GB2312736A - A warming system to protect body tissue from unintended cooling - Google Patents

Abstract

Fluid (12) contained in a vessel (10) is heated by means of infra-red light sources (20 and 22). The infra-red light passes through the wall of the vessel (10) and is absorbed directly by the fluid. A control circuit (34) controls the operation of the light sources (20 and 22) to maintain the fluid at a desired temperature, or within a desired temperature window. A probe (24) carries at least one temperature sensor (26 and 28). The sensor measures the temperature of the fluid, and the behaviour of the sensor output also indicates whether the fluid level has fallen below the level of the sensor (28), whereupon an alarm signal is produced by the control circuit (34). The system is used in a warming system to protect healthy body tissue from unintended freezing during cryosurgery.

Description

IMPROVEMENTS RELATING TO THE SUPPLY OF HEATED FLUID FOR MEDICAL USE This invention relates to the supply of heated fluid for medical use.

The invention is particularly suitable for a warming system to protect sensitive and/or healthy body tissue from unintended freezing during cryosurgery. Such a warming system is described, for example, in our published International patent application WO-A-95/29643. However, the present invention is not limited exclusively to such use.

A common technique for heating the fluid is to stand a vessel containing the fluid on an electrically heated hot plate.

The fluid temperature is monitored, and the supply of electrical power to the hot plate is controlled in dependence on the difference between the measured temperature, and a desired temperature.

However, it is difficult to maintain the fluid at a constant predetermined temperature. On heating, the thermal mass of the hot plate slows the temperature rise of the fluid, because the hot plate has to be heated as well as the fluid. Once the desired temperature has been reached, and the power supply turned off, the heat in the hot plate continues to radiate into the fluid, possibly causing the temperature to overshoot above that desired.

In a warmer system for use in cryosurgery, the temperature of the fluid is critical. If the temperature is too high, the healthy tissue may suffer severe and irreparable damage.

Although of less of a problem, if the temperature is too low, the efficiency of the warming is significantly reduced; in extreme cases the protection may be insufficient to prevent unintended freezing of all of the healthy tissue. Generally, the temperature needs to be maintained within the range 44-45 "C. The preferred constant temperature is 44.5 "C. However, in view of the risk of the temperature overshooting and causing heat damage, the warming system often has to be run at several degrees below the desired optimum temperature, merely as a safety precaution.

A further disadvantage is that the hot plate only heats the fluid at the bottom of the vessel. In order to obtain a uniform temperature distribution it is usually necessary to stir or agitate the fluid. Since the fluid must remain sterile, a relatively expensive non-contact stirrer, such as a magnetically movable stirrer, has to be used.

Other difficulties exist in detecting loss of fluid. This can occur if the surgeon has accidentally punctured the warming device (typically a bladder) during insertion into a body cavity, causing the fluid to leak away. It is important that a warning be given of any significant loss of fluid.

The present invention has been devised bearing the above problems in mind.

In contrast to the prior art technique of heating fluid by a hot plate, one aspect of the present invention is to heat fluid by using a light source which illuminates the fluid with radiation at least partly in the infra-red range.

Such an arrangement allows the fluid to be heated directly by absorption of the infra-red light. A primary advantage is that there is no hot plate thermal mass which affects the controllability of the heating. Heating begins almost immediately when the light source is switched on, and terminates almost immediately if the light source is switched off.

Moreover, the rate of supply of heat to the liquid is corresponds directly with the amount of electrical power supplied to the light source. It is therefore possible to control the temperature extremely accurately, and to maintain an optimum desired temperature with little, if any, temperature fluctuation.

In other words, the problem in the prior art of overshooting a maximum desirable temperature can be completely avoided.

Preferably, a control arrangement monitors the temperature of the fluid, and controls the light source in response to the measured temperature. Preferably, the control arrangement is operable to maintain the fluid at a desired temperature, or within a desired temperature window.

The fluid can be contained in a vessel which is transparent to infra-red light. For example, the vessel can be made of transparent thermoplastics material, or of glass.

