CEREBRAL EFFECTS OF INHALATIONAL ANESTHETICS

Neurosurgical patients often display important changes in the physiology of the central nervous system. Cerebral blood flow may become an important determinant of cerebral blood volume and intracranial pressure. The efficiency of the autoregulatory mechanism may acquire paramount importance for the maintenance of an adequate cerebral perfusion. Knowledge of the influence of anaesthetic agents on the brain is an absolute requirement for safe neuroanaesthetic practice. Nitrous oxide has clear neurostimulatory characteristics. All volatile anaesthetic agents are cerebral vasodilators in spite of their metabolic suppressant activity. Secondary to its greater metabolic depression, isoflurane has the least potential to increase cerebral blood flow. The new inhalational agents, desflurane and sevoflurane, appear to have similar characteristics as isoflurane. The undesirable vasodilatory effects of inhalational anaesthetics can often be overcome by hyperventilation. Neuroprotective activity, which would be due to the extensive cerebral metabolic depression, has been attributed to isoflurane. However, recent evidence suggests that metabolic depression is beneficial, but not sufficient to provide cerebral protection in ischaemia severe enough to cause tissue damage. Nitrous oxide has been incriminated to decrease the neuroprotective effect of barbiturates. Although these observations have been challenged by other investigators it seems prudent to avoid the use of nitrous oxide in patients with compromised brain perfusion. Drugs used in neuroanaesthesia should be devoid of epileptogenic activity. From an extensive review of the literature it appears that all inhalational anaesthetics are safe except for enflurane. It is clear that inhalational anaesthetics are not ideal drugs for neuroanaesthesia. Nevertheless, nitrous oxide and isoflurane are still widely used in neuroanaesthetic practice. In most cases the undesirable effects of these agents can be easily overcome by judicious anaesthetic management. In patients with a very badly compromised brain perfusion, the use of an intravenous anaesthetic technique may, however, be the best choice. © 1993 Baillière Tindall.