WO2004105848A1

WO2004105848A1 - Heaters for breathing tubes

Info

Publication number: WO2004105848A1
Application number: PCT/GB2004/002285
Authority: WO
Inventors: Stuart Corner; Paul Berwick
Original assignee: E.M.E. (Electro Medical Equipment) Limited
Priority date: 2003-05-30
Filing date: 2004-05-28
Publication date: 2004-12-09

Abstract

A heating wire assembly for reducing condensation in a breathing tube (4) delivering humidified gas to a patient for breathing comprises an electrically resistive heating wire (2) and a ribbon-shaped carrier (26) for supporting the heating wire (2) in use in the breathing tube (4). The ribbon-shaped carrier (26) is configured to hold the heating wire (2) at the periphery of the breathing tube (4) such that it is held against the inner wall of the tube. The ribbon-shaped carrier (26) may be designed to carry two or more strands of heating wire (2) spaced around the circumference of a breathing tube (4) and being supported across a diameter of the tube by a web (24). The carrier (26) may also support one or more temperature sensors such as the resistive wire (28) for sensing the temperature of the gas in the breathing tube (4).

Description

Heaters for Breathing Tubes

This invention relates to heaters for conduits which supply humidified air from a ventilator to a patient for reducing condensation in the conduit .

It is well known in the art to deliver moist, possibly warmed air to a patient via a plastics conduit i.e. a breathing tube but that there is a problem with moisture condensing out of the supplied gas on the edges of the tube . This phenomenon is also known as "rainout" . Rainout is undesirable since the water collects in the tube which can cause blockages and provide a breeding ground for bacteria. Such water must therefore be periodically drained.

There have been several previous proposals to provide a heater in such breathing tubes in order to reduce or prevent rainout . An example of such a proposal is shown in GB-A-2284356. Another example of a heater for a breathing tube is shown in WO 97/18001.

In order to enhance the safety of such systems and to prevent the delivery of gas at an excessively high temperature to a patient, which could be dangerous, one such system which is currently available commercially has a specially modified breathing tube with apertures moulded into it at certain points into which encased thermistors are inserted. The thermistors monitor the temperature of the gas in the tube and may therefore be used to regulate the power supply to the heating wire if the temperature of the supplied gas rises too high.

Although when used properly such a system can work effectively, the Applicant has realised that there are a number of problems associated with it. Firstly, the encased thermistor assembly is relatively expensive such that it must be reused from one patient to the next . However, it is not sufficiently robust to be sterilised in a clinical autoclave and must therefore be cleaned using wipes. This is generally unsatisfactory and compromises the overall hygiene of the system. By contrast, the breathing tube itself and the heating wire are disposable.

A further problem with the system described above is that the encased thermistors must be inserted into the apertures in the breathing tube each time the equipment is set up. In order to achieve a good seal, the thermistors are designed to be a tight fit in the apertures, but this can make them difficult to insert properly. It is therefore not uncommon for the thermistors not to be pushed fully home. This makes them inaccurate as they are not then in the degree of thermal contact with the gas flow that they were designed for. This is a potentially dangerous situation since it will result in the thermistor returning a reading corresponding to a lower temperature than the actual temperature of the gas . This results in the power to the heating wire being increased which exacerbates the problem. In the most extreme case, the temperature of the heating wire could be raised to such an extent that it melts the plastic of the breathing tube. This would clearly be very dangerous. It is an object of the present invention to mitigate at least some of the drawbacks mentioned above.

When viewed from a first aspect the present invention provides a heating wire assembly for reducing condensation in a breathing tube delivering humidified gas to a patient for breathing, said assembly comprising: an electrically resistive heating wire; and carrying means for supporting said heating wire in use in the breathing tube, the carrying means being configured to hold the heating wire at the periphery thereof .

When viewed from a second aspect the invention provides a heated breathing tube assembly for delivering humidified gas to a patient for breathing, said assembly comprising: a breathing tube for conveying breathing gas; an electrically resistive heating wire; carrying means supporting said heating wire, the carrying means being disposed in said breathing tube so as to extend across its axis, the carrying means being configured to hold the heating wire at the periphery thereof such that the heating wire is held against the inner wall of the breathing tube.

