GB2029572A

GB2029572A - Determination of anaesthetic vapour in gas

Info

Publication number: GB2029572A
Application number: GB7928947A
Authority: GB
Original assignee: TENDERINI TOLLER A
Current assignee: TENDERINI TOLLER A
Priority date: 1978-08-29
Filing date: 1979-08-20
Publication date: 1980-03-19
Also published as: GB2029572B; IT1162176B; FR2435031B3; SE7907094L; FR2435031A1; DE2933224A1; IT7882542A0

Abstract

An anaesthetic mixture is produced by introducing volatile anaesthetic liquid into a gas flow and vaporizing the liquid on a vaporizing surface heated within the gas flow. The proportion of vapor in the gas is determined by measuring the temperature of the gas upstream and downstream of the vaporizing surface. A control valve for the anaesthetic liquid may be adjusted automatically in dependence on the measured temperature difference in order to bring the proportion of anaesthetic vapor in the gas to a predetermined value.

Description

SPECIFICATION A method of, and apparatus for, determining the proportion of anaesthetic vapor present in a gas The present invention relates to a method of, and apparatus for, determining the proportion of anaesthetic vapor present in a gas.

In current anaesthetic techniques the proportion of anaesthetic vapors present in the gases inhaled is assumed, at the place of use, to be dependent on the volumetric ratio between the mixture of gases containing no vapors and other gases saturated with vapors. In any case no account is taken of the temperature of the liquid which produces the anaesthetic vapors, and of the gases.

Away from the place of immediate use the proportion can be determined in previously prepared mixtures by analytic methods, such as refractometry, spectrophotometry, mass spectrometry, gas chromatography, and the variation of elastic characteristics of special gums immersed in the fluid.

At the present time the automatic regulation of proportions is not effected at the place of use because of the lack of suitable signals which are proportional to the variable to be controlled.

According to the invention, there is provided a method for determining the proportion of anaesthetic vapor present in a gas, comprisng the steps of providing a gas flow zone, feeding the gas through said zone, vaporizing a volatile anaesthetic liquid in said zone, and measuring the gas temperature upstream and downstream of the said zone, the difference between said temperatures being indicative of the proportion of anaesthetic vapor in the gas.

In a preferred embodiment, the anaesthetic liquid which is to be vaporized is caused to flow with a regulated rate of flow to a spongy or reticular body, or other body having a large surface area, which is bathed by the current of gas. The difference in temperature of the gas immediately upstream and downstream of the zone in which the vaporization of the liquid takes place, is determined while, at the same time, heat exchange with other bodies is minimized. If the liquid has been brought before hand to the temperature of the gas, the percentage of vapor in the gas is also proportional to the latent heat of vaporization of the liquid and to the tempera- ture of the gas.

The application of the method explained above permits the use of vaporizing apparatus for volatile anaesthetics equipped with means of indicating the percentage of anaesthetic.

Such apparatus may comprise three chambers which intercommunicate and which are thermally insulated, and through which the current of gas which is to be enriched with the anaesthetic vapors flows in successive stages. The anaesthetic liquid is introduced into an intermediate one of the chambers at a regulated rate of flow and is distributed over a vaporizing surface of a size such as to permit the complete vaporization of the liquid so as to avoid the production of liquid accumulations. In each of the other chambers is disposed a temperature sensor.

The ratio of the difference of the readings of the two sensors supplies either an indication of the percentage of vapor in the gas or, in the case of automatic regulation, a signal proportional to the variable to be controlled.

One suitable type of temperature sensor can operate on the basis of variation in electric resistance, or in a voltage threshold, or in the electromotive force produced by contact between conductors of different compositions, which occurs upon variations of temperature.

Another suitable type of temperature sensor can operate on the basis of the deformation of a structure composed of substances having different coefficients of thermal expansion, or the variation in pressure of a gas contained in a closed container having deformable portions. This type of sensor can supply through its movement an indication of the ratio between gases and vapors, or may also directly effect a controlled throttling of the flow of liquid into the vaporization zone to provide for automatic regulation.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawings, in which: Figure 1 is a schematic section of one embodiment of vaporizing apparatus in accordance with the invention; and Figures 2 and 3 are schematic sections, similar to Figure 1, showing modified embodiments of the apparatus.

With reference to Figure 1 the apparatus comprises a duct A into which is fed a volatile anaesthetic liquid and a duct B into which is fed a gas which may consist of a single gas or a gas mixture. The ducts A and B are separated by a wall I.

A porous surface D is disposed in the duct B, and the duct A communicates with the duct B via a passage in the vinicity of the porous surface D so that the liquid from the duct A bathes the surface D and is vaporized, and mixes with the gas in duct B. A common duct C serves to discharge the mixture of gas and vapor.

