values to correlate the two parameters.

[0156] In a second exemplary method (or second exemplary stimulated mode), the device 10, 210 allows an instantaneous (or static pressure), such as an instantaneous compartment pressure, to be measured. The second exemplary method is also suitable for measuring trends in compartment pressure over extended periods. The second exemplary method is further suitable for determining pulse rates, and amplitudes of the cardiac variation.

[0157] On initialisation, the processor applies a stimulus, such as an electrical or ultrasound stimulus, to the sensor 22. The frequency of the stimulus is ramped in stages from a low frequency to a high frequency (or vice versa), and the resonant frequency, electrical impedance and/or capacitance of the sensor 22 determined. The applied frequency can be an AC frequency. However, pulsed or other waveforms are used in other embodiments.

[0158] The resonant frequency of the sensor 22 is indicative of (and dependent upon) the pressure differential (P.sub.ext−P.sub.bore). Therefore, it is possible to correlate the change in resonant frequency to the pressure differential (P.sub.ext−P.sub.bore) or external load pressure (P.sub.ext). The resonant frequency of the pressure sensor can be calibrated by precisely machining the dimensions of the pressure sensor. For a precisely dimensioned pressure sensor, it will be possible to determine a known characteristic function that allows the direct correlation of the measured resonant frequency with a pressure differential.

[0159] In addition to the resonant frequency, the electrical impedance and/or capacitance of the pressure sensor can be indicative of the pressure differential in accordance with methods of the second exemplary stimulated mode.

[0160] In some embodiments the piezoelectric material used has a narrow resonance peak, or a high Q-factor, which helps to provide an increased sensitivity in the pressure measurement. In other embodiments, a low Q-factor is used to give a broad range of resonant frequency with good sensitivity. It may be preferable to use a Q-factor having an intermediate value. This compromise provides a broad based of resonance, whilst giving a higher resonance response.

[0161] In the first embodiment, the sensor has two resonance modes: (i) a thickness mode (parallel to the electric field); and (ii) a transverse mode (orthogonal to the electric field).

[0162] In the second exemplary method, the frequency of the stimulus applied to the sensor is ramped in stages over the frequency range, and the resonant peak detected through the electrical excitation. The lower and upper limits of the frequencies are selected by the user and may depend on the size and geometry of the sensor, and on the pressure differential values to be monitored. In some exemplary methods, the frequency range is between 0.1 and 30 MHz. In other exemplary methods, the frequency range is between 1 and 7 MHz. In other exemplary methods, the frequency range is between 16 and 22 MHz. The lower and upper limits of the frequency may be selected based on the pressure differential measured in the immediately preceding measurement. For example, the lower and upper limits of the frequency may be a localised ranged spanning the frequency that provided the maximum resonance of the immediately preceding measurement. For example, the localised range may be about ±5 Hz, ±2 MHz, ±1 MHz, or ±0.5 MHz around the frequency that provided the maximum resonance of the previous measurement.

[0163] In some exemplary methods, the measured resonant frequency is indicative of a mean static pressure. The measured resonant frequency may be a measured over several frequency cycles. For example, the resonant frequency may be measured over about 100 frequency cycles. In some exemplary methods, the resonant frequency is measured over a few milliseconds. In some exemplary methods, the sampling rate is up to about 1000 samples per second. The invention is not limited by the range of frequency used. In some exemplary methods, the resonant frequency of the pressure sensor is mapped against calibrated values to correlate the two parameters.

[0164] FIG. 7A is a proof-of-concept example showing how the absolute impedance of a piezoelectric sensor, (with an active area of 9×9 mm.sup.2), varied with applied frequency in a range of 0.1-30 MHz. The external load pressure in this example was varied using a column of water with a height from 0 cm to 120 cm (i.e. approximately 0-90 mmHg). In this example, the thickness mode has a resonant frequency around 18 MHz; and the transverse mode has a resonant frequency in the low MHz range.

[0165] The pressure sampling rate may be fast, for example, 10 samples per second. This allows rapid monitoring of changes in instantaneous pressure. For example, this beneficially allows a user (or clinician) to monitor changes in compartment pressure during specific tests, such as when a limb is raised. Again as an example, the pressure in a muscular compartment may increase if a limb is raised. Further, this beneficially allows instantaneous pressure measurements to be monitored during activity. Pressure measurement during activity allows a clinical diagnosis of exertional compartment syndrome.