Thus it will be seen that in accordance with the invention a heater wire may be supported in the breathing tube. This is advantageous as it allows for a predictable temperature profile across the width of the tube. By contrast in some known arrangements where the heating wire simply lays in the tube, the temperature profile is unpredictable and depends upon the path of the tube and thus the heater inside it. Furthermore, by having the heating wire held at the periphery of the carrying means, the temperature profile across the tube may be made more even.

The carrying means is preferably of a thermally conductive material such as a loaded polycarbonate. This improves the transfer of heat from the heating wire to the breathing gas passing through the tube. The dimensions of the carrying means will be such as to allow it to support itself generally across a diameter of a standard breathing tube and thus its greatest lateral dimension will be e.g. approximately 10 mm.

The carrying means could comprise a plurality, of discrete elements - either formed integrally on the heating wire or attached afterwards by clamping or clipping. Such discrete elements may be advantageous in promoting turbulent flow of gas over them which improve the transfer of heat to the gas. Indeed in some preferred embodiments the elements may comprise suitable formations for promoting turbulent flow.

In a preferred example of such an arrangement the elements comprise one or more peripheral clips for receiving the heating wire which of course ensures that the heating wire is supported at the periphery of the elements. In such embodiments the heating wire is held in use directly against the inner wall of the breathing tube . In other embodiments there may be a thin wall of the material of the carrying element between the heating wire and the inner wall of the breathing tube. However it will still be the case that the heating wire will be located closer to the wall of the breathing tube than to its centre.

In alternative embodiments the carrying means comprises a continuous length of carrier. In a preferred example the carry means is generally ribbon- shaped. Again the heating wire could be formed integrally with the carrier, e.g. by co-extrusion, or the carrier may comprise means such as a clip for receiving the wire after formation. The advantages of a continuous length is that it is easier to manufacture and assemble than discrete elements and easier to insert into a breathing tube. If desired it could be formed with openings so as to mimic the effect of discrete elements so as to achieve any advantages to be had thereby, although it is currently preferred to form a continuous carrier by extrusion.

The carrying means could be designed to carry just one strand of wire, but preferably is designed to carry two e.g. both arms of a loop of heating wire. In some embodiments, more than two strands may be carried - e.g. four where two loops of heating wire are provided. By spacing the strands around the circumference of the breathing tube, an even more uniform temperature profile could be achieved.

The carrying means may carry just the heating wire. In preferred embodiments however the carrying means also carries temperature sensing means for sensing the temperature of the gas in the breathing tube. This has several advantages over the prior art temperature sensing arrangement described hereinabove. Firstly the assembly of heating wire, thermal sensing means and carrying means may be simply inserted into one end of a or the breathing tube to extend along at least part of the length of the tube. It will be appreciated therefore that there is no need for sensors to be inserted into apertures so avoiding the problems with incorrect insertion outlined above . Indeed by holding the heating wire and thermal sensing means in fixed relationship. to one another in the carrying means, and having the carrying means in a predetermined part of the breathing tube, accurate and reliable temperature measurements may be made .

Furthermore, the heater wire assembly in accordance with the invention may be manufactured sufficiently inexpensively that it may be disposable. The wire assembly may therefore be inserted into the tube at the time of manufacture and the entire system - i.e. the breathing tube and the heating wire sensing assembly may be disposed of together after a single use . Not only does this facilitate setting up the ventilator apparatus, it avoids the potential hygiene problems outlined above with respect to the prior art.

Whilst in accordance with the invention the heating wire is disposed at the periphery of the carrying means and so against the inner wall of the. breathing tube, it is preferred that the temperature sensing means is provided substantially centrally thereof . This gives some degree of thermal isolation between the heating wire and the temperature sensor so that the latter may more accurately reflect the temperature of the gas, whilst at the same time maintaining a degree of thermal contact with the wire so as to allow the supply of power to the latter to be reduced or interrupted in the event that it should overheat excessively, thereby preventing dangerous melting of the breathing tube.

It has been appreciated that the arrangements above embody a further novel and inventive concept that is not necessarily dependent on the first aspect of the invention. When viewed from a second aspect therefore the present invention provides a heating wire assembly for reducing condensation in a breathing tube delivering humidified gas to a patient for breathing, said assembly comprising: an electrically resistive heating wire; carrying means for supporting said heating wire in use in the breathing tube; and temperature sensing means for sensing the temperature of gas in the tube, wherein the temperature sensing means is also supported by the carrying means .

