GB2124387A

GB2124387A - Oxygen sensor

Info

Publication number: GB2124387A
Application number: GB08317832A
Authority: GB
Inventors: Dr Dawood Parker
Original assignee: POLYSTAN
Current assignee: POLYSTAN
Priority date: 1982-07-08
Filing date: 1983-06-30
Publication date: 1984-02-15
Also published as: GB2124387B; GB8317832D0

Abstract

A polarographic oxygen sensor intended primarily for measuring levels of oxygen in blood during by- pass surgery has a sensor body comprising a rod-like cathode 12 mounted in a tubular anode 10, an electrolyte film covering flush end surfaces of the cathode and the anode, and a low oxygen permeability membrane 22 trapping the electrolyte on the sensor body. The preferred material for the membrane is a polypropylene film. By using a relatively low permeability membrane, the cathode can be made significantly larger and hence less expensively than in prior art sensors without creating unwanted consumption of the oxygen being measured. The sensor is potted in a standard conduit section 26, the combination of the two constituting a sterile, disposable item. An electrical connector or cable termination includes contact elements for engaging the electrodes of the sensor. One of the elements has a recess for a thermistor which is in close thermal contact with the cathode when fastened to the sensor so that the temperature of the blood in close promixity to the sensor can be accurately monitored simultaneously with the oxygen level. <IMAGE>

Description

SPECIFICATION A polarographic sensor This invention relates to a polarographic sensor intended primarily for in-line monitoring of partial oxygen pressure (pO2) during by-pass surgery.

During operations on for example the heart or lungs, and particularly during open-heart surgery, it is important to be able to monitor accurately the level of oxygen dissolved in the patient's blood.

This can be conveniently carried out by placing an electrochemical measuring cell in contact with blood circulating through a by-pass tube. One of the difficulties involved in measuring the concentration of a gas in a liquid with an electrochemical sensor is that the sensor itself must not affect the concentration in its immediate vicinity by consuming the gas being measured.

This problem is overcome in a known device by using a very small electrode as part of a measuring cell so that chemical action at the electrode produces a measurable electrical current in the sensor without using significant quantities of oxygen. The disadvantage of such a cell is that it is relatively expensive to make.

Furthermore, after prolonged use deterioration of the electrolyte and/or the electrodes results in less accurate measurements. The cell therefore has a limited operational life.

A second difficulty arises from the effect of temperature on the level of oxygen in the blood brought about by the release of oxygen from the haemoglobin as the temperature of the blood rises. Measurement of temperature is therefore generally performed simultaneously using for example a thermistor mounted adjacent the cell or at a separate location in the by-pass tube. This can lead to inaccuracies because the temperature of the cell (or of the blood at a separate location) may be different from the actual temperature of the blood at the point where the oxygen concentration is being measured.

It is an object of the invention to provide a sensor which offers an improvement in respect of at least one of the difficulties mentioned above.

According to one aspect of this invention there is provided a polarographic sensor for measuring the concentration of a gas in a fluid, wherein the sensor comprises.~ a sensor body having a sensing surface, the body comprising at least two mutually adjacent electrodes, each electrode having a surface forming a portion of the sensing surface; a film of electrolyte in the form of a liquid or gel on the sensing surface to provide an electrically conductive bridge between one of the electrodes and another of the electrodes; and a membrane covering the sensing surface to retain the electrolyte thereon, the membrane having a low permeability to the gas being measured.

In the known device mentioned above, the membrane separating the electrolyte of the cell from the blood is a Teflon (RTM) PTFE sheet of relatively high oxygen permeability. By restricting the passage of oxygen to the electrolyte using a low permeability membrane, a much larger - electrode can be used largely without affecting the concentration of oxygen in the blood adjacent the sensor. Cathode surface areas in excess of 0.0075 mm2 can be used, with a surface area in the range 0.05 to 2 mm2 being preferred. An electrode of this size is relatively inexpensive to manufacture. The preferred materials for the membrane are a high-density polyethylene film such as Bartuf (Trade Mark) or a polypropylene film.These materials both have an oxygen permeability in the range of from 7.5x 10-12 to 1.1 x 10-' cm3cm/sec/cm2/cmHg, and are both gamma ray sterilisable. It has been found that a suitable response time for the sensor is provided if the membrane has a thickness of less than 30 microns.

in a preferred embodiment of the invention the polarographic cell comprises a hollow cylindrical silver anode housing an inner rod-like platinum cathode surrounded by an epoxy resin insulating sleeve. The end surfaces of the cathode, the sleeve and the anode all form flush surface portions of the sensing surface of the sensor body. The sensing surface is coated with a polyhydroxy-alcohol based electrolyte film which is trapped beneath the low permeability membrane. A second high oxygen permeability outer membrane may be provided for protecting the inner membrane and holding it in position under varying pressure conditions.

