WO1996018340A1

WO1996018340A1 - Monitoring pain responsiveness in patients under anaesthesia

Info

Publication number: WO1996018340A1
Application number: PCT/AU1995/000857
Authority: WO
Inventors: Peter Standish St. Vincent Welch; Wolf-Dieter Herrmann
Original assignee: Commonwealth Scientific And Industrial Research Organization
Priority date: 1994-12-16
Filing date: 1995-12-18
Publication date: 1996-06-20
Also published as: AUPN007594A0

Abstract

Pain responsiveness in animal or human patients under anaesthesia is measured to determine depth of anaesthesia by monitoring jaw tension. A load cell (1) located by a plate (2) and a scroll (3) is placed between the jaws of the patient so that jaw tension is applied via actuating members (4) to the load cell (1). The output from the load cell is used to present a graphical representation of a function of jaw tension over time.

Description

MONITORING PAIN RESPONSIVENESS IN PATIENTS UNDER ANAESTHESIA TECHNICAL FIELD This invention relates to a method of and apparatus for monitoring pain responsiveness in patients under anaesthesia.

BACKGROUND ART

Assessment of the depth of anaesthesia has always been important . There are the obvious reasons such as the need to avoid causing pain and the need for the patient to be immobilised to reduce interference with surgical procedures, but there is also increasing recognition of the value of limiting the amount of anaesthetic administered in order to minimise the depressive affect which all anaesthetics exhibit to some degree towards a range of body systems.

Although general reference is sometimes made in medical and veterinary literature of "depth of anaesthesia" there does not appear to exist any simple known parameter by which "anaesthetic depth" may be measured. As one of the prime functions of anaesthesia is to prevent pain awareness

(together with prevention of patient movement) it is generally more accurate to refer to depth of anaesthesia as a level of pain awareness or pain responsiveness. Conventionally the depth of anaesthesia has been assessed by monitoring a variety of parameters. These include monitoring gross signs, such as the response to peripheral nerve stimulation, loss of reflexes, reduction of muscle tone, skin incision, changes in respiration, ECG and EEG, which together can tell the experienced anaesthetist a lot about the condition of the patient, but which considered independently can be misleading.

Additionally reference may be made to autonomic parameters, such as pulse rate, arterial blood pressure, sweating, tear production and pupil size. However, questions remain about the reliability of these criteria, and there have been instances where all such parameters pointed to satisfactory anaesthesia but patients have been aware of proceedings, or have been overdosed.

Alveolar anaesthetic concentration is recognised as offering a useful standard of the potency of inhalation anaesthetics. The minimum alveolar anaesthetic concentration to eliminate movement in response to surgery in 50% of patients (MAC) is fairly constant for a given anaesthetic, nevertheless the need for relatively expensive measuring equipment has discouraged its widespread use for the control of the administration of anaesthetics. Also, the MAC is affected by factors such as the age of the patient, hypothermia and the use of narcotic drugs and tranquillisers.

In recent years there has been limited use of electroencephalogram (EEG) activity as an aid in determining the depth of anaesthesia, however considerable experience is required to be able to interpret EEG data quickly and reliably, as this depends on the type of agent used.

There is not, therefore, a generally accepted procedure for determining the depth of anaesthesia or pain responsiveness in man and animals which is simple to carry out, reliable, and does not involve the use of expensive equipment.

The present invention meets the need for such a procedure by taking advantage of the correlation between muscle tone and depth of anaesthesia.

It has been recognised for many years that muscle tone changes according to the depth of anaesthesia. Hitherto, however, the practising anaesthetist has had to rely on visual, ie essentially subjective judgement of a patient's muscle tone. The present invention now makes it possible to provide the anaesthetist with continuous, instantaneous, objective readings of muscle tone. Moreover, by means of a novel procedure based on sensing jaw tension, the invention is able to make these readings available without the use of invasive technology.

DISCLOSURE OF THE INVENTION Accordingly, the present invention provides a method of monitoring pain responsiveness in an animal or human patient under anaesthesia, comprising the steps of positioning a pressure sensitive member in a location responsive to the pressure exerted by the jaws of the patient and measuring the pressure exerted on that member.