The temperature sensing means may comprise any thermally responsive sensor giving an electrical signal. In one set of preferred embodiments the temperature sensing means comprises a plurality of temperature sensors at discrete locations along the heater wire. Preferably the sensors comprise thermistors since these are relatively inexpensive and give an easily measured electrical response by their change in resistance . Preferably sensors are provided at two locations along the wire, most preferably approximately at the proximal and distal ends thereof respectively.

As mentioned above, avoiding the need for clinical personnel to assemble the sensors into the breathing tube avoids the possibility of incorrect insertion and therefore inaccurate readings which can lead to the gases being heated excessively. Furthermore, it will be appreciated that in accordance with the invention the thermal sensors will be in relatively close proximity to the heating wire. They will therefore at least partially measure the temperature of the heating wire as well as the gas passing over them. This provides an additional safety benefit in that if for any reason the wire should begin to increase in temperature, this will be sensed immediately by the sensors and the power thereto may be reduced. A single sensor may be provided at each location.

In a preferred embodiment, however, a pair of sensors is provided at the proximal end, with one being in closer thermal contact with the heating wire than the other. This allows the temperature of the heating wire and the temperature of the gas passing through the tube to be measured independently.

In another set of preferred embodiments the temperature sensing means comprises an elongate sensor extending along a substantial portion of the heating wire . In a preferred example the sensor comprises a resistive wire having a significant temperature coefficient of resistance, either positive or negative, changes in which may be measured to measure temperature . An example of a suitable material would be Nichrome . Such an arrangement is beneficial as it can be sensitive to hot spots anywhere along the heater wire and requires only one signal wire, or may not require one at all if, as is preferred, the sensor is looped.

Whether comprising discrete elements or a continuous length, the temperature sensing means may be arranged to have a sensing zone that is just coextensive with the heating wire. In some preferred embodiments though it extends beyond the heating wire . This allows the temperature of the gas leaving the tube - i.e. to be delivered to a patient - to be measured.

Certain preferred embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which: Fig. 1 is a cross-sectional view through a known heating wire and breathing tube arrangement, shown for reference only;

Fig. 2 is a schematic view of a breathing tube arrangement in accordance with the present invention;

Fig. 3a is a transverse cross-section through the breathing tube of Fig . 2 ;

Fig. 3b is a cross-section similar to Fig. 3a showing the temperature profile in use;

Figs. 4a and 4b are respectively longitudinal cross-sections in perpendicular planes through a breathing tube arrangement in accordance with a second embodiment of the invention; Fig. 5 is a diagram showing a breathing tube arrangement in accordance with a further embodiment of the invention;

Fig. 6 is a transverse cross-section through the tube of Fig. 5; Fig. 7 is a view of a further embodiment of the invention;

Fig. 8 is a transverse cross-section through Fig. 7;

Fig. 9 is a perspective view of another embodiment of the invention;

Fig. 10 is a view of yet another embodiment of the invention; and

Fig. 11 is a view of yet another embodiment of a heating wire assembly. Turning to Fig. 1, there may be seen a transverse cross-section through a breathing tube and heating wire in accordance with a known arrangement . In this arrangement, it will be seen that the two arms A of a looped heating wire are relatively close together. The resultant temperature profile B is of relatively small extent and it does not effectively heat across the. whole width of the breathing tube C. To make matters worse, although the two heating wires A are shown in the centre of the tube C for clarity, in fact in use they will normally both lie on the bottom of the breathing tube C so that the temperature profile across the width is even more uneven. Fig. 2 shows schematically an embodiment of the present invention. In this embodiment, a looped heating wire 2 is provided in a standard breathing tube 4. The two arms of the heating wire 2 are held apart against opposite sides of the interior wall of the breathing tube 4 by carrying means in the form of a series of spaced retaining clips 6. As will be seen more clearly in the cross-section of Fig. 3a, each clip 6 comprises an elongate central shaft 6a at each end of which is a part-circular resilient clip 6b. The clip 6 is made from a suitable flexible plastics material so that the heating wire 2 may be clipped into and retained in the two peripheral clips 6b. It will be seen that the width of the clip 6 is such that when the heating wire 2 is received therein, it is approximately equal to the interior diameter of the breathing tube . This means that the clip 6 will extend across the middle of the tube 4 and will support the heating wire 2 at diametrically opposed points on the inner wall of the tube .

