The basis of general anesthesia is to establish the "triad" of hypnosis, muscle relaxation and suppression of sympathetic reflexes. This, together with manipulation of mechanical ventilation, fluid therapy, temperature and the circulation by the use of anesthetic and vasoactive drugs, can produce the required operating conditions for complex neurosurgery.
Most intravenous anesthetics decrease cerebral metabolism and blood flow and tend to have a cerebral protective effect and decrease intracranial pressure. The inhala-tional agents are all cerebral vasodilators that can be offset by the induction of hypocapnia through to hyperventilation. The overall effect on cerebral blood flow (CBF) depends on a balance between the concentration of the inhalational agent and the degree of hyperventilation.
Moderate hyperventilation reduces CBF and brain volume. Extreme hyperventilation may be associated with critical reduction in flow to compromised areas and focal ischemia. It is likely that barbiturates offer some protection for the brain against ischemia, but there is evidence that mild hypothermia has a cerebral protective effect that exceeds that of the barbiturates and which is out of proportion to the degree to which the cerebral metabolic rate is lowered.
The induction of general anesthesia depresses normal protective reflexes, and patients are at risk of aspiration of gastric contents. Those with raised intracranial pressure or who have suffered recent trauma causing vomiting are at particular risk.
Manipulation of the blood pressure may facilitate some procedures (e.g. the induction of hypotension during aneurysm surgery). Patients must be appropriately monitored and the risks of the failure of normal autoregulation of the cerebral circulation in patients with cerebral vasospasm must be considered. Careful monitoring control of the arterial pressure is also required where there is potential for cord ischemia.
A significant number of patients suffer moderate or severe pain after craniotomy. Morphine appears to be a safe analgesic and is more effective than codeine in post-craniotomy patients.
Management of the multiply injured patient must initially focus upon the ABC (airway, breathing and circulation) of basic life support.
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