The direct absorption of the infra-red light can achieve greater uniformity of heating in the fluid, without the prior art need for stirring (although a stirring arrangement can still be retained if desired). Preferably, the infra-red light is incident on the fluid from more than one direction. For example, in the preferred embodiment, two light sources are used which face each other on opposite sides of the vessel. This can achieve extremely uniform heating.

Preferably, the or each light source is in the form of a light bulb. Types of bulb are available which emit light predominantly in the infra-red range, but other types of bulb may be used as desired. Spot-light bulbs are available with front face diameters of about 10-15 cm (around 4 to 6 inches), which provide a wide beam of good directionality.

Another aspect of the invention relates to detecting the level of fluid without requiring a dedicated fluid level sensor.

In this aspect of the invention, a temperature sensor measures the temperature of the fluid, and means are provided for monitoring the output of the sensor to detect a thermal characteristic indicative of the fluid level having fallen below the level of the temperature sensor, and no longer being in working contact with the sensor.

This aspect of the invention is particularly suitable for use with the first aspect described above, because the temperature sensor produces a markedly different output signal when the fluid level drops below the sensor level, and the sensor becomes illuminated directly by the infra-red light.

However, the second aspect may be used with other forms of heater, provided that it is possible to detect a behaviour abnormality which is characteristic of the sensor being no longer in contact with, or immersed in, the fluid.

In the preferred embodiment, two temperature sensors are used, one arranged above the other. The lower temperature sensor provides a direct indication of the fluid temperature, and the upper temperature sensor is used to detect a fall in the fluid level. When the fluid level is high, the outputs of the temperature sensors are virtually identical. However, if the fluid level drops below the level of the upper sensor, the output from the upper sensor becomes abnormal and can easily be detected.

Embodiments of the invention are now described by way of example only, with reference to the accompanying drawings, in which: Fig. 1 is a schematic view of a fluid heater; Fig. 2 is a graph illustrating the output signals from the thermo-couples used in Fig. 1; Fig. 3 is a schematic view of a warming system for use with cryosurgical apparatus; and Fig. 4 is a schematic plan view of an integrated unit for use in the warming system of Fig. 3.

Referring to Fig. 1, a vessel 10 contains fluid 12 (liquid in this embodiment) which is heated above ambient room temperature, for medical use. This embodiment is intended for use as part of a body tissue warming system (illustrated in Fig.

3), and so the vessel is sealed in sterile conditions by a plug 14. The warming system operates by circulating the liquid in a closed circuit coupled at one end to an outlet or supply conduit 16, and at the other end to an inlet or return conduit 18. The supply conduit extends below the surface of the liquid, and the return conduit terminates above the surface of the liquid.

The liquid is heated by first and second infra-red light bulbs 20 and 22 arranged facing each other on opposite sides of the vessel 10. The infra-red light passes through the wall of the vessel 10 and is absorbed by the liquid, thereby producing the desired heating effect. Such an arrangement heats the liquid more uniformly than in the prior art, because the infra-red light is absorbed progressively as it passes into the liquid.

Furthermore, the arrangement of the bulbs 20 and 22 on opposite sides of the vessel 10 enables a large volume of liquid to be illuminated, further improving the uniformity of the heating in the centre of the vessel as well as in the outer regions adjacent to the bulbs 20 and 22.

The use of infra-red bulbs instead of a hot plate greatly improves the controllability of the heating process. There is no lag in the heating operation, because the liquid is heated directly by absorption of the infra-red light, and not by heat transfer from a body of large thermal mass (i.e. large thermal inertia).

In this embodiment, spot-light bulbs are used as they provide broad beams of light, and have good directionality so that virtually all of the light is incident on the vessel 10.

The vessel 10 is made of thermo-plastics material which is transparent to infra-red light. The liquid may be any suitable liquid. However, in the case of a body tissue warming system, since the liquid is pumped (within closed tubing) into a person's body, it is preferred that a bio-compatible liquid be used in case of internal leakage. For optimum heating efficiency, the liquid should also be a good absorber of infra-red light. Water is an eminently suitable liquid, and isotonic saline is a preferred liquid for use in the body.