[0166] In some exemplary methods, the measured resonant frequency is processed to provide information regarding dynamic pressure changes. For example, the calibrated baseline and/or the static compartment pressures may be removed from the measured response to provide dynamic pressure values. This allows pulse rates and amplitudes to be determined (with respect to the baseline value).

[0167] Alternatively the pressure sampling may be slow, for example, 1 sample per minute. This beneficially allows trends in static pressures to be monitored over an extended period of time. In one exemplary method, the pressure is monitored over a period of many hours. A slow sampling rate requires a lower power input, which also preserves battery life.

[0168] FIG. 7B is a magnified view of FIG. 7A between a range of 16 and 22 MHz with a column height from 0 cm to 60 cm (i.e. approximately 0-45 mmHg). These frequencies correspond to the resonant frequency of the thickness mode. As the load pressure increases, the impedance of the sensor decreases systematically.

[0169] FIG. 7C is a magnified view of FIG. 7A between a range of 16 and 22 MHz with a column height from 70 to 120 cm (i.e. approximately 50-90 mmHg). These frequencies correspond to the resonant frequency of the thickness mode. As the load pressure increases (above 50 mmHg), the impedance of the sensor was found to increase. Without wishing to be bound by any theory or conjecture, it is believed that this is due to the large size of the sensor, and a change in contact area and electrode connection due to deformation under these high pressures. These effects were not observed for sensors of a small size.

[0170] FIG. 8A is a proof-of-concept example showing how the absolute impedance of a piezoelectric sensor (with an active area of 4×4 mm.sup.2) varied with applied frequency in a range of 0.1-30 MHz. The external load pressure in this example was varied using a column of water with a height from 0 cm to 120 cm (i.e. approximately 0-90 mmHg). In this example, the thickness mode has a resonant frequency around 18 MHz; and the transverse mode has a resonant frequency in the low MHz range.

[0171] FIGS. 8B and 8C are magnified views of FIG. 8A between a (high) frequency range of 16 and 22 MHz across the same pressure range. These frequencies correspond to the resonant frequency of the thickness mode. As the load pressure was increased, the absolute impedance of the sensor decreased systematically.

[0172] FIG. 8D is a magnified view of FIG. 8A between a (low) frequency range of 0.4 and 2 MHz. These frequencies correspond to the resonant frequency of the transverse mode. At this lower frequency, the absolute impedance increased systematically as the load pressure increased.

[0173] The results of the proof-of-concept experiments can be used to generate calibration curves for a device having the same sized active area.

[0174] In a third exemplary method (or passive operational mode), the device 10, 210 provides a dynamic pressure measurement. A dynamic pressure measurement is, for example, suitable for measuring delta pressure, arterial pulse rate, and for monitoring an attenuated and transformed cardiac cycle. In the third exemplary method, the sensor is not electrically (or ultrasonically) stimulated. However, the piezoelectric sensor 22 will provide a measurable response, such as an electrical or ultrasound signal, in response to a change in the pressure differential. In some exemplary methods, the electrical signal is measured as a change in impedance across the sensor 22. In the passive operational mode, there is no change in electrical signal if the pressure differential is a constant value. Therefore, the passive mode is not suitable for measuring static pressures. However, pulsatile pressure waves generated as a result of the cardiac cycle may be detected using the passive operational mode. The device is able to switch between the stimulated and passive modes.

[0175] The processor correlates a change in the measured signal with a change in pressure differential between the exterior of the needle (P.sub.ext) and the internal bore (P.sub.bore). In some exemplary methods, temporal data in the measured signal is used to determine the pulse rate.

[0176] In one exemplary method, the pressure sampling rate is 10 samples/second. This sampling rate is sufficiently fast to accurately record pulse rates of approximately 60 bpm (beats per minute). Faster sampling rates could be employed to record the pulsatile pressure waves in greater resolution. In some exemplary methods, the sampling rate is at least 10 times as fast as the pulse rate.

[0177] FIG. 9 shows the pressure variation in a muscular compartment during two arterial pressure waves at a pulse rate of 60 bpm (beats per minute). The pressure is sampled at a rate of 10 samples per second. The peak pressure can be analysed to determine the pulse rate. The mean arterial pressure (MAP) may be determined using known formulae. The delta pressure and the compartment pressure may be determined using known formulae.

