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  • Print publication year: 2006
  • Online publication date: December 2009

2 - Principles of anaesthesia


Anaesthesia is a non-therapeutic intervention. It is particularly important, therefore, to determine potential benefits and estimated risks. Complications of anaesthesia are poorly tolerated and it is necessary to place safety before pereceived efficacy.



Anaesthesia is a reversible state of pharmacologically controlled sleep with reduction in cortical activity. At sufficient anaesthetic depth there is absence of conscious awareness and recall, and no sensory, motor or autonomic response to stimulation.

Balanced anaesthesia

This term is used to illustrate an equilibrium between the three constituents of an anaesthetic: anaesthesia (sleep), analgesia and paralysis. The three interact and the separation is more conceptual than clinical.


Sedation is a state of sleepiness but preserved consciousness. Ideally, there is awareness and response to simple commands with verbal contact, but also a degree of amnesia and reduced anxiety.


In the 1920s four stages of progressively deeper anaesthesia were described in relation to inhalational induction. Drugs and techniques have changed considerably since and it is difficult to distinguish these stages clearly in modern anaesthesia.


This is the stage of inhalational sedation prior to loss of consciousness.


The breathing slowly becomes more erratic; the airway is irritable. There may be uncontrolled movements of the limbs.

Surgical anaesthesia

Classically, this consists of four ‘planes’:

Small, pinpoint pupils; the tidal breaths are large and regular; the pharyngeal and vomiting reflexes are depressed.

Pupils are slightly larger; breathing remains regular; the corneal reflex is depressed.


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