Pharmacology Notes

Stages and Planes of Anesthesia
Before patients reach surgical anesthesia, they go through several stages. Th e use of these stages and planes of anesthesia helps to describe the levels and progression of anesthesia produced by anesthetics. Th ere are four stages of general anesthesia:

Stage I: Th is stage begins when the agent is administered and lasts until loss of consciousness. Stage I is characterized by:

• Analgesia

• Euphoria

• Perceptual distortions

• Amnesia

Stage II: Delirium begins with loss of consciousness and extends to the beginning of surgical anesthesia. Th ere may be excitement and involuntary muscular activity. Th e skeletal muscle tone increases and breathing is irregular. At this stage, hypertension and tachycardia may occur. It is important that the passage from Stage I to Stage III be attained as quickly as possible. Sudden death can occur during Stage II.

Stage III: Surgical anesthesia lasts until spontaneous respiration ceases. It is further divided into four planes based on:

• Respiration

• Th e size of the pupils

• Refl ex characteristics

• Eyeball movements

Th is stage is characterized by progressive muscular relaxation. Muscle relaxation is important during many surgical procedures as refl ex movements can occur when a scalpel slices through the tissues.

Stage IV: Medullary paralysis begins with respiratory failure and can lead to circulatory collapse. Th rough careful monitoring, this stage is avoided.