GB2187845A

GB2187845A - Measuring sensory responses

Info

Publication number: GB2187845A
Application number: GB08629651A
Authority: GB
Inventors: Gerd Kobal; Christoph Maurer
Original assignee: Individual
Current assignee: Individual
Priority date: 1986-03-14
Filing date: 1986-12-11
Publication date: 1987-09-16
Anticipated expiration: 2006-12-11
Also published as: GB8629651D0; DE3608475A1; GB2187845B; FR2599977A1

Abstract

For measuring sensory qualities of responses, for example for measuring analgesia in the monitoring of anaesthesia, a gas vapour which triggers a specific stimulus is passed over the nasal mucosa in the form of a series of pulses. and stimulus-correlated electrical signals are derived from an electroencephalogram (EEG) with the aid of a recording apparatus. These signals represent the sensory quality or response. The pulses of stimulating gas are supplied in a manner such that their shape can be altered, for example as shown, and the alteration of this pulse shape enables the EEG-derived signals which serve as the basis of the measurement to be adjusted to optimum values. CO2 and air are supplied to a patient by means of two quick-acting ball valves which are controlled to keep the total flow of gas constant while varying the concentration of CO2. <IMAGE>

Description

SPECIFICATION Method and apparatus for measuring sensory qualities or responses The invention relates to a method for measuring sensory qualities or responses, for example for measuring analgesia in the monitoring of anaesthesia, in which method a substance which triggers a specific stimulus is passed over the nasal mucosa in the form of series of pulses. The substance is introduced as gas or vapour, as the particular case requires, and stimulus-correlated electrical signals are derived from an electroencephalogram (EEG) with the aid of a recording apparatus.

These signals are measured as unknown quantities representing the sensory quality or response. The invention also relates to an apparatus suitable for carrying out the method.

G. Kobal and K. H. Plattig have published a paper (Zeitschrift für Elektro-Encephalographie, Elektro-Myographie und verwandte Gebiete, 9 (1978), 135-145), where they disclose experiments in which it proved possible to check qualities or responses relevant to the sense of smell, in that a stimulus which could be identified in the EEG was triggered by a substance supplied via a nasal probe.

A method has been proposed in which a continuous series of pulses of a stimulating gas is produced by means of a change-over device. At the same time, evaluation in an EEG is effected by means of an evaluating unit, within an evaluation window possessing a width corresponding to a predetermined number of stimulation pulses, this window being displaceable across a number of stored signals.If constant characteristics, particularly with regard to flow rate, temperature and humidity, are exhibited by the stream of gas or vapour which is supplied to the nasal mucosa, and into which the series of pulses of stimulating gas are introduced, it can be assumed that the stimulus-correlated electrical signals, occurring in the EEG, derive exclusively from the response to the pulses of stimulating gas which have been incorporated into the stream. CO, is one of the gases which can be used as the stimulating gas.

However, in the measurement of sensory qualities or responses it has been found that the shape and duration of the pulses are significant influencing variables. In such measurements, or rather in the measurement of pain sensations, the accuracy of the measurements, and hence their usability or validity, also depends on whether the stimuli which are being administered exhibit a pulse shape that matches the natural conditions under which stimuli act.

According to one aspect of the present invention there is provided a method for measuring sensory qualities or responses, for example for measuring analgesia in the monitoring of anaesthesia, in which a substance which triggers a specific stimulus is passed over the nasal mucosa in the form of a series of pulses, this substance being introduced as gas or vapour as the particular case requires, and stimulus-correlated electrical signals are derived from an electroencephalogram (EEG) with the aid of a recording apparatus and are measured as quantities representing the sensory quality or response, the pulses of stimulating gas or vapour being supplied in a manner such that their shape can be altered so as to enable the EEG-derived signals to be adjusted to optimum values which serve as the basis of the measurements.

The invention may provide a method in which the differentiability of the EEG signals and the detection of change trends are improved.

The alteration of the pulse shape may be concerned particularly with the curve shape, comprising steepness, pulse repetition rate, amplitude and pulse duration. At the same time, it appears to be advantageous to select values which approximate to the natural physiological conditions under which stimuli act. It is thus advantageous if the initial rise rate can exceed 5 %/ms, referred to the maximum amplitude value, and if the pulse repetition rate can be between 1 and 0.01 Hz, in association with a pulse duration in the range between 50 ms and 5 seconds.

