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.
- Plane 1: "light" anaesthesia - the animal still has blink and swallowing reflexes, and regular respiration.
- Plane 2: "surgical" anaesthesia - the animal has lost blink reflexes, pupils become fixed and respiration is regular.
- 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.
- 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.