A central probe 24 extends downwardly into the vessel from the plug 14. The probe houses one or more temperature sensors; in this embodiment two sensors are used, a first sensor 26 being positioned near the lower end of the probe 24, and a second sensor 28 being positioned about mid-way along the length of the probe. The outputs from the sensors are fed along respective signal lines 30 and 32 to an electronic control circuit 34.

A first function of the temperature sensors 26 and 28 is to provide a direct temperature measurement of the liquid in the vessel. In this embodiment, only the lower temperature sensor 26 is used for this purpose, although the signals from the two sensors could be combined (e.g. averaged) if desired. The circuit 34 is coupled to the bulbs 20 and 22 by respective driver lines 36 and 38, and operates to control the bulbs to maintain the liquid at a desired predetermined temperature. A control input is illustrated schematically at 40 by which a operator can input information to the control circuit 34 to set the desired temperature.

In this embodiment, the control circuit simply turns the bulbs on and off at appropriate times to maintain the desired temperature. However, in other embodiments, the control circuit may control variably the intensity of the emitted light by regulating the amount of power supplied to each bulb.

A second function of the temperature sensors 26 and 28 is to provide signals which can be analyzed to detect whether the level of the liquid has fallen below the position of the upper sensor 28. Fig. 2 illustrates typical output signals from the temperature sensors. The upper curve 44 represents the output from the upper sensor 28, and the lower curve 46 represents the output from the lower sensor 26.

When the level of the liquid is above the upper sensor 28, both sensors are, of course, immersed in the liquid, and they produce almost identical output signals. These are illustrated on the left of the graph in part (a). The temperature fluctuates very slightly as the bulbs are turned on and off (the amount of fluctuation is exaggerated in the drawing so as to be visible).

If the level of the liquid in the vessel 10 falls below the upper sensor 28, a markedly different signal is produced by that sensor, as illustrated in part (b) of the graph. During infrared illumination, the output signal rises significantly because there is no liquid to absorb the infra-red light before the light reaches the temperature sensor. Similarly, during a dark period or cycle, the output signal drops significantly because there is no surrounding liquid to keep the temperature sensor 28 warm.

The control circuit 34 is configured to detect the occurrence of the part (b) output waveform from the upper sensor 28, and to produce an output 48 to indicate that the liquid level has fallen below acceptable limits. For example, the output 48 could be used to sound an alarm, or to shut down operation of the fluid heater automatically.

It will be appreciated that the control circuit 34 could generate the output 48 simply as a result of comparing the output signals from the temperature sensors 26 and 28. Any significant difference can be taken as being indicative of a low liquid level. Alternatively, the control circuit 34 could be arranged to sense the magnitude of fluctuations in the output signal from the upper sensor 28 without reference to the lower sensor 26.

It will also be appreciated that, if desired, the lower temperature sensor could be omitted, leaving the upper temperature sensor 28 to provide a signal indicative of liquid temperature while immersed in liquid, and a fluctuating signal if the liquid level falls. Alternatively, the lower temperature sensor 26 could be retained as the only sensor, and other means used to monitor liquid level. For example, an optical liquid level monitor may be suitable, either using the infra-red light as the level sensing optical source, or using a separate light source.

Referring to Fig. 3, the warming system is very similar in principle to that described in the earlier referenced WO-A95/29643. Indeed, the urethral and rectal inflatable bladders described in WO-A-95/29643 are suitable for use as the bladder denoted generally at 50 in Fig. 3. The bladder 50 is coupled to the supply conduit 16 of the vessel 10 by means of first tubing 52, and to the return conduit 18 by second tubing 54, to form the closed circuit.

In addition to, or as an alternative to, the temperature sensor(s) in the vessel 10, a temperature sensor 56 may be employed to measure the liquid temperature at some point in the circuit outside the vessel 10. In Fig. 3, the sensor is illustrated in the bladder 50 to measure accurately the liquid temperature in the bladder 50. The temperature sensor is coupled as an input to the control circuit 34 by a line 58.

