General anaesthesia are CNS depressant that produce anaesthesia, which extends to the entire body and are characterised by a state of unconsciousness, analgesia and Amnesia with skeleton muscle relaxation and loss of reflexes.

General anaesthetic are employed for surgical operations and four stages of anaesthesia may be recognised as:

Stage 1 (Analgesia):

The patient is conscious and experience sensation of warmth, renowned, drifting, taking and giddiness. There is a marked reduction in the perception of painful stimuli. This stage is used often in obstetrics and minor surgery.

Stage 2 (Delirium):

This stage begins with the loss of consciousness. Depression of higher centres produce a variety of effects including excitement, involuntary activity and increased skeletal muscle tone and respiration.

Stage 3 (Surgical Anaesthesia):

This is the stage of unconsciousness and paralysis of reflexes. Respiration is regular And blood pressure is maintained. All surgical procedures are carried out in the stage.

Stage 4 (Medullary Paralysis):

Respiratory and circulatory failure occurs as depression of the vital centres of the medulla and brain stem occur.

Mechanism of action

The wide variation in structures led to several theories of Anaesthesic action. The mechanism by which inhalation Anaesthetics manifest their effect is not exactly known. Since they do not belong to one chemical class of compounds, the correlations between structure and activity are also not known. There are a number of hypothesis that have been advanced to explain the action of general aesthesia. However, none of them can adequately describe the entire spectrum is effects caused by general anaesthetic.

The action of general anaesthetic can be explained as a blockage of in channels or add specific changes in mechanism of release of neurotransmitters. Three of the  proposed mechanism are discussed here.

1. Hydrate Hypothesis: Anaesthetic molecules can form hydrate with structured water, which can stop brain function in corresponding areas. However, the correlation between the ability to form hydrates and activity of inhalation Anaesthetic is not known.
2. Ion channel hypothesis: Anaesthetic block in channels by interacting with cellular membranes and reduced the flow of Na+ ions and increasing the flow K+ ions into the cell, which leads to the development of anaesthesia.
3. Fluid membrane hypothesis: Anaesthetics stabilize or rather immobilize the cell membrane, hampering membrane fluidity which produces changes in the ion channel action.

Classification

1) Inhalation Anaesthetic

i) Liquid: enflurane, isoflurane, halothane, methoxyflurane, ether.

ii) Gas: cyclopropane, nitrous oxide.

2) Intravenous Anaesthetic:

i) Ultrashort action barbiturates: thiopentone, thiamylal, methohexital.

ii) Arylcyclohexylamine: ketamine.

iii) Benzodiazepine: midazolam.

iv) Narcotic analgesic: alfentanil, fentanyl.

v) miscellaneous: etomidate, propofol.

3) Newer drugs:

desflurane, sevoflurane, minaxolone.