Effect of halothane on the cerebral circulation in young children: a hysteresis phenomenon

To determine the effect of halothane on the cerebral blood flow velocity (C BFV) with increasing then decreasing concentrations, 11 children scheduled for minor surgery were studied. Anaesthesia consisted of halothane, vecuron ium, nitrous oxide in oxygen and a caudal block. End-tidal carbon dioxide, temperature, heart rate and systolic arterial pressure were maintained cons tant. CBFV increased significantly between 0.5 and 1.0 MAC (p<0.001), and 0 .5 and 1.5 MAC of halothane (p<0.001), but was not different after increasi ng concentration from 1.0 to 1.5 MAC. During the decreasing phase, CBFV dec reased significantly from 1.5 to 1.0 MAC of halothane (p<0.001), whereas th ere was no difference in CBFV when decreasing halothane MAC from 1.0 to 0.5 MAC. In children, the decrease in CBFV during decreasing halothane concent ration is not superimposable to the increase in CBFV seen when increasing h alothane concentration, suggesting the presence of cerebrovascular hysteres is to halothane.