In accordance with the invention the sensor may be coupled to an electrical connector which houses a temperature transducer. This transducer is arranged so as to be in thermal contact with one of the electrodes, whereby the gas concentration and the temperature of a fluid can be measured simultaneously at a single location using a 4-way cable terminating in the connector.

According to another aspect of the invention a polarographic sensor may form part of a disposable sensor assembly comprising a fluid conduit section with a lateral opening housing the sensor. This conduit section is preferably a standard luer fitting, which is a widely used and well proven article. The assembly can be treated as a disposable sterile item since both components are relatively cheap to manufacture and are easily connected in an extra-corporeal circuit. This largely avoids sterilisation problems and any effects caused by deterioration of the sensor after prolonged and repeated use. The location and method of securing the sensor complet with membranes in a disposable fitting minimises the potential risk to the patient of any component in the extra-corporeal circuit working loose.

In the preferred embodiment of the invention, the sensor has a rear face which, as well as providing an electrical contact surface for connecting the cell to the connector, provides a thermal contact surface for conducting heat from the sensing surface to a temperature sensor housed in the connector.

S 10 15 20 25 30 35 40 45 50 55 60 Although the primary use of the sensor in accordance with the invention is as a device for measuring oxygen in the bloodstream, it can be used to measure other gases by for example varying the applied voltage. The sensor can also be used to monitor respiratory gas concentrations or gas concentrations in other relatively low tempera~ure environments.

One advantage of the relatively large size of the electrodes is that the sensing surface can, as an alternative, be manufactured by thick film deposition techniques.

The invention will now be described by way of example with reference to the drawings in which: Figure 1 is a cross-section of an oxygen sensor mounted in a conduit for measuring the oxygen content in blood flowing through the conduit; Figure 2 is a longitudinal section of the sensor and the conduit of Figure 1 together with an electrical connector; and Figure 3 is a block diagram of an electronic instrument to which the sensor may be connected.

Referring to Figure 1, a polarographic sensor in accordance with the invention has a sensor body comprising a tubular silver anode 10, and a central platinum cathode 12 in the form of a rod bonded coaxially inside the anode 10 by an epoxy resin insulating sleeve 14. The cathode 12 has a brass extension 16 extending beyond a flanged rear face 18 of the anode 10. The front end surfaces of the anode 10, cathode 12 and sleeve 14 are flush, and constitute a sensing surface 20.

A film of liquid electrolyte, not visible in the drawings, is applied during manufacture to the sensing surface 20 to provide a conductive electrical bridge between the cathode and the anode. To trap the electrolyte on the sensing surface, two membranes 22A and 22B cover the surface 20, their edges being secured beneath a collar 24 whihch is a tight fit around the anode 10. The sensor body as a whole, comprising anode 10, cathode 12, electrolyte, membranes 22A and 22B, and collar 24, is potted in a conduit 26 using a polyurethane potting compound (shown at 28 in Figure 1) which bonds the rear flange of the anode 10 to an externally threaded ring 30 fixed on the conduit 26.The sensor is thus mounted with the sensing surface 20 approximately flush with the interior surface of the conduit 26 so that the outer (22B) of the two membranes is in contact with blood flowing through the conduit 26.

In accordance with the invention, the inner membrane 22A is a low oxygen permeability membrane comprising, in this embodiment, a 12.5 micron biaxially oriented polypropylene film such as that manufactured under the trademark BEXPHANE P by the Hercules Powder Company Limited of Brantham, Manningtree, Essex, England. This material has an oxygen permeability of 7.29x10-11 65 70 75 80 85 90 95 100 105 cm 3 cm sec cm2 cmHg which, for a 12.5 micron membrane becomes 0.1596 litre m2 hour atm BEXPHANE P can be sterilised by gamma radiation without deterioration. The outer membrane 22B which acts merely as a protective membrane and for holding the inner membrane 22A in position, has a relatively high oxygen permeability and therefore affects the total permeability of the two membranes together only to a negligible degree.The material used for the outer membrane is a 50.8 micron dimethyl silicone film manufactured by the General Electric Company, 1 River Road, Schenectady, NY 12345, U.S.A. This material has an oxygen permeability of 5xlO-8 cm 3 cm sec cm2 cmHg which becomes 26.93 litre m2 hour atm for a 50.8 micron film.

