Since general anesthesia first became widely used in late 1846, assessment of anesthetic depth was a problem. To determine the depth of anesthesia, the anesthetist relies on a series of physical signs of the patient. In 1847, John Snow (1813-1858) and Francis Plomley attempted to describe various stages of general anesthesia, but Guedel in 1937 described a detailed system which was generally accepted.
This classification was designed for use of a sole inhalational anesthetic agent, diethyl ether (commonly referred to as simply "ether"), in patients who were usually premedicated with morphine and atropine. At that time, intravenous anesthetic agents were not yet in common use, and neuromuscular-blocking drugs were not used at all during general anesthesia. The introduction of neuromuscular blocking agents (such as succinylcholine and tubocurarine) changed the concept of general anesthesia as it could produce temporary paralysis (a desired feature for surgery) without deep anesthesia. Most of the signs of Guedel's classification depend upon the muscular movements (including respiratory muscles), and paralyzed patients' traditional clinical signs were no longer detectable when such drugs were used. Since 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 general anesthesia can now be estimated using devices such as the BIS monitor, however, the use of BIS monitoring remains controversial.
Stages of Anesthesia 
Stage I (stage of analgesia or disorientation): from beginning of induction of general anesthesia to loss of consciousness.
Stage II (stage of excitement or 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 (apnea).
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.
- 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 analgesia and amnesia
See also 
- Oropharyngeal airway
- Anesthetic agents
- Instruments used in anesthesiology
- Anesthesia machine
- Inhalational anesthetic
- John Snow and Meyer Joubert. Five stages of narcotism; On the inhalation of ether in surgical operation, London, 1847
- Plomley F. Lancet 1847,p. 134 (reprinted in classical file, Survey of Anesthesiology 1970, 14, 88)
- Lunn, J.N. (1982) Lecture Notes on Anaesthetics, 2nd edn. Blackwell Scientific Publications, Oxford.
- Guedel AE. Inhalation anesthesia, Ed 2, New York, 1951, Macmillan
- Bhargava AK, Setlur R, Sreevastava D. (2004 Jan). "Correlation of bispectral index and Guedel's stages of ether anesthesia". Anesth Analg. 98 (1): 132–4. PMID 14693605.
- ULTIMATE COMPUTING
- Carlsson C, Karlsson JP, Daniels FB, Harwick RD. The end of ether anesthesia in the USA. In: Fink BR, Morris LE, Stephen CR, eds. Proceedings 3rd International Symposium on the history of Anesthesia. Atlanta, Georgia. Wood Library - Museum of anesthesiology, Illinois. 1992: 100–2.
- Anesthesia and Analgesia April 2005 vol. 100 no. 4 1221. doi:10.1213/01.ANE.0000149022.48021.24
- Artusio JF. Di-ethyl ether analgesia: a detailed description of the first stage of ether analgesia in man. J Pharmacol Exp Ther 1954, 111, 343-334