Fig. 3b shows the temperature profile resulting from the arrangement in Figs. 2 and 3a. It will be seen that the heating effect of the wire 2, shown schematically by the dashed line 8, extends across the whole width of the tube and is relatively even.

Figs. 4a and 4b show respectively two schematic longitudinal cross-sections through a heated wire arrangement in accordance with another embodiment of the invention. This embodiment is identical to the previous one except that the clips 6' each comprise a pair of transversely projecting wings 10. As will be seen from the arrows, these act to deflect the gas passing over them to encourage turbulence and therefore better heat transfer from the heating wire 2 to the gas in the tube 4.

Fig. 5 shows another embodiment of the invention. In this embodiment, rather than a series of discrete spaced clips, the heating wire 2 is supported in a similar position by a continuous extruded ribbon 12. This may be seen more clearly in the cross-section of Fig. 6. From here it will be seen that the cross- sectional profile is similar to that of the clips of the previous embodiment with the two arms of the wire loop 2 being supported across a diameter of the tube 4 by a thin web 14. At each edge of the web 14 however is an enclosed lumen 16 into which the heating wire 2 is received. The heating wire is therefore separated from the interior wall of the breathing tube 4 by the lumen wall thickness, but is at either side still essentially at the periphery of the ribbon carrier 12. It will be seen that the temperature profile 18 is very similar to that of the previous embodiment. The advantage of the embodiments shown in Figs. 5 and 6 is that the ribbon 12 may be extruded onto the heating wires 2 and the assembly then simply inserted into the breathing tube. Indeed, since the heating wire 2 is received in an enclosed lumen 16, the wire does not require its own insulation.

Figs . 7 and 8 show a further embodiment of the invention. This embodiment is similar to the previous one except that the heating wire is in the form of a double loop. The carrying means is now cross shaped in profile with two perpendicular webs 20 supporting the four arms of the wire loops 2. It will be seen that the temperature profile 22 gives yet more even and efficient heating of the interior of the breathing tube 4 than in the previous embodiment .

Fig. 9 shows schematically an embodiment similar to that shown in Figs. 5 and 6 except that in this embodiment the central web 24 of the ribbon 26 is slightly thicker and has embedded in it a temperature sensor in the form of a single loop of resistive wire 28. The resistive wire 28 has a significant positive temperature coefficient of resistance (PTC) so that as - li the temperature it experiences increases, its electrical resistance will increase by a measurable amount . Since the relationship between temperature and resistance for the sensor wire 28 is known, the measured change in resistance may be used to calculate its temperature. Since the web 24 is still relatively thin, the temperature of the wire 28 will give a reasonably good indication of the temperature of the gas passing through the breathing tube 4. The sensor wire 28 therefore allows electrical power to the heating wire 2 to be regulated to maintain the temperature of the gas being supplied to the patient within a required range. Although the sensor wire 28 is to some degree thermally isolated from the heater wire 2, it will also sense overheating of this wire in the event of a malfunction before the heater wire 2 reaches a temperature at which the heater tube 4 would be in danger of melting.

Fig. 10 shows a yet further embodiment of the invention which is similar to that shown in Fig. 9 except that the temperature sensor is in the form of a thermistor 30 rather than a continuous length of PTC wire. Signal wires 31 for the thermistor are embedded into the central web 24.

Also different in this embodiment is that an overmoulding 32 is provided over the end of the ribbon 26 in order to encase and support the looped end of the heating wire 2 but particularly the thermistor 30. A further thermistor (not shown) is provided at the proximal end of the heater ribbon 26. By monitoring the temperature of these two thermistors, both the gas entry and gas exit temperatures may be monitored and an ■ appropriate power supplied to the heating wire 2 to give the required exit temperature .