The liquid is pumped around the circuit by a peristaltic pump 60 which can either be upstream of the bladder 50 (as illustrated in Fig. 3), or downstream. The pump can be controlled either manually, or be coupled to the control circuit 34. The latter enables the pump rate to be controlled by the circuit 34, thus providing a further controllable variable for regulating the rate of supply of heat to the bladder 50.

Fig. 4 illustrates an integrated unit suitable for use in the warming system. A housing 62 contains the infra-red bulbs 20 and 22 which are positioned on either side of an open well 64 (also shown schematically in Fig. 1). The well 64 serves as a chamber for receiving the vessel 10. The top of the housing 62 is formed with an opening 63 through which the vessel 10 can be placed in the well, and allowing access for flying connections (for example, the tubing 52 and 54, and the signal lines 30 and 32). Apertures 66 formed in the walls of the well 64 adjacent to the bulbs 20 and 22 permit the infra-red light to illuminate the vessel 10 when positioned in the well 64. The control circuit 34 is divided into three user controllable modules 34a-34c positioned behind the front panel 67. The first control module 34a controls operation of the bulbs 20 and 22 to maintain a desired temperature. The second control module 34b detects a low liquid level in the vessel (as described above). The third control module 34c regulates the pump speed. A pump motor 68 is fitted in the housing 62, and drives a removable rotary peristaltic pump head 70 through an aperture 72. The first and second control modules 34a and 34b may conveniently comprise Cal Controls digital controllers type 3200 (RS model 730-004).

Although the fluid is liquid in the above embodiment, the invention can also be used to heat gas by infra-red absorption.

Although the heater has been described above in relation to a warming system for use in conjunction with cryosurgical apparatus, the skilled man will appreciate that the invention has many other applications in the medical field.

It will be appreciated that many modifications may be made within the scope and/or principles of the invention.

Whilst endeavouring in the foregoing description to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.

Claims (16)

1. Apparatus for heating a fluid for medical use, the apparatus comprising a light source for illuminating the fluid with light at least partly within the infra-red range, a temperature sensor for measuring the temperature of the fluid, and control means for controlling the light source in response to the measured temperature.

2. Apparatus for heating a fluid for medical use, comprising an electrically controllable infra-red light source, the fluid being heated by direct absorption of infra-red light emitted by the source.

3. Apparatus for circulating a supply of fluid through a warming device insertable into a body cavity, the apparatus comprising a vessel containing fluid to be heated, conduits for coupling the vessel to the warming device to form a closed circuit loop, pump means for circulating the fluid in the circuit, and a light source for illuminating the fluid in the vessel with light at least partly in the infra-red range to heat the fluid.

4. Apparatus according to claim 2 or 3, further comprising control means for controlling the light source to control the temperature to which the fluid is heated.

5. Apparatus according to claim 4, further comprising a temperature sensor for measuring the temperature of the fluid and for providing a measurement signal to the control means.

6. Apparatus according to claim 1, 4 or 5, wherein the control means is operable to maintain the fluid at desired temperature, or within a desired temperature window.

7. Apparatus according to any preceding claim wherein the light source comprises light source means for illuminating the fluid from at least two directions.

8. Apparatus according to claim 7, wherein the light source means illuminates the fluid from two opposite sides.

9. Apparatus according to any preceding claim, wherein the light source comprises one or more light bulbs.

10. Apparatus according to claim 9, comprising one or more infra-red type light bulbs.

11. Apparatus according to any preceding claim further comprising means for detecting a low-fluid-level condition.

12. Apparatus for heating fluid for medical use and comprising a fluid level detector, the fluid level detector comprising a temperature sensor located at a predetermined fluid level, and means responsive to the output of the temperature sensor for detecting a characteristic in the sensor output indicative of the fluid having fallen below the level of the temperature sensor, and no longer being in working contact with the temperature sensor.

13. Apparatus according to claim 12, further comprising a second temperature sensor mounted below the level of the first mentioned temperature sensor.

14. A method of controllably heating a fluid for medical use, the method comprising illuminating the fluid controllably with light at least partly in the infra-red frequency range.

15. Use of an electrically controllable infra-red light source to heat fluid for medical use.

16. A method or apparatus substantially as hereinbefore described with reference to any of the accompanying drawings.