Continuous EEG and ICP monitoring as a guide to the administration of althesin sedation in severe head injury

The effects of 142 intravenous boluses of althesin (0.05 ml/kg) on cerebral perfusion pressure (CPP) were studied in twelve head injured comatose patients. The data were divided into those where the mean pre-bolus intracranial pressure (ICP) was above or below 20 mmHg and then subdivided into those where the minimum pre-bolus voltage of the cerebral function monitor (CFM) was above or below 5 μV (representing marked reduction in cortical electrical activity). The pre-bolus ICP influenced the extent of the fall in ICP and thus the direction of the resulting change in CPP. A mean reduction in CPP was observed in both groups in which ICP was below 20 mmHg irrespective of the CFM voltage. In the subgroups with high ICP, CPP increased when CFM was not depressed and fell when CFM was below 5 μV. The effects of individual boluses varied within and between the subgroups and, occasionally, severe and unpredictable hypotension occurred. If althesin administration had been restricted to the high CFM and high ICP group, 90% of the episodes of reduced CPP would have been avoided. In fact, when cortical electrical activity is already severely depressed, further administration of hypnotic anaesthetic agents produces only small reductions in ICP and usually a fall in CPP. These findings suggest that the minimum voltage of the CFM trace is a clinically useful guide to the administration of intravenous anaesthetic agents and offers a relevant prediction of their effects on CPP.