In a development of the invention, arrangements can be provided for storing the EEG signals and for evaluating them in order to measure the unknown quantities. This evaluation can be effected with the aid of an electronic evaluation window possessing a width which corresponds to a predetermined number of stimulating pulses, this window being displaceable across the number of stored signals.

The use of an evaluation window facilitates early detection of trends involving a change in the magnitude of the EEG signal representing the quantity which is to be measured, detection of such trends being possible after even only a few stimulus signals.

Window comparators of the abovementioned type can be designed in any of several known ways. General design principles are contained in Section 9.6.3 of the book "Operational amplifiers, Design and Applications", by Jerald G. Graeme, Gene E. Tobey and Lawrence P. Huelsman PhD, published in 1971 by the McGraw-Hill Book Company (ISBN 07 064917-0).

If required, a measuring process of the above mentioned type can be exploited as a method for controlling the supply of an anaesthetic gas as a function of measured sensory variables or responses. This can be achieved by administering a painful nasal stimulus at regular intervals by means of pulses of a stimulating gas or vapour as appropriate, and by using the response signals from the EEG as quantities for controlling the supply of anaes thetic gas.

In cases where series of pulses of a stimu lating gas are introduced into a stream of neu tral gas, an effective measuring process can be implemented by arranging for the pulse shape to be such that a steep initial rise in the concentration of the stimulating substance at a rate exceeding 5 %/ms, precedes the maximum value which corresponds to 100% stimulating gas and is maintained over a hold ing period not exceeding 30% of the pulse duration. This holding period is then followed by a period of falling concentration values, lasting to the end of the pulse. Furthermore, it can be expedient if the pulses of stimulating gas are superimposed on a constant base quantity of stimulating gas, while the pulses can themselves have various shapes, including even sinusoidal ones.

According to a second aspect of the invention there is provided apparatus suitable for carrying out the method according to the first aspect of the invention, comprising a twinvalve assembly connected to a control unit operable to control the desired timebased pulse pattern, one of the two valves being connected to a source of neutral gas and the other being connected to a source of stimulating gas, and the outlets of the two valves being connected to a nasal supply line via a common gas discharge connection.

The two valves are expediently designed as fast-responding electrically actuated ball valves that can be controlled by the control unit, on which it is possible to set the parameters defining the pulse shape, and hence the pulse shape itself. Examples of this type of electrically controllable control valve have been disclosed in German Patent Document No.28 31 856 B2.

For a better understanding of the invention and to show how it may be put into effect, diagrammatic representations of illustrative embodiments of the subject matter of the invention are presented in the drawings in which: Figure 1 shows one shape that may be exhibited by the pulses of stimulating gas; Figure 2 shows an alternative shape that may be exhibited by the pulses of stimulating gas; and Figure 3 shows a twin-valve assembly for producing series of pulses exhibiting shapes of the kinds shown in Figures 1 and 2.

Figure 1 shows one shape that may be exhibited by the pulses of stimulating gas (e.g.

CO2), series of which are introduced into a stream of neutral gas (e.g. air). This pulse shape comprises a steep leading edge, followed by a comparatively long period over which the concentration of the stimulating gas is maintained at 100%, after which it falls to an intermediate plateau at 40%. A further fall in the concentration of the stimulating gas fol lows the intermediate plateau, and terminates at 0%. The 100% total gas flow value has to be maintained at all intermediate concentrations of stimulating gas, and this is achieved by topping-up with the neutral gas.

Figure 2 shows a pulse shape which is based on a constant base quantity of stimulating gas, namely 20%, the stimulating gas in this case being 002. This means that at no time during the investigation is the person under test supplied with pure neutral gas, but rather that the proportion of the component represented by the stimulating gas is merely reduced to 20% over the periods between the pulses. A pulse shape of this kind, or rather a series of pulses exhibiting such a shape, appears to be particularly suitable for the investigation of pain sensations which extend over comparatively long periods of time.This is particularly advantageous for monitoring the depth of anaesthesia if the control or, as the case may be, the monitoring of the supply of anaesthetic gas is made dependent on the response signals which are derived from the EEG as measured quantities, or as control quantities, as the case may he, these signals occurring in response to a powerful nasal stimulus which is administered at regular intervals.