In a further aspect the invention provides apparatus for monitoring pain responsiveness in an animal or human patient under anaesthesia, the apparatus comprising a pressure sensitive member adapted to be positioned in a location responsive to the pressure exerted by the jaws of the patient, and to transmit signals indicative of jaw pressure to means for displaying information based on those signals .

In a still further aspect the invention provides apparatus suitable for use in a method of monitoring pain responsiveness in an animal or human patient under anaesthesia, the method comprising the steps of positioning a pressure sensitive member in a location responsive to the pressure exerted by the jaws of the patient and measuring the pressure exerted on that member.

In a still further aspect the invention provides the use of apparatus in monitoring pain responsiveness in an animal or human patient under anaesthesia, which method comprises the steps of positioning a pressure sensitive member in a location responsive to the pressure exerted by the jaws of the patient and measuring the pressure exerted on that member.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a graph of jaw tension over time from the onset of unconsciousness;

Fig. 2(a) is a side view of a load cell secured between the jaws of a patient;

Fig. 2(b) is a similar view to Fig. 2(a) showing a lever arrangement by which the load cell may be positioned outside the mouth of the pa ient;

Fig. 3 is a graph of jaw tension over time of a sheep anaesthetised for eight minutes; and

Fig. 4 is a similar graph to Fig. 3 showing response to Phalanges Pressure Reflex test.

MODES FOR CARRYING OUT THE INVENTION Characteristically a plot of jaw tension in animals and man from the onset of unconsciousness follows a curve of the type shown in Figure 1. This shows how from an initial sharp decrease in tension the curve levels out (point A) when the patient has reached the stage of surgical anaesthesia (Guedel A E. "Inhalation Anaesthesia, a Fundamental Guide." Macmillan: New York. 1937) - from which base level any further decrease in jaw tension can be taken to be indicative of an unnecessary and undesirable level of anaesthesia.

Clearly the actual jaw tension at any stage of anaesthesia will vary according to factors such as species, age and weight of the patient, and according to the anaesthetic agent being used. While it should be possible, by trial and error, to determine jaw tension values for a given set of circumstances, relating observed values to a curve having a shape as depicted in Figure 1 is believed to offer a reliable and practical guide for the administration of an anaesthetic.

Another aspect of this invention is a device for monitoring the muscle tone of an animal or human patient under anaesthesia, which device comprises a pressure sensitive member adapted to be positioned to be responsive to the pressure exerted by the jaws of the patient, and to transmit signals indicative of jaw pressure to means for displaying information based on those signals.

Typically the pressure sensitive member for making measurements in the practice of this invention will be a load cell, and this will transmit signals to a meter, CRT or other device allowing the anaesthetist to monitor progress of the anaesthesia. The load cell may be any load detecting and measuring element utilizing electrical or hydraulic effects which are remotely indicated or recorded. The load cell will need to be secured in place between the patient's jaws or in some other position responsive to jaw pressure to ensure consistency of measurement conditions, to prevent accidental swallowing and to avoid interference with intubation. Figure 2(a) is a side view of a suitable arrangement. In this figure, 1 is the load cell, which is held in position by the plate 2. Preferably this plate is shaped to approximately conform with the mouth configuration of the patient in order to locate the load cell between the upper and lower jaws. Preferably it is made from a durable, sterilisable resilient plastic, and is provided with a channel-shaped member 4 adapted to receive the upper teeth of the patient and thereby afford additional locating means for the load cell. In the case of animals with limited jaw movement it may be preferable for the load cell to be positioned outside the mouth and to be acted upon by a lever arrangement, as in Figure 2(b) . Optionally, plate 2 will incorporate a scroll or substantially non-deformable member 3 to prevent any intubation elements from being crushed or severed by the plate or by the jaws of the patient.

While in the simplest form of the invention the load cell, or other pressure sensitive device, might be wired to a meter displaying a direct pressure reading, more sophisticated embodiments are contemplated wherein the readings are processed by computer software, together with information about the patient and the anaesthetic, to generate warnings or other messages relevant to the condition of the patient. The output may be presented in many different ways but will typically incorporate a graphical presentation of a function indicative of jaw pressure over time. The invention will now be described with reference to specific examples: Example 1

A four year old Merino sheep (weight approximately 50 kg) was anaesthetised to the stage of surgical anaesthesia with fluothane and using a mechanical ventilator. The fluothane was vaporised at a maximum concentration of 5.0% and with an oxygen flow of 0.5 litres (ventilator control setting: 25 breaths per minute, total gas flow to the animal: 6.0 litres/minute) .

