Sensor

Abstract

A physiological sensing device comprises an electrical sensor (4) for insertion into the tissue of a live animal and which measures the partial pressure of carbon dioxide in the animal tissue. The device also includes an electrical cable (6) connected electrically at its distal end to the sensor (4). The cable (6) is surrounded by a sheath (28). The sheath (28) has several flexible portions (30) separated by longitudinal slits (29). Movement of the proximal end of the sheath (28) towards its distal end shortens the distance between the ends of the flexible portions (30) and causes them to bow outwardly. The sensor (4) can be retained in animal tissue by the bowed flexible portions (30).

Description

An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

FIG. 1 is a schematic diagram of a complete sensing system incorporating the sensor of the invention;

FIG. 2 is a schematic diagram illustrating the measurement principle for the sensor in the system of FIG. 1;

FIG. 3 is a partially cutaway view of a sensor according to the invention;

FIG. 4 is a cross-sectional view along line A-A of FIG. 3;

FIG. 4 a is a magnified view of the detail indicated by the circle in FIG. 4;

FIG. 5 is a view of the sensor of FIG. 3 with the membrane removed; and

FIG. 6 illustrates a variant of the sensor of FIG. 3 wherein the attachment mechanism is visible.

In accordance with the invention, a pCO₂ sensing system comprises a disposable sensor unit 1, an electronic surface unit 2, and a monitor unit 3, as shown in FIG. 1.

The disposable sensor unit 1 is delivered packaged and sterilised. It consists of a membrane-protected conductometric sensor 4 with a diameter of less than 1 millimetre, and a temperature probe 5 integrated in the sensor unit. Wires 6 connect the sensor 4 and probe 5 electrically by means of a connector to the electronic surface unit 2. Alternatively, a wireless connection may be provided between the sensor unit 1 and the surface unit 2.

The electronic surface unit 2 sends and receives signals to and from the sensor unit 1. It is placed on the patient's skin, performs signal processing and transmits the conditioned signal to the monitor unit 5.

The monitor unit 3 is based on a portable personal computer 7 with PCMCIA input/output card 8 and Labview software (available from National Instruments Corporation of Austin, Tex.).

The pCO₂ sensor 4 is used for measurements of the level (partial pressure) of CO₂ (pCO₂) in a fluid, according to the measurement principle illustrated in FIG. 2. The measurement chamber consists of two small cavities 9 with one electrode 10 positioned in each. The two cavities 9 are connected by one or more passageways 11 enclosed by a semi-permeable membrane 12, i.e. a membrane that only allows transport of CO₂ in and out of the volume of the sensor 4. The whole volume is filled with de-ionised water. The conductivity in the water depends upon the pCO₂, and by measuring the conductivity between the electrodes 10 in the volume, information about pCO₂ may be extracted.

As shown in FIGS. 3 to 5, the sensor unit 1 comprises an injection moulded plastics support 23, which is substantially cylindrical and surrounded by the semi-permeable membrane 12. The support 23 has a conical tip 24 at its distal end and a body portion 25 which extends proximally from the tip 24. On the body portion 25 are mounted, by glueing, two gold electrodes 10. The electrodes 10 extend longitudinally along opposed sides of the body portion 25 and are received in respective recesses in the body portion 25.

Between the tip 24 and the body portion 25, a frustoconical projection 26 is provided for securing the membrane 12 by frictional fit. A corresponding projection 26 is provided at the proximal end of the body portion 25. The membrane 12 may be glued to the support 23, but it is important that the glue used to secure the membrane 12 and electrodes 10 is selected such that it does not bleed ions into the water-filled chamber formed between the body portion 25 of the support 23 and the membrane 12. Furthermore, the sealing faces of the support 23 may be made selectively hydrophobic in order to avoid the formation of a water film into which ions may bleed.

The membrane 12 may also be secured to the support 23 by means of crimp connection and a soft gasket, if necessary. The membrane 12 may act as the gasket, particularly where the membrane 12 is formed of silicone rubber. A heat shrink sleave may be used to form the crimp connection, as is the case in FIG. 6. Alternatively, metal crimp rings may be used in locations corresponding to those of the sealing projections 26.

The body portion 25 of the support 23 is provided with a plurality of ribs 27, which are formed with a saw tooth profile for easy moulding. The ribs 28 provide mechanical support to the membrane 12 and also define the fluid passageways 11 required for the sensor 4 to function effectively. Between each electrode 10 and the fluid passageways formed between the ribs 27 is provided a reservoir 9 formed by the recess in which the electrode 10 is located. The reservoir 9 provides a region of relatively low current density around the electrodes 10 in order to reduce electropolarisation effects.

