WO2005021076A2

WO2005021076A2 - Heater for ventilator conduit

Info

Publication number: WO2005021076A2
Application number: PCT/GB2004/003688
Authority: WO
Inventors: Stuart Corner; Paul Berwick
Original assignee: E.M.E. (Electro Medical Equipment) Limited
Priority date: 2003-08-28
Filing date: 2004-08-27
Publication date: 2005-03-10
Also published as: GB0320194D0; WO2005021076A3

Abstract

A heating wire assembly (16) for reducing condensation in a breathing tube (18) delivering humidified gas to a patient for breathing comprises an electrically resistive heating wire (20), a plurality of thermally responsive sensors (4,10) at discrete locations along the heating wire (20), and at least one signal wire (6,8,12) connected to each of the sensors (4,10). The heating wire (20) and the signal wires (6,8,12) are terminated for connection thereto at a common end of the assembly and are bound together to facilitate insertion of the assembly into a breathing tube (18). A method of assembling a wire (20) or wire assembly (16) into a flexible ventilator tube (18) comprises cooling the wire (20) to a predetermined temperature in order to reduce its flexibility and inserting the wire (20) axially into the flexible tube (18).

Description

Heater for Ventilator Conduit 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 that when moist, possibly warmed air is delivered to a patient via a plastics conduit i.e. a breathing tube 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 or 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. 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 therefore an object of the present invention at least partially to mitigate 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; a plurality of thermally responsive sensors at discrete locations along the heating wire assembly; and at least one signal wire connected to each of said sensors, wherein said heating wire and said signal wires are terminated for connection thereto at a common end of the assembly and are bound together to facilitate insertion of the assembly into a breathing tube. Thus it will be seen by those skilled in the art that in accordance with the present invention a self- contained heating wire assembly with thermal sensors is provided that may be simply inserted into one end of a breathing to tube to extend along at least part of the length of the tube . It will be appreciated therefore that there is no need for the sensors to be inserted into apertures thereby avoiding the problems with incorrect insertion outlined above. Furthermore, the 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 and sensors 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. Any thermally responsive sensor giving an electrical signal may be used, but preferably the sensors comprise thermistors since these are relatively inexpensive and give an easily measured electrical response by their change in resistance. In preferred embodiments, sensors are provided at two locations along the wire assembly, 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. The heating and sensor wires may be bound together by any suitable means. For example, they may simply be inter-engaged with one another - e.g. by twisting. Alternatively, they may be held together by a sleeve, tape, clips or the like. Since insertion of the heating wire assembly into the breathing tube may be carried out at the manufacturing stage, this may be optimised for maximum efficiency. Indeed, the Applicants have further devised a novel and inventive method for assembling a wire into a tube which is particularly beneficial in accordance with the first aspect of the invention. Thus when viewed from a second aspect, the invention provides a method of assembling a wire or wire assembly into a flexible ventilator tube comprising cooling said wire to a predetermined temperature in order to reduce its flexibility and inserting said wire axially into said flexible tube. As mentioned above, although this novel procedure is beneficial in the insertion of wires generally into a flexible breathing tube, it is particularly beneficial when used with a heating wire assembly in accordance with the first aspect of the invention. The appropriate degree of cooling will depend upon the materials present in the wire or wire assembly, particularly the insulation on the individual wire or wires. In one exemplary embodiment, the wire comprises polypropylene insulation and is cooled to a temperature of approximately -20°C A preferred embodiment 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 schematic view of a sensor sub-assembly for use in a heating wire assembly in accordance with the invention; Fig. 2 is a schematic circuit diagram showing how the two thermistors are connected; and Fig. 3 is a schematic view of the heating wire assembly after insertion into a breathing tube. Fig. 1 shows a thermal sensor sub-assembly 2. At the distal end is a standard bead thermistor 4. Electrical connection to the thermistor 4 is made by a pair of insulated low voltage electrical wires comprising feed and return signal wires 6,8 respectively. The signal wires 6,8 run the length of the sensor sub-assembly 2. Towards the proximal end of the assembly is a further bead thermistor 10. One side of this second thermistor 10 is connected electrically to the same signal feed wire 6 as the first thermistor 4. The other side is connected to a third signal wire 12. The electrical connections are shown schematically in Fig. 2. All three signal wires are