Without any change in gas flow the fluothane supply was stopped and the animal was allowed to recover. This procedure was repeated twice with mechanical ventilation and once with spontaneous ventilation.

Throughout the procedure the jaw tension of the sheep was measured using a load cell positioned and secured as illustrated in Figure 2 (b) to detect pressure between the gum of the upper jaw and the central incisors of the lower jaw.

When the animal was deeply anaesthetised the first time with mechanical ventilation, the force registered by the load cell was approximately 225 g. After the fluothane was turned off, the animal remained in deep stage anaesthesia for approximately 25 minutes and then, as the level of anaesthesia decreased, the jaw tension increased to approximately 600 g, after which the tension oscillated but with an overall increasing trend till it peaked at approximately 1400 g. Light chewing reflexes returned at approximately 1000 g. Heart rate during this period was above 100 BPM, but only increased to >150 once jaw tension was >750 g.

Similar jaw tension response was observed when the animal was subsequently reanaesthetised, irrespective of whether ventilation was mechanical or spontaneous.

Example 2

Figure 3 is a graph showing the jaw tension of a sheep after being anaesthetised with fluothane for eight minutes under the same conditions as described in Example 1. The jaw tension was observed to remain essentially constant for nearly nine minutes, after which it increased as the animal, lying on the operating table and not subject to artificial stimulus, started to respond to the environment.

Example 3 Figure 4 shows the response of an anaesthetised animal to the Phalanges Pressure Reflex test (depression of the phalanges of the fore foot to their full deflexion backwards) after being anaesthetised under the conditions described in Example 1 for eight minutes; reflex tests were applied at one minute intervals. Jaw tension response to the tests over the first three minutes was insignificant, but it then increased to between 450 and 650g during the next four minutes. At seven minutes jaw tension response was greater than lOOOg and the animal responded to the reflex tests by involuntary movement of its body. When the fluothane flow was restored to 5% at 7.5 minutes, jaw tension decreased rapidly as anaesthetic depth increased.

Claims

CLAIMS :

1. A method of monitoring pain responsiveness in an animal or human patient under anaesthesia, comprising the steps of positioning a pressure sensitive member in a location responsive to the pressure exerted by the jaws of the patient and measuring the pressure exerted on that member.

2. A method as claimed in claim 1, wherein the pressure exerted on the pressure sensitive member is measured and analysed over time to monitor changes in pain responsiveness.

3. A method as claimed in claim 2 wherein reduction in jaw tension followed by levelling out of jaw tension over time after administration of anaesthetic is monitored to determine a base level of jaw tension which is then further monitored to indicate necessary application of further anaesthetic to the patient on an on-going basis.

4. Apparatus for monitoring pain responsiveness in an animal or human patient under anaesthesia, the apparatus comprising a pressure sensitive member adapted to be positioned in a location responsive to the pressure exerted by the jaws of the patient, and to transmit signals indicative of jaw pressure to means for displaying information based on those signals.

5. Apparatus as claimed in claim 4 wherein the pressure sensitive member comprises a load cell.

6. Apparatus as claimed in claim 5 wherein the load cell is provided with actuating members operable to transmit pressure from the jaws of the patient to the load cell, said actuating members being adapted to be secured in position between the patient's jaws in a manner ensuring consistency of measurement conditions.

7. Apparatus as claimed in claim 6 wherein the actuating members are adapted to be secured in position between the jaws of the patient by a plate shaped to conform with the mouth configuration of the patient.

8. Apparatus as claimed in any one of claims 4 to 7 including said means for displaying information which means incorporates a graphical presentation of a function indicative of jaw pressure over time.

9. Apparatus suitable for use in a method as claimed in either claim 1 or claim 2.

10. The use of apparatus in monitoring pain responsiveness in an animal or human patient under anaesthesia, which method comprises the steps of positioning a pressure sensitive member in a location responsive to the pressure exerted by the jaws of the patient and measuring the pressure exerted on that member.