During manufacture, the membrane 12 is fixed onto the support 23, while immersed in the de-ionised water, so that the chamber bounded by the membrane 12, the electrodes 10, and the ribs 27 is completely filled with the de-ionised water. Thus, this chamber forms a pCO₂ sensor as shown schematically in FIG. 2.

It is possible for the sensor 1 to include more than one sensing chamber. For example, two parallel electrodes 10 separated by a wall member may be provided on each side of the support 23. A sensing chamber is thereby formed between one electrode 10 on one side of support 23 via the fluid passageways 11 between the ribs 27 on the top of the support 23 to one of the electrodes 10 on the other side of the support 23. A corresponding sensing chamber is provided between the remaining electrodes 10 and the fluid passageways 11 on the bottom of the support 11. An electrode 10 from each of these chambers may be electrically connected to the corresponding electrode from the other chamber, such that the electrical signal from the sensor reflects the conductivity of both chambers.

Embedded in the proximal end of the support 23 is a temperature sensor 5 in the form of a thermocouple. The temperature sensor 5 is used both for pCO₂ corrective calculations and for the measured tissue temperatures to be displayed on the monitor 3, which is informative for medical diagnosis. The temperature sensor 5 has a minimum measuring range of 33-42° C. and a minimum accuracy of +/− 0.2° C.

A ribbon cable 6 is electrically and mechanically connected to the electrodes 10 and the temperature sensor 5. The electrodes 10 are formed as extensions of the conductors of the ribbon cable 6. Alternatively, the electrodes may be formed by plating onto the support 23. Where the cable 6 and the connection to the support 23 are sufficiently strong, the cable 6 can be used to pull the sensor unit 1 from its position of use. Alternatively, a Kevlar line may be provided, for example incorporated with the ribbon cable 6, to provide a strong external mechanical connection.

The membrane 12 may extend proximally from the support 23 with the cable 6 to form a catheter around the cable 6. Alternatively, a separate catheter 28 may be provided, as shown in FIG. 6. In this case, the catheter 28 is bonded to the support 23 proximally of the electrodes 10 and the membrane 12.

As shown in FIG. 6, the catheter 28 may be provided with a plurality of slits 29 in order to fix the sensor unit 1 in position in tissue. The slits 29 are arranged such that when the catheter 28 is pushed distally (in the direction of the arrow B in FIG. 6), relative to the cable 6 (or Kevlar line) the portions 30 of the catheter 28 between the slits 29 are forced outwardly and assume the shape shown in phantom in FIG. 6. The radially projecting portions 30 of the catheter 28 retain the sensor unit 1 in the tissue in which it is embedded. The relative position of the catheter 28 and the cable 6 can be maintained with a locking mechanism (not shown) until it is time for the sensor unit 1 to be removed from the tissue. At this time, the locking mechanism can be released and the portions 30 of the catheter 28 will return to their relaxed position so that the sensor unit 1 can be removed from the tissue.

The catheter tip with the integrated sensor 4 is placed 2-3 cm into organ tissue during surgical procedures to monitor ischemia during a period of up to two weeks. The sensor may be used in orthopaedic and reconstructive surgery, and in organs such as the liver, kidneys, heart muscle, brain and intestines. An insertion tool (not shown) may be used for the placement of the sensor 4, and there is a fixation aid (portions 30 of the catheter 28) to keep the sensor tip in position.

The sensor unit 1 has a maximum diameter of 1 mm and the maximum distance from the catheter tip to the sensor element is 2 mm. The sensor 4 has a minimum pCO₂ measuring range of 4-25 kPa, with a minimum detectable pCO₂ difference of 0.2 kPa. The maximum response of the sensor 4 is 20 seconds. The maximum allowable measurement current i in any area of the fluid chamber is such that j<1 ma/cm² while the measuring input voltage is not more than 50 mV RMS.

The electrodes 10 are gold plated and their total area is approximately 0.3 mm². The measurement frequency f_meas should be higher than 100 Hz. At lower frequencies, polarisation effects in the measurement chamber dominate the measurements. At frequencies above 10 kHz, the low impedance of the capacitances become a significant issue. The measurement resistance R_—measure is in the range of 500 kOhm to 7 MOhm.