The apparatus shown in Figure 3 incorporates a twin-valve assembly and is capable of being used for producing various pulse shapes, particularly those shown in Figures 1 and 2. However, the apparatus can also be used quite generally wherever it is intended, in connection with the measurement of sensory qualities or responses, to introduce a stimulating gas in the form of a series of pulses into a stream of neutral gas, with or without a base quantity of the stimulating gas.

The twin-valve assembly comprises two fast responding electrically actuated ball valves 1,2 which, as elements of a control circuit, can be triggered by a control unit 3 serving as the means for setting the pulse shape. Setting of the desired time-based pulse pattern, namely the setting of the pulse shape and repetition rate, is effected at the control unit 3 by fixing the electrical control values for-the electrically actuated ball valves 1,2.

Pressure sensors 4,5 are located on the inlet pressure sides of the electrically actuated ball valves 1,2, the sensor 4 measuring the pressure at which a stimulating gas is supplied from a source 6, and the sensor 5 measuring the pressure at which a neutral gas is supplied from a source 7, the sensors 4,5 then transmitting the measured pressure values to the control unit 3. The pressure values required in the delivery spaces 8,9 in order to produce the desired time-based pulse pattern are fixed by appropriately controlling the electrically actuated ball valves 1,2 in accordance with the measured pressure values P1 and P2.

The outlets of the two electrically actuated ball valves 1,2 are led to a common gas discharge connection 10 which can be connected to a nasal supply line 11 inserted into the nose of a person 12 who is to be tested.

The EEG signals are detected by a recording apparatus 13 which, if appropriate, serves as an evaluating unit housing the appropriate circuit components for storing and evaluating the signals. The latter operation is effected within an electronic evaluation window possessing a width which corresponds to a predetermined number of stimulating pulses, this window being shiftable across the number of stored signals.

The evaluating unit associated with the recording apparatus 13 outputs values which represent measurements of the sensation intensity, and which can be used for controlling the supply of an anaesthetic gas to the person 12.

Claims

1. A method for measuring sensory qualities or responses, for example for measuring analgesia in the monitoring of anaesthesia, in which a substance which triggers a specific stimulus is passed over the nasal mucosa in the form of a series of pulses, this substance being introduced as gas or vapour as the particular case requires, and stimulus-correlated electrical signals are derived from an electroencephalogram (EEG) with the aid of a recording apparatus and are measured as quantities representing the sensory quality or response, the pulses of stimulating gas or vapour being supplied in a manner such that their shape can be altered so as to enable the EEG-derived signals to be adjusted to optimum values which serve as the basis of the measurements.

2. A method according to claim 1, wherein the EEG signals are stored and are evaluated, in order to measure said quantities, with the aid of an electronic evaluation window possessing a width which corresponds to a predetermined number of pulses of stimulating gas, this window being shiftable across the number of stored signals.

3. A method according to claim 1 or 2, wherein signals derived from the EEG are used to control a supply of anaesthetic gas, and wherein a painful nasal stimulus is administered at regular intervals by means of the pulses of stimulating gas or vapour.

4. A method according to any preceding claim, wherein a series of pulses of a stimulating gas is introduced into a stream of neutral gas and the pulse shape is arranged to be such that a steep initial rise in the concentration of the stimulating substance, at a rate exceeding 5 %/ms, precedes a maximum concentration value which corresponds to 100% stimulating gas, this maximum value being maintained over a holding period not exceeding 30% of the pulse duration, and this holding period being followed by a period of falling concentration values lasting to the end of the pulse.

5. A method according to any one of claims 1 to 3, wherein the pulses of stimulating gas are superimposed on a constant base quantity of stimulating gas.

6. Apparatus suitable for carrying out the method according to any preceding claim, comprising a twin-valve assembly connected to a control unit operable to control the desired time-based pulse pattern, one of the two valves being connected to a source of neutral gas and the other being connected to a source of stimulating gas, and the outlets of the two valves being connected to a nasal supply line via a common gas discharge connection.

7. Apparatus according to claim 6, wherein the two valves are fast-responding electrically actuated ball valves.

8. Apparatus according to claim 6 or 7, wherein the pulse shape can be altered by means of the control unit.

9. A method for measuring sensory qualities or responses substantially as hereinbefore described with reference to Figure 1 or 2 of the accompanying drawings.

10. A method for controlling the supply of anaesthetic gas on the basis of the sensory qualities or responses measured in accordance with claim 9.

11. Apparatus suitable for carrying out the method of claim 9 or 10 and substantially as hereinbefore described with reference to Figure 3 of the accompanying drawings.