Assessing anaesthetic depth

Withdrawal reflexes

We can assess withdrawal reflexes in most animals. In rodents, the tail pinch and the pedal withdrawal reflexes are usually assessed. The tail pinch response is usually lost at light to medium planes of anaesthesia, and this is followed by loss of the pedal withdrawal response at medium to deep planes of anaesthesia. Most surgical procedures can be carried out when the pedal withdrawal reflex is absent or barely detectable.

Eye position/reflex

The use of ocular reflexes is useful in the dog, cat, pig and sheep. With most anaesthetic regimens, the eye gradually rotates downwards as anaesthesia deepens, and the blink reflex when the eyelids are gently brushed is also lost. At very deep planes of anaesthesia, the eye rotates back up and becomes fixed centrally. In rodents, the position of the eye remains fixed, and the blink reflex may still be present at surgical planes of anaesthesia. In rabbits, there is also considerable variation in loss of ocular reflexes, however, at deep planes of anaesthesia, the eye may rotate and protrude.

Ear pinch reflex

In rabbits and guinea pigs, the ear pinch reflex is an additional useful measure of anaesthetic depth. At surgical planes of anaesthesia, the response to pinching the ear is lost.