The sensor 4 is electrically connected to an electronic surface unit 2 located on the patient skin by the ribbon cable 6, which has a length between 5 cm and 1 metre. The maximum diameter of the cable/catheter is 1 mm and the preferred length of the cable/catheter is 25 cm. The cable/catheter is soft and flexible so that it does not excessively disturb the neighbouring tissue and organs. The cable/catheter and its connections are also sufficiently robust to withstand the strong pulling forces which may be caused by both normal and “abnormal”, use.

During sterilisation, storage and transport the sensor unit 1 is covered by deionised, sterile and endotoxin-free water to make sure that there is substantially no net loss of water from the sensor reservoir.

As shown in FIGS. 1 and 2, the electronic surface unit 2 comprises a sine generator 13 which provides a voltage of at least 5 Volts and a current supply of 50 mV, and is powered by batteries 14. A filter 15 is provided for filtering or averaging the input of the lock-in amplifier 16. A passive filter can be used which reduces the current consumption. A pre-amplifier 17 is combined with a servo mechanism to remove DC current from the signal to reduce electrolysis effects. According to the servo arrangement, the output of the pre-amplifier is fed back to its input via a low pass filter. Thus, only DC components of the output are fed back and cancel any DC current drawn through the pCO₂ sensor. In this way, it is ensured that there is no DC current through the pCO₂ sensor which would degrade the electrodes. The op-amp used in this stage consumes minimal current and has a large CMMR value. At the same time, the bias current is minimal. A lock-in amplifier 16 amplifies the AC signal from the sensor 4. This may be built with op-amps or using an IC package with at least 1% accuracy for the signal detection at frequencies lower than 1 kHz. A galvanic division 19 such as an optocoupler or a coil coupler is provided to prevent noise transfer from the monitor unit 3 and associated cabling 18. The optocoupler is normally favoured due to the noise signal ratio. A temperature signal amplification and conditioning unit 20 is provided to amplify the signal from the temperature sensor 5. The electronic unit 2 is powered by a rechargeable and changeable standard type battery 14. The battery capacity is sufficient for 14 days continuous monitoring. The surface unit 2 is also provided with an on/off indicator LED 21, and a battery status indicator (not shown). Communication between the surface unit 2 and the monitor 3 is analogue through a shielded cable 18. However, the surface unit 2 may include an analogue to digital converter such that communication between the surface unit 2 and the monitor 3 may be digital, for example by digital wire transmission or digital wireless transmission. The cable 18 is at least 4 m long and light and flexible.

As shown in FIGS. 1 and 2, an AC current is generated by sine generator 13 and fed to one of the pCO₂ sensor electrodes 10 and to a lock-in amplifier 16. The high-pass signal from the other pCO₂ electrode 10 is passed through a filter 15 to a low noise amplifier 17 and from there to the lock-in amplifier 16 where it is compared to the reference signal generated by the sine generator 13. Out of phase components, i.e undesired components, of the signal are rejected and the remaining portion of the signal is amplified. The amplified signal is proportional to pCO₂ (or conductance) and is passed on for recordal or further manipulation to the monitor 3.

The surface unit 2 may also be electrically connected to a reference electrode (not shown) that is electrically connected to the patient's skin. The signal from the reference electrode can be used to compensate the signals from the sensor unit 1 for the effect of electromagnetic noise generated by the patient.

A single surface unit 2 may receive signals from several sensor units 1 and provide a multiplexed output to the monitor unit 3.

The monitor unit 3 comprises a portable PC 7 including CD RW and IR port, and a PCMCIA I/O card 8 which can collect signals from at least 4 different surface units 2 simultaneously. The PCMCIA card 8 may have an integrated non-galvanic coupling. The power supply 22 for the monitor unit 3 is of a medically approved type operating on both 110V and 230V.

The software functions of the monitor unit 3 may be implemented in Labview, a software package available from National Instruments of Austin, Tex. and capable of handling up to 4 different surface units simultaneously. The software provides the facility for calibration of the sensor(s) with three calibration points and a second order calibration function. The software can be modified to support any other number of calibration points and type of calibration function. The software also has the facility to smooth the signal from the sensor 4 over defined time intervals. It is possible to have at least two alarm levels for the measurement values and two alarm levels for their gradients. The measurement value gradients are calculated for individually defined time intervals. The alarm is both visible and audible. It is possible to stop an alarm indication while keeping the other alarms active. The monitor 3 can log all measured values, parameter settings and alarms throughout a session. With a 30 second logging interval there should be a storage capacity for at least 10 two week sessions on the hard disc. The session log can be saved to a writeable CD in a format readably by Microsoft Excel.