By choosing a relatively low permeability membrane, the cathode, where reduction takes place can have a relatively large diameter, in this case 0.4 mm. Successful experiments have also been carried out with an even lower permeability membrane, such as a high density polyethylene having a permeability of approximately 1.3 xl 0-li cm3 cm sec cm2 cmHg in combination with a 1 mm diameter cathode.

It should be noted that reducing the overall permeability by increasing the thickness of the inner membrane 22A has a detrimental effect on the response time of the sensor.

The electrolyte is preferably a solution of potassium chloride in propylene glycol. Other solvents could be used, including water, but polyhydroxy-alcohols such as propylene or ethylene glycol or glycerene are preferred due to their low evaporation characteristics. Other soluble metal halides could be substituted for potassium chloride.

When the sensor is used to measure oxygen concentrations, a voltage of 650 mV or thereabouts is applied across the electrodes.

Oxygen which permeates through the membranes 22A and 22B causes the formation of OH- ions at the cathode, and at the same time Cl-ions from the electrolyte combine with the silver of the anode to form silver chloride. A net transfer of negative charge thus occurs between the cathode and the anode, giving rise to a detectable current in the order of 10-8 of 10-9 amps. The rate of the chemical reaction, and hence the detected current, is proportional to the concentration of oxygen adjacent the sensing end of the cell. No other medical gases affect the sensor at 650 mV, with the exception of nitrous oxide and halothane.

Referring to Figure 2, the sensor and the conduit 26 constitute a sterilised, disposable item which is supplied with protective caps 32 at each end of the conduit (only one of these is shown in Figure 2). Prior to use, the end caps 32 are removed, and the conduit is connected into a bypass circuit during a surgical operation. To measure the oxygen content of blood in the conduit 26, a current measuring instrument is coupled to the sensor by an electrical connector 34 having contact elements for engaging the sensor electrodes 12 and 10.

The connector 34 has a brass contact ring 36 mounted in a plastics tube 38, which in turn houses a S-way cable 40. A central socket 42 is mounted inside the contact ring 36 by an epoxy resin insulator 44. The ring 36 and the socket 42 engage respectively the rear flanged surface 18 of the anode 10 and the extension 16 of the cathode 12, to provide electrical connection with a pair of wires in the cable 40. A threaded sleeve 46 is provided for securing the connector 34 to the sensor by engagement with the ring 30 on the conduit 32.

An important feature of the connector 34 is the incorporation of a temperature measuring device.

A first bead thermistor 46 is housed in a recess bored in one side of the contact ring 36. In this position, it is in close thermal contact with the anode 10 when the connector is fitted to the sensor, enabling simultaneous measurement of the temperature of the blood immediately adjacent the sensing surface 20. Since the oxygen level in the blood is affected by temperature, the temperature signal fed from the thermistor 46 to the measuring instrument by a second pair of wires in the cable 40 can be used to compensate the oxygen measurement with comparative accuracy. A second thermistor 48 at the other end of the connector provides an ambient temperature reference to enable correction for any temperature difference between the blood adjacent the sensor surface and the thermistor 46.

The electrical circuitry of a measuring instrument suitable for use with the sensor described above is shown in the simplified block diagram of Figure 3. As has been mentioned above, the level of oxygen in the blood is affected by temperature. To compensate the oxygen level (PO2) signal for this effect, the incompensated current signal from the sensor or inputs 50 and 52 is first amplied and then fed as a voltage signal to a chopper 52 having a mark-to-space ratio variable in response to the temperature signals received from the thermistors at inputs 53, 54 and 55. A compensating network 56 processes the temperature signal to vary the mark-to-space ratio of the controlling signal on the chopper control input 57 in a non-linear manner corresponding to a known relationship between oxygen concentration and blood temperature. The compensated PO, signal is obtained by filtering the chopper output in a low pass filter 58 to remove the square wave and yield a d.c. signal which is the average level of the chopper output.

The instrument shown in Figure 4 is arranged to display a compensated PO2 reading and a temperature reading simultaneously on displays 59 and 60.

Claims

1. A polarographic sensor for measuring the concentration of a gas iri a fluid, wherein the sensor comprises a sensor body having a sensing surface, the body comprising at least two mutually adjacent electrodes, each electrode having a surface forming a portion of the sensing surface; a film of electrolyte in the form of a liquid or gel on the sensing surface to provide an electrically conductive bridge between one of the electrodes and another of the electrodes; and a membrane covering the sensing surface to retain the electrolyte thereon, the membrane having-a low-permeability-to the gas being measured.

2. A sensor according to claim.1, where the said surface of one of the electrodes forms a substantially annular surface portion of the sensing surface, and the said surface of another of the electrodes forms a surface portion substantially surrounded by the annular surface portion.

