Guedel's classification

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Guedel's classification is a simple way to judge how deep general anesthesia is. It was developed by Arthur Ernest Guedel (1883–1956). The idea came after anesthesia began in the mid-1800s, with early attempts by John Snow and Francis Plomley in 1847; Guedel published a detailed system that became widely accepted in 1937. The classification was designed for ether anesthesia, with patients usually premedicated with morphine and atropine. At that time, intravenous drugs and muscle relaxants were not yet used.

Because many signs depend on muscle movement and breathing, the introduction of neuromuscular blocking agents later made some signs unreliable. Since 1982, ether is no longer used in the United States. Today, depth of anesthesia is often monitored with devices such as BIS monitors, though their use is controversial.

Stages of Guedel’s classification:
- Stage I (analgesia or disorientation): from the start of induction to loss of consciousness.
- Stage II (excitement or delirium): from loss of consciousness to the onset of automatic breathing. The eyelash reflex disappears, but other reflexes may remain; coughing, vomiting, and struggling can occur; breathing may be irregular.
- Stage III (surgical anesthesia): from the start of automatic breathing to respiratory paralysis. This stage is divided into four planes (I–IV), from light to deep anesthesia.
- Stage IV (medullary paralysis): from respiratory arrest to death, with pupil dilation and relaxed muscles.

In 1954, Artusio further divided Stage I into three planes.