In summary, a physiological sensing device comprises an electrical sensor for insertion into the tissue of a live animal and which measures the partial pressure of carbon dioxide in the animal tissue. The device also includes an electrical cable connected electrically at its distal end to the sensor. The cable is surrounded by a sheath. The sheath has several flexible portions separated by longitudinal slits. Movement of the proximal end of the sheath towards its distal end shortens the distance between the ends of the flexible portions and causes them to bow outwardly. The sensor can be retained in animal tissue by the bowed flexible portions.

Claims

1. A physiological sensing device comprising: an electrical sensor dimensioned for insertion into the tissue of a live animal with minimal disruption to the tissue and configured to measure electrically at least one physiological parameter of the tissue, such as the partial pressure of carbon dioxide, the partial pressure of oxygen, temperature, pH or glucose concentration;an electrical cable for communicating signals from the sensor and connected electrically at its distal end to the sensor; anda sheath mechanically connected to the sensor and extending with and surrounding at least a portion of the length of the cable,wherein the sheath comprises a plurality of substantially longitudinally extending flexible portions separated by a plurality of longitudinal slits, such that movement of the proximal end of the sheath towards the distal end of the sheath shortens the distance between the ends of the flexible portions and causes the flexible portions to project outwardly and thereby increase the effective diameter of the sheath in the region of the flexible portions, such that the sensor can be retained in animal tissue by the projecting flexible portions.

2. A device as claimed in claim 1 further comprising a line mechanically connected to the distal end of the sheath and extending longitudinally with the cable for assisting in pulling the distal end of the sheath towards the proximal end thereof.

3. A device as claimed in claim 1, wherein the cable is surrounded only by the sheath.

4. A device as claimed in claim 1 having a maximum diameter, with the flexible portions flush with the sheath, of 2 mm, preferably 1 mm.

5. A device as claimed in claim 1, wherein the sensor is a sensor for the partial pressure of carbon dioxide (pCO2) and comprises two spaced electrodes in a chamber containing water, the chamber being bounded at least partially by a carbon dioxide permeable membrane.

6. A device as claimed in claim 5, wherein the sheath forms the carbon dioxide permeable membrane.

7. A physiological sensing device comprising: a sensor for the partial pressure of carbon dioxide (pCO2) having two spaced electrodes in a chamber containing water, the chamber being bounded at least partially by a carbon dioxide permeable membrane;an electrical cable connected electrically at its distal end to the electrodes; anda sheath extending with and surrounding at least a portion of the length of the cable,wherein the sheath forms the carbon dioxide permeable membrane.

8. A device as claimed in claim 7 comprising a plurality of sensors for respective physiological parameters.

9. A device as claimed in claim 7 comprising a temperature sensor.

10. A physiological sensing device comprising: an electrical sensor dimensioned for insertion into the tissue of a live animal with minimal disruption to the tissue and configured to measure electrically at least one physiological parameter of the tissue, such as the partial pressure of carbon dioxide, the partial pressure of oxygen, temperature, pH or glucose concentration;a signal processing device connected to the electrical sensor and arranged to process signals from the electrical sensor to generate a measurement of the physiological parameter; anda reference electrode for electrical connection to a patient,wherein the reference electrode is connected to the signal processing device and the signal processing device is configured to compensate the electrical signals from the electrical sensor for electromagnetic noise from the patient by reference to signals from the reference electrode.

11. A physiological sensor comprising: a sensor body having a longitudinal axis;at least two electrodes spaced in a direction transverse to the longitudinal axis of the sensor body;a plurality of support members extending outwardly from the axis of the sensor body and defining between adjacent support members at least one liquid channel that provides a fluid pathway between the electrodes; anda gas-permeable, liquid-impermeable membrane supported by the support members and providing an outer wall of the liquid channel(s).

12. A sensor as claimed in claim 11, wherein the electrodes extend longitudinally.

13. A sensor as claimed in claim 11, wherein the liquid channel(s) are transverse to the longitudinal axis of the sensor body.

14. A sensor as claimed in claim 11, wherein the support members are transverse to the longitudinal axis of the sensor body.

15. A sensor as claimed in claim 11, wherein the support members are formed integrally with the sensor body.

16. A sensor as claimed in claim 11, wherein the electrodes are located in a recess in the sensor body that has a greater cross-sectional area than the liquid channels.

17. A method of manufacturing a physiological sensor comprising a sensor body having defined therein a water-filled chamber closed by a semi-permeable membrane, the method comprising: immersing the sensor body in water; andattaching the membrane to the sensor body to close the chamber while the sensor body is in the water.