Electroencephalographic changes during vital capacity breath induction with halothane

We investigated the EEG responses in 17 patients during induction of anaesthesia with vital capacity breaths of 4% halothane. The control alpha activity changed to a low voltage, 5-25 muV, fast, 20-28 Hz activity at the time of loss of consciousness at 78 (SD 27) s and typical spindle bursts (20-75 muV, 12-15 Hz) developed at 223 (51) s. All patients remained haemodynamically stable. This sequence of EEG changes was similar to that observed during induction with the conventional method of normal tidal volume breathing and a gradual increase of the inspired concentration of halothane.