Stages and Phases of Anaesthesia

Assessment of the depth of anaesthesia 

Anaesthesia has been described as a series of four Stages. 

Stage 1

The period between administration of an anaesthetic and loss of consciousness. 

Stage 2

The period after loss of consciousness, which may include actions such as uncontrolled movement, delirium, vocalization.

Stage 3 

The level at which surgery can be performed. Stage 3 anaesthesia is divided into four planes. 

1. Plane 1: "light" anaesthesia - the animal still has blink and swallowing reflexes, and regular respiration.
2. Plane 2: "surgical" anaesthesia - the animal has lost blink reflexes, pupils become fixed and respiration is regular.
3. Plane 3: "deep" anaesthesia - the animal starts losing the ability to use the respiratory muscles and breathing becomes shallow; may require assisted ventilation. The surgeries are performed in this plane.
4. Plane 4: the animal loses all respiratory effort, and breathing may stop entirely. 

Stage 4

Anaesthetic crisis! Respiratory arrest and death from circulatory collapse imminent.