Anaesthesia: Induction and maintenance of general anaesthesia
General anaesthesia is a state of induced reversible unconsciousness. In part two of the series, Jonathan M Behar and colleagues explain the clinical principles of inducing and maintaining general anaesthesia
The administration of a general anaesthetic comprises the induction of a reversible state of unconsciousness with amnesia, as well as muscle relaxation and analgesia. Knowledge of the physiological changes is essential to manipulate and optimise the body's homoeostasis mechanisms. It is the role of the anaesthetist to endeavour to compensate for and mitigate the effects of these changes. In box 1 we list some effects of general anaesthesia on physiological function-an example in this context is the taping shut of eyelids (because of absent corneal reflexes) to avoid the development of ulcers.
The effects of a general anaesthetic on respiratory function are such that anaesthetists usually use an inspired fraction of 30% oxygen as opposed to air (21% oxygen), even in healthy patients. An anaesthetised patient has a reduced tidal volume and respiratory drive and the loss of the hypoxic pulmonary vasoconstriction reflex.
The latter is a reflex which when