3. A sensor according to claim 2, wherein the said one electrode comprises an outer hollow cylinder, and wherein the said another electrode comprises an inner rod mounted coaxially inside the outer electrode and separated from the outer electrode by an electrically inulating sleeve, the end surfaces of the outer electrode, the sleeve, and the inner electrode forming flush surface portions of the sensing surface.

4. A sensor according to claim 3, wherein the membrane has an edge portion which overlaps at least a portion of the outer curved surface of the outer electrode and is held in position by a collar fitted around the outer electrode.

5. A sensor according to claim 2, wherein the electrode forming the annular surface portion is an anode and the said another electrode is a cathode.

6. A sensor according to claim 5, wherein the anode is silver.

7. A sensor according to claim 6, wherein the cathode is platinum.

8. A sensor according to claim 1, wherein the electrolyte is a solution of a metal halide, the metal being an alkali metal or an alkaline earth metal.

9. A sensor according to claim 8, wherein the metal halide is dissolved in a polyhydroxy-alcohol.

10. A sensor according to claim 1, having a cathode surface area forming part of the sensing surface, the area being greater than 0.0075 mm2.

11. A sensor according to claim 10, wherein the said surface area is in the range of from 0.05 mm2 to 2.00 mm2.

12. A sensor according to claim 10, wherein the oxygen permeability of the membrane is less than 1.1x10-10 cm3 cm sec cm2 cmHg

13. A sensor according to claim 12, wherein the oxygen permeability is greater than 7.Sx 10-12 cm3 cm sec cm2 cmHg

14. A sensor according to claim 12, wherein the thickness of the membrane is less than 30 microns.

15. A sensor according to claim 12, wherein the membrane is a polypropylene film.

16. A sensor according to claim 12, wherein the membrane is a high-density polyethylene film.

17. A sensor according to claim 12, wherein the membrane is a polyurethane film.

18. A sensor according to claim 1, including a second outer membrane covering the first, inner membrane and having a higher oxygen permeability than the inner membrane.

19. A sensor according to claim 18, wherein the outer membrane is a silicone film.

20. Sensor apparatus comprising the combination of the sensor of claim 1 and a temperature transducer in thermal contact with at least one of the electrodes.

21. Sensor apparatus comprising the combination of the sensor of claim 3 and a temperature transducer for measuring the temperature of the outer electrode.

22. Sensor apparatus comprising the combination of the sensor of claim 1 and an electrical connector having an electrical contact element for each electrode of the sensor, wherein a temperature transducer is mounted in one of the contact elements for measuring the temperature of the corresponding electrode.

23. Apparatus according to claim 22 comprising the combination of (a) a sensor which has an anode in the form of a hollow cylinder, a cathode in the form of a rod mounted coaxially inside the cylinder, and a sleeve of insulating material between the cathode and the anode, wherein the end surfaces of the anode, the sleeve and the cathode form flush surface portions of the sensing surface, and wherein the other end of the cathode projects beyond the other ends of the anode and the sleeve, and (b) a removable electrical connector having a socket for receiving the projecting end of the cathode, and an annular contact element for abutting the other end surface of the anode, the temperature transducer comprising a thermistor housed in a recess in the said contact element.

24. A disposable sensor assembly comprising: a fluid conduit section having a lateral opening; a polarographic sensor fitted in the lateral opening, the sensor including a sensor body comprising a first electrode in the form of a hollow cylinder, a second electrode in the form of a rod bonded coaxially inside the first electrode, and an insulating sleeve around the second electrode to insulate the second electrode from the first electrode, whereby the end surfaces of the first and second electrodes and the sleeve constitute flush surface portions of a sensing surface facing the interior of the conduit section, the sensor further including a liquid or gel electrolyte on the sensing surface, and an oxygen- permeable membrane covering-the electrolysis surface for trapping the electrolyte on the surface.

25. An assembly according to claim 24, wherein the sensor is mounted in a branch pipe portion of the conduit section extending transversely with respect to a longitudinal axis of the conduit section, and wherein the said pipe portion has an external screw thread for attaching an electrical connector to the sensor electrodes.

26. An assembly according to claim 24 including an electrical connector housing a temperature transducer.

27. An assembly according to claim 24, wherein the fluid conduit section includes removable sterility end caps.

28. An assembly according to claim 24, wherein the sensor is potted into the conduit section.

29. An assembly according to claim 24, wherein the sensing surface of the sensor body is substantially flush with an interior surface of the conduit section.

30. A polarographic sensor constructed and arranged substantially as herein described and shown in the drawings.

31. Sensor apparatus constructed and arranged substantially as herein described and shown in the drawings.

32. A disposable sensor assembly constructed and arranged substantially as herein described and shown in the drawings.