Theoretical considerations on the potential hazards of hyperventilation during anaesthesia

The purpose of this paper is to evaluate the effect of hyperventilation on cerebral oxygenation. The best estimate of cerebral oxygenation is cerebral venous PO2 (PVO2). PVO2 has been considered in relation to PaCO2 during normoxaemia. In order to compare the effects of hyperventilation with the effects of hypoxaemia, PVO2 has also been considered in relation to PaO2 when PaCO2 is constant. The results indicate that the low PaCO2 values which may be seen in the operating room with manual ventilation have the same effect on cerebral oxygenation as severe hypoxaemia. It is emphasized that the brain is better protected against threats to cerebral oxygenation, e.g. hypotension, if PaCO2 is kept close to normal during anaesthesia. During neurosurgical anaesthesia the advantages of excessive hyperventilation must be weighed against the threat it poses to oxygenation in healthy areas of the brain.