Guedel's classification

Since the invention of anesthesia in 1846, assessment of its depth was a problem. To determine the depth of anesthesia, anesthetist must rely on a series of physical signs of the patient. In 1847, John Snow (1813-1858) and Francis Plomley attempted to describe various stages of anesthesia, but Arthur E. Guedel (1883-1956) in 1937 described a really detailed system which was generally accepted.

Stage I (Stage of Analgesia or the stage of Disorientation): from beginning of induction of anesthesia to loss of consciousness.

Stage II (Stage of Excitement or the stage of Delirium): from loss of consciousness to onset of automatic breathing. Eyelash reflex disappear but other reflexes remain intact and coughing, vomiting and struggling may occur; respiration can be irregular with breath-holding.

Stage III (Stage of Surgical anesthesia): from onset of automatic respiration to respiratory paralysis. It is divided into four planes:
 * Plane I - from onset of automatic respiration to cessation of eyeball movements. Eyelid reflex is lost, swallowing reflex disappears, marked eyeball movement may occur but conjunctival reflex is lost at the bottom of the plane
 * Plane II - from cessation of eyeball movements to beginning of paralysis of intercostal muscles. Laryngeal reflex is lost although inflammation of the upper respiratory tract increases reflex irritability, corneal reflex disappears, secretion of tears increases (a useful sign of light anesthesia), respiration is automatic and regular, movement and deep breathing as a response to skin stimulation disappears.
 * Plane III - from beginning to completion of intercostal muscle paralysis. Diaphragmatic respiration persists but there is progressive intercostal paralysis, pupils dilated and light reflex is abolished. The laryngeal reflex lost in plane II can still be initiated by painful stimuli arising from the dilatation of anus or cervix. This was the desired plane for surgery when muscle relaxants were not used.
 * Plane IV - from complete intercostal paralysis to diaphragmatic paralysis (apnoea).

Stage IV: from stoppage of respiration till death. Anesthetic overdose cause medullary paralysis with respiratory arrest and vasomotor collapse. Pupils are widely dilated and muscles are relaxed.

In 1954, Artusio further divided the first stage in Guedel's classification into three planes.


 * 1st plane	The patient does not experience amnesia or analgesia
 * 2nd plane	The patient is completely amnesic but experiences only partial analgesia
 * 3rd plane	The patient has complete amnesia and analgesia

This classification was designed for use of a sole inhalational anesthetic agent, ether, with patients usually premedicated with morphine and atropine. Till that time, muscle relaxants were not used during anesthesia and intravenous induction agents were not common. Introduction of neuromuscular blocking agents (tubocurarine) in 1942 changed the concept of anesthesia as it could produce temporary paralysis (a desired feature for surgery) without deep anesthesia. But most of the signs of Guedel's classification depnds upon the muscular movements (including respiratory muscles), therefore paralyzed patients’ traditional clinical signs were no longer detectable when such drugs were used. Scince 1982, ether is not used in United States. Now, because of the use of intravenous induction agents with muscle relaxants and discontinuation of ether, Guedel’s classification is regarded as obsolete. Depth of anesthesia is now measured by Bis monitor.