terminated in a common three-pin connector 14 for connection to an existing temperature monitoring and power supply unit . Fig. 3 shows the final heater wire assembly 16 in a breathing tube 18. To form the wire assembly 16, the sensor sub-assembly 2 of Fig. 1 is bound to a heating wire in the form of a single loop of insulated nickel- chromium resistance wire 20 which is terminated in a known two-pin electrical connector 22. The heating wire 20 is bound at its distal end to the signal wires 6,8 of the sensor sub-assembly by means of a clip 24. Further clips may be provided along the length of the heater wire 20 as necessary. Additionally or alternatively, the heater wires 20 may be twisted together with the signal wires 6,8,12 to bind them together. The breathing tube has standard Luer connectors 25 at either end. At the distal end the tube 18 is connected in use to a face mask or the like (not shown) for supplying the gas for a patient to breath. At the proximal end the tube 18 is connected to a connector piece 26 which has an inlet port 26a for gas from a humidified gas source and two airtight grommets 28 through which the wires 6,8,12,20 pass to permit external access to the two electrical connectors 14,22. It will be observed that the sensor sub-assembly is longer than the heating wire 20 so that the distal thermistor 4 is spaced from the looped end of the heating wire 20. This means that it will sense accurately the temperature of the gas being delivered to the patient without being directly influenced by the temperature of the heating wire 20. The proximal thermistor 10 on the other end will be influenced by the temperature of the heater which provides for a safety backup that prevents the heater wire 20 becoming too hot and potentially melting the breathing tube in the event of a malfunction. Together the temperatures sensed at the two thermistors 4,10 may be used to regulate the electrical power to the heating wire to provide reliable rainout prevention whilst ensuring that gas is delivered to the patient at a constant comfortable temperature. In order to manufacture the arrangement shown in Fig. 3, the heating wire 20 and sensor sub-assembly 2 are fabricated separately including the grommets 28 which are moulded onto the respective wires. The heating wire 20 and sensor sub-assembly 2 are then bound together by one or more clips 24 and/or twisted together. The resultant wire assembly is then cooled to -20^CC so that it becomes stiff. The assembly may then easily be inserted into the tube 18. Thereafter the grommets are pushed into apertures in the connector piece 26. Of course a sterile environment must be maintained during manufacture until the finished assembly has been packaged. The whole of the arrangement shown in Fig. 3 may be made sufficiently inexpensively as to be disposable after a single use, thereby maximising hygiene. In use in a hospital or clinic, the tube 18 simply needs to be connected into the breathing circuit by connecting it to a gas source and face mask as is well known in the art. The two connectors 14,22 are then plugged into sockets on a corresponding power supply unit and the breathing circuit may be safely operated. Set up is therefore straightforward and intuitive. Once the breathing tube 18 has come to the end of a particular use, it and the electrical connectors 14,22 are simply disconnected and discarded. A new, sterile breathing tube, heating wire and sensor assembly may therefore be used for each patient, reducing the danger of infection. It will be appreciated by those skilled in the art that the embodiments described above are only specific examples of how the principles of the invention may be implemented and there are many possible variants within the scope of the invention as set forth in the accompanying 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; a plurality of thermally responsive sensors at discrete locations along the heating wire assembly; and at least one signal wire connected to each of said sensors, wherein said heating wire and said signal wires are terminated for connection thereto at a common end of the assembly and are bound together to facilitate insertion of the assembly into a breathing tube.

2. A heating wire assembly as claimed in claim 1 wherein at least one sensor is provided at each of two locations along said heating wire assembly.

3. A heating wire assembly as claimed in claim 2 wherein said two locations are approximately at the proximal and distal ends of said heating wire assembly.

4. A heating wire assembly as claimed in any preceding claim wherein one of the sensors is spaced longitudinally from the end of the heating wire.

5. A heating wire assembly as claimed in any preceding claim wherein a pair of sensors is provided at the proximal end of said heating wire assembly, one of said pair of sensors being in closer thermal contact with the heating wire than the other sensor.

6. A heating wire assembly as claimed in any preceding claim wherein said sensors comprise thermistors.

7. A method of assembling a wire or wire assembly into a flexible ventilator tube comprising cooling said wire to a predetermined temperature in order to reduce its flexibility and inserting said wire axially into said flexible tube.

8. A method as claimed in claim 7 wherein said wire assembly is as claimed in any of claims 1 to 6.

9. The method as claimed in claim 7 wherein the wire or wire assembly comprises polypropylene insulation and is cooled to a temperature of approximately -20°C.