Temperature sensors E and F are positioned in the ducts B and C, respectively upstream and downstream of the surface D. The flow of anaesthetic liquid through the passage from the duct is control led by a valve H which is operated by an indicator and/or actuator G. The difference in the readings of the two temperature sensors will provide an indicator of the percentage of vapor present in the gas. For automatic regulation, the sensors can provide a signal which is compared with a reference signal representative of the required percentage and this is used to control the valve H to alter the liquid flow rate so that the required percentage is attained.

In operation, the readings of the two temperature sensors E and F, suitably processed electrically or electronically together with a predetermined electrical magnitude which is dependent on the latent heat of vaporization of the particular anaesthetic liquid that is to be vaporized, operate the indicator and/or actuator G which, with the aid of the valve H, effects the regulation of the rate of flow of the liquid flowing to the surface D. The wall I permits heat exchange between the incoming liquid and gas, while the other walls are poor conductors of heat and are optionally transparent for inspection purposes.

Figure 2 shows a mechanically operated system.

Reference letters A, B, C, D and I designated equivalent parts to those in Figure 1. E and F are two suitably profiled thermosensitive bimetal strips fastened to a block G, which is pivotal about a pin H; the thickened edge indicates the strip having the greater coefficient of the thermal expansion. The strip E is held supported on a movable manual regulation block M and its point of support is displaced along a profile L in order to compensate for variations in the proportion indication in dependence on the variation of temperature of the incoming gas and liquid.

The displacement of the strip F provides an indication which can be read directly, or through a mechanical linkage, or through partial interception or deviation of a beam of light. With the aid of a suitable transducer, this indication can control the throttling of the flow of volatile liquid or be compared with the reference magnitude in the case of automatic regulation.

For the sake of clarity in the drawings, in this and the following Figure an elastic element which ensures contact between members E and M has not been shown.

Figure 3 shows a possible automatic regulation device in which the reference magnitude consists of the position of the movable element M, which constitutes the zeroising means. In this Figure, letters common to it and to Figure 2 have the same meanings. The body N is the movable element of a spindle valve which regulates the flow of liquid into a chamber containing the surface D. The operation of the element M makes it possible also to prevent, in certain circumstances, the passage of the anaesthetic liquid.

Claims

1. A method for determining the proportion of anaesthetic vapor present in a gas, comprising the steps of providing a gas flow zone, feeding the gas through said zone, vaporizing a volatile anaesthetic liquid in said zone, and measuring the gas temperature upstream and downstream of the said zone, the difference between said temperatures being indicative of the proportion of anaesthetic vapor in the gas.

2. A method according to claim 1, comprising the step of making the temperature of the incoming liquid equal to the temperature of the incoming gas.

3. A method according to claim 1 or claim 2, further comprising the steps of providing an electric- al signal representative of the difference between the two temperatures, providing a reference signal representative of the required proportion of anaesthetic vapor, and comparing said difference signal with said reference signal.

4. A method according to claim 3, further comprising the step of using the difference signal to modify the rate of flow of the liquid.

5. Apparatus for determining the proportion of anaesthetic vapor present in a gas, comprising a duct for the passage of the gas, a body having a large vaporization surface bathed by the flow of gas in the said duct, means for directing anaesthetic liquid onto said surface to vaporize said liquid, and first and second temperature sensing means disposed in the duct upstream and downstream of the said body for determining the temperature of the incoming gas and that of the latter mixed with the vapor of the liquid, the difference between said temperature being indicative of the proportion of anaesthetic vapor.

6. Apparatus according to claim 5, further comprising means for providing heat exchange between the incoming gas and the incoming liquid in order to equalize their temperature.

7. Apparatus according to claim 5 or claim 6, wherein each said temperature sensing means comprises a bimetal strip, said apparatus further comprising an angularly movable member mounting both said strips, and manual control means for adjusting the position of one of said strips, said control means including a contact profile contacted by said one strip at a point of contact which is displaced through the deformation of the said strip and through the effect of the manual control.

8. Apparatus according to claim 7, further comprising valve means for controlling the flow of liquid, said valve means being controlled by the said other strip.

9. Apparatus according to claim 5, further comprising means operative to generate a reference signal representative of an electrical signal to the required proportion of vapor, and means operative to generate a signal representative of the difference in said temperatures and to compare same with said reference signal.

10. A method for determining the proportion of anaesthetic vapor present in a gas substantially as hereinbefore described with reference to the accompanying drawing.

11. An apparatus for determining the proportion of anaesthetic vapor present in a gas substantially as herein before described with reference to the accompanying drawing.