Finally, Fig. 11 shows another embodiment similar to that shown in Fig. 10 but with the thermistor 30 being spaced from the end of the heating wire 2. In one particular example, the thermistor is approximately 190mm from the end of the heating wire. It will be seen that the overmoulding 34 is correspondingly elongate to support the thermistor 30. Signal wires 31 are buried in the heater ribbon 26 and connect the thermistor 30 to a terminating connector (not shown) at the other end of the ribbon. The advantage of the embodiment shown in Fig. 11 is that the thermistor 30 will accurately reflect the temperature of the gas leaving the breathing tube without being influenced by the temperature of the heater. This allows the proximal thermistor (not shown) to monitor mainly the temperature of the heating wire 2, whilst the distal thermistor 30 measures the exit temperature of the gas .

It will be appreciated by those skilled in the art that only certain specific embodiments of the invention have been shown and that many variations are possible within the scope of the invention.

Claims

Claims :

1. A heating wire assembly for reducing condensation in a breathing tube delivering humidified gas to a patient for breathing, said assembly comprising: an electrically resistive heating wire; and carrying means for supporting said heating wire in use in the breathing tube, the carrying means being configured to hold the heating wire at the periphery thereof.

2. A heated breathing tube assembly for delivering humidified gas to a patient for breathing, said assembly comprising: a breathing tube for conveying breathing gas; an electrically resistive heating wire; carrying means supporting said heating wire, the carrying means being disposed in said breathing tube so as to extend across its axis, the carrying means being configured to hold the heating wire at the periphery thereof such that the heating wire is held against the inner wall of the breathing tube.

3. A wire or tube assembly as claimed in claim 1 or 2 wherein the carrying means comprises a thermally conductive material.

4. A wire or tube assembly as claimed in claim 3 wherein the carrying means comprises a loaded polycarbonate .

5. A wire or tube assembly as claimed in any preceding claim wherein the carrying means comprises a plurality of discrete elements.

6. A wire or tube assembly as claimed in claim 5 wherein said elements comprise suitable formations for promoting turbulent flow.

7. A wire or tube assembly as claimed in 5 or 6 wherein each element comprises one or more peripheral clips for receiving the heating wire.

8. A wire or tube assembly as claimed in any of claims 1 to 4 wherein the carrying means comprises a continuous length of carrier.

9. A wire or tube assembly as claimed in claim 8 wherein said carrier is generally ribbon-shaped.

10. A wire or tube assembly as claimed in any preceding claim wherein the carrying means is designed to carry two strands of heating wire .

11. A wire or tube assembly as claimed in any of claims 1 to 9 wherein the carrying means is designed to carry more than two strands of heating wire spaced around the circumference of the breathing tube .

12. A wire or tube assembly as claimed in any preceding claim wherein the carrying means also carries temperature sensing means for sensing the temperature of the gas in the breathing tube .

13. A wire or tube assembly as claimed in claim 12 wherein the temperature sensing means is provided substantially centrally of the breathing tube.

14. A heating wire assembly for reducing condensation in a breathing tube delivering humidified gas to a patient for breathing, said assembly comprising: an electrically resistive heating wire; carrying means for supporting said heating wire in use in the breathing tube; and temperature sensing means for sensing the temperature of gas in the tube, wherein the temperature sensing means is also supported by the carrying means .

15. A wire or tube assembly as claimed in any of claims 12 to 14 wherein the temperature sensing means comprises a plurality of temperature sensors at discrete locations along the heater wire .

16. A wire or tube assembly as claimed in claim 15 wherein the temperature sensors comprise thermistors .

17. A wire or tube assembly as claimed in claim 15 or 16 wherein sensors are provided at two locations along the wire.

18. A wire or tube assembly as claimed in claim 17 wherein said sensors are arranged to be approximately at the proximal and distal ends of the tube thereof respectively.

19. A wire or tube assembly as claimed in claim 17 or 18 wherein a pair of sensors is provided at the proximal end of the tube, with one being in closer thermal contact with the heating wire than the other.

20. A wire or tube assembly as claimed in any of claims 12 to 14 wherein the temperature sensing means comprises an elongate sensor extending along a substantial portion of the heating wire.

21. A wire or tube assembly as claimed in claim 20 wherein the sensing means comprises a resistive wire having a significant temperature coefficient of resistance.

22. A wire or tube assembly as claimed in claim 20 or 21 wherein the temperature sensing means is arranged to have a sensing zone that extends beyond the heating wire.