In the 1920s four stages of progressively deeper anaesthesia were described in relation to inhalational induction. Drugs and techniques have changed considerably since and it is difficult to distinguish these stages clearly in modern anaesthesia.
This is the stage of inhalational sedation prior to loss of consciousness.
The breathing slowly becomes more erratic; the airway is irritable. There may be uncontrolled movements of the limbs.
Classically, this consists of four 'planes':
1. Small, pinpoint pupils; the tidal breaths are large and regular; the pharyngeal and vomiting reflexes are depressed.
2. Pupils are slightly larger; breathing remains regular; the corneal reflex is depressed.
3. Pupils are size 3-4; respiration becomes more diaphragmatic and the tidal volumes decrease; the lacrimation and laryngeal reflexes are depressed.
4. Pupils are dilated; respiration is irregular and shallow; the carinal reflex is depressed.
Surgery is normally conducted in plane 3 of surgical anaesthesia.
The pupils are dilated and fixed; the brainstem reflexes are depressed and there is profound cardiopulmonary depression.
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