Stages of anesthetic depth

Stage I	Analgesia state: Patient is conscious and rational, with decreased perception of pain.
Stage II	Delirium stage: Patient is unconscious; body responds reflexively; irregular breathing pattern with breathholding.
Stage III	Surgical anesthesia: Increasing degrees of muscle relaxation; unable to protect airway.
Stage IV	Medullary depression: There is depression of cardiovascular and respiratory centers.

Adapted from: Hewer CL. The stages and signs of general anesthesia. Br Med J 1937; 2:274.

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