[0178] The first, second and/or third exemplary method may measure blood flow characteristics in terms of acoustic and pressure sensed signals which can be interpreted to yield information on the state of a blood vessel, for example, at the site of a cardiac procedure and in postoperative care to monitor internal blood pressure levels and conditions.

[0179] In a fourth exemplary method, the first, second, and/or third exemplary methods are alternated. That is, the electrical impedance (or resonant frequency) is measured alternately with measurements of changes in electrical impedance.

[0180] FIG. 10 shows the pressure variation in a muscular compartment at a pulse rate of 60 bpm (beats per minute). The pressure sensor is electrically stimulated during the period indicated by reference numeral 50. The electrical impedance (or resonant frequency) of the pressure sensor is measured at 52. The electrical impedance (or resonant frequency) is measured at a sampling rate of 6 samples per minute. The sample duration is 2 s. That is, the pressure sensor receives electrical stimulation during the electrical impedance (or resonant frequency) measurement for a duration of 2 s.

[0181] The device is not electrically (or ultrasonically) stimulated during the period indicated by reference numeral 54. Whilst the pressure sensor is not being electrically (or ultrasonically) stimulated, the device can measure dynamic changes in pressure differential. Changes in electrical resistance of the pressure sensor are measured at 56. For example, changes in electrical impedance of the pressure sensor may be measured in accordance with the third exemplary method. This can provide clinical information such as pulse rate.

[0182] The peak pressure can be analysed to determine the pulse rate. The mean arterial pressure (MAP) and the delta pressure may be determined using known formulae. Alternating between first, second and/or third exemplary methods reduces power consumption and helps to preserve battery life.

[0183] The fourth exemplary method may measure blood flow characteristics in terms of acoustic and pressure sensed signals which can be interpreted to yield information on the state of a blood vessel, for example, at the site of a cardiac procedure and in postoperative care to monitor internal blood pressure levels and conditions.

[0184] In some exemplary methods, the device delays or suspends pressure measurements during needle insertion into a body.

[0185] In some embodiments, the device is switchable between the first, second, third, and/or fourth exemplary methods. In some embodiments, the sampling rate is user adjustable.

[0186] In some embodiments, the tube comprises a flexible tube. The flexible tube may be hollow, comprising an internal bore, a closed first end (or tip) and an open second end, as described in relation to previous embodiments. The flexible tube may have any cross-sectional geometry. The flexible tube may comprise a pressure sensor as described in previous embodiments. In some embodiments, the flexible tube (or needle) has a blunt end. The flexible tube may be positioned at a location in a body for pressure measurement during a surgical procedure or via a cannula, rather than being inserted into a body by penetrating body tissue. Embodiments incorporating a flexible tube have applications in, for example, measuring post-operative intracranial pressure; and monitoring arterial pressure at locations along a blood vessel to indicate arterial narrowing. Identifying arterial narrowing provides an indication that angioplasty or similar treatments may be required.

[0187] In some embodiments, the pressure sensor provides the closure for the closed first end.

[0188] In some embodiments, the pressure sensor comprises at least two pressure sensors. For example, the pressure sensor may be a dual-segment hybrid sensor. The first pressure sensor is operated in the first and/or second exemplary method, and the second pressure sensor is operated in the third exemplary method.

[0189] In some embodiments, an insertion site guide is used to indicate the appropriate needle insertion site. The insertion site guide may comprise a wearable sleeve or sheet for position over the body. The o insertion site guide may be made from a transparent, flexible material. The insertion site guide may comprise an indication of the location of muscular compartments. Correct positioning of the insertion site guide may be achieved using reference points provided on the insertion site guide. The insertion site guide may provide an indication of the necessary needle penetration depth for a muscular compartment. The insertion site guide beneficially facilitates insertion of the needle into the correct location and at the correct penetration depth to measure the pressure differential value of interest. The insertion site guide also beneficially facilitates insertion of the needle at the same location, which reduces experimental variation during repeated pressure measurements.

[0190] Whilst the present invention has been described above in relation to medical applications, in particular the measurement of a compartment pressure and intracranial pressure, embodiments of the device and associated methods of use are suitable for measuring pressure differentials in other medical applications, such as bone marrow investigations, and in non-medical applications alike. Non-medical applications of the present invention include, but are not limited to, measuring a pressure differential: in a pressurised chamber, such as a tyre; in a sealed chamber, such as a sealed reaction vessel; or in a low pressure chamber, such as a vacuum chamber.