Assessing anaesthetic depth during surgical or prolonged procedures

Introduction

Depth of anaesthesia is usually assessed from a combination of measures including the presence or absence of responses to painful stimuli, changes in the pattern and depth of respiration, changes in muscle tone, and changes in heart rate and blood pressure. More sophisticated techniques for assessment of depth of anaesthesia have been developed for use in people, for example, measurement of the electroencephalogram (EEG) and of sensory or somatic evoked potentials. Although these techniques are not yet widely applied in animals, they may be of value in some circumstances, for example during prolonged periods of anaesthesia and when using neuromuscular blocking drugs.

Although the effects of different anaesthetics vary, the initial effects are broadly similar. After administration of a volatile anaesthetic agent or the intraperitoneal injection of a drug such as pentobarbital, most animals become ataxic (wobbly), lose the ability to turn over when placed on their back (loss of righting reflex) and eventually remain immobile. At this depth of anaesthesia, they can easily be roused by painful stimuli, so anaesthesia must be allowed to deepen until these responses are lost if a surgical or other painful procedure is to be undertaken. This gradual onset of anaesthesia will not be seen if an injectable anaesthetic such as propofol is given intravenously since a loss of consciousness occurs within a minute or less. 

Responses to Painful Stimuli

Anaesthetics are often administered to produce both unconsciousness and to block the perception of pain. Consequently, the response to painful stimuli is an essential part of the assessment of the depth of anaesthesia. In most species, this can be done by pinching the toes, tail, ears or nose, and observing the animal’s response. To assess limb withdrawal reflexes, the hindlimb should be extended and the web of skin between the toes pinched between the anaesthetist’s fingernails. If the limb is withdrawn, or the animal vocalises, it indicates that the depth of anaesthesia is insufficient to allow surgical procedures to be carried out. A slight, barely perceptible movement may remain, and in most circumstances, this indicates a sufficient depth of anaesthesia to carry out the surgery. 

In small rodents, the tail pinch response is often used, but this response is often lost at lighter planes of anaesthesia than the toe pinch reflex. Hence, loss of this response may not guarantee a lack of response to surgery. In many species, the hindlimb withdrawal response is lost at lighter planes of anaesthesia than the forelimb response, but loss of hindlimb responses is normally sufficient to allow surgery to be undertaken without the animal responding. Besides using the limb withdrawal response, the reaction to pinching an ear can be observed in rabbits or guinea pigs. At light levels of anaesthesia, the animal responds to ear pinching by shaking its head and at very light levels by vocalizing. The loss of a response to painful stimuli does not occur uniformly in all body areas. 

On occasion, it may be possible to begin to perform a laparotomy without eliciting either any movements or any autonomic responses, such as an increase in heart rate, in an animal that still shows a limb withdrawal reflex. However, further surgical stimulation, such as cutting or clamping the abdominal muscles or handling the abdominal viscera, may produce reactions indicating an inadequate depth of anaesthesia. If this occurs, then surgery should stop, and the depth of anaesthetic increased, for example by increasing the vaporiser setting if an inhalant anaesthetic is in use.

Alterations in Eye Reflexes

In larger species, such as the dog, cat, pig, sheep and primates, the palpebral reflex (blinking when the edge of the eyelid is lightly touched) is lost during the onset of light surgical anaesthesia with barbiturates, volatile anaesthetics and some other drugs. Use of ketamine causes the loss of this reflex at lighter levels of anaesthesia, and the use of neuroleptanalgesic combinations has unpredictable effects on the reflex. The palpebral reflex is difficult to assess in small rodents, and in rabbits it may not be lost until dangerously deep levels of anaesthesia have been attained. In larger species, as anaesthesia deepens the corneal reflex is lost (blinking in response to brushing the cornea with a damp swab). In small mammals, loss of this reflex is very variable, but loss usually indicates a very deep plane of anaesthesia. 

The position of the eyeball can also be of use once experience has been gained with the species of animal and the particular anaesthetic technique that is to be used. For example, in dogs anaesthetized with a volatile anaesthetic such as isoflurane, or with propofol, the eye rotates downward as a surgical plane of anaesthesia is attained. At very deep planes of anaesthesia, the eye rotates back to a central position, but the palpebral reflex is absent, which enables this stage to be distinguished from very light anaesthesia. Since the position of the eyeball, and other changes such as the degree of pupillary dilatation and the occurrence of side-to-side movement of the eye (nystagmus) vary both between species and with different anaesthetics, they cannot be relied upon as indicators of the depth of anaesthesia and should always be combined with observation of other clinical signs. 

Alterations in Cardiovascular and Respiratory Functions

Most anaesthetics cause a dose-dependent depression of the cardiovascular and respiratory systems. The way in which this depression is manifested can vary considerably with different anaesthetics. Both the rate and depth of breathing may change, and so may the pattern of breaths. Cardiovascular system depression usually results in a fall in arterial blood pressure, but this may be associated with either a fall or a rise in heart rate.

Given these wide variations in response, it is dangerous to generalize about the effects of different depths of anaesthesia. However, once experience has been gained with a particular anaesthetic technique in a particular animal species then these changes can be used to help assess the depth of anaesthesia. Methods of monitoring the cardiovascular and respiratory system are discussed as follows.