Stages of general Anaesthesia

Inhalant anaesthetics such as ether, chloroform when used to induce and maintain anaesthesia typically exhibit the following four Stages of general Anaesthesia-

• Stage 1: Stage of voluntary movement or excitement
• Stage 2: Stage of involuntary movement or delirium
• Stage 3: Stage of surgical anaesthesia
• Stage 4: Stage of medullary paralysis and death

General anaesthesia is defined as the state of controlled and reversible loss of consciousness, characterized by loss of pain(analgesia), amnesia, muscle relaxation and relatively depressed reflex responses.

Stage 1: Stage of voluntary movement or excitement

Stage of voluntary movement or excitement lasts for initial administration of anaesthetics drug to loss of consciousness, characterized by epinephrine release resulting in increased heart rate and papillary dilation. Voluntary breaths holding.

Stage 2: Stage of involuntary movement or delirium

Stage of involuntary movement or delirium lasts from loss of consciousness to the onset of a regular pattern of breathing. Loss of all voluntary control. Violent reflex struggling to external stimuli Nystagmus in horses. Vocalization may occur in all animals. Excessive salivation in ruminants and cats. Vomiting in dogs and cats. Presence of jaw tone.

Stage 3: Stage of surgical anaesthesia

Stage of surgical anaesthesia characterized by unconsciousness with progressive depression of reflexes. Muscular relaxation develops and ventilation become slow and regular.

Stage of surgical anaesthesia further divided in three plans-

• Plan 1: or plane of light anaesthesia persist until eyeball movement cases
• Plan 2: or plan of medium anaesthesia progressive intercostals paralysis strong corneal reflex
• Plan 3 or deep anaesthesia diaphragmatic depression, weak corneal reflexes, cantered and dilated pupil

Stage 4: Stage of medullary paralysis and death

Stage of medullary paralysis and death include-

• Extreme CNS depression
• Apnea
• CRT markedly delayed
• Anal and bladder sphincter relax