Comparison of pattern of breathing with other measures of induction of anaesthesia, using propofol, methohexital, and sevoflurane.

We assessed change of the pattern of breathing as a marker of induction of anaesthesia, using a method of maintaining spontaneous breathing throughout the induction period. We compared this index with a measure used clinically, the lash reflex, and measures used for drug investigations such as loss of grip of an object, cessation of finger tapping, and loss of arm tone. Ninety female patients (mean age 32 (17-63) yr, mean weight 63 (10) kg) were randomly allocated to induction of anaesthesia using propofol, methohexital, or sevoflurane. The i.v. agents were given by slow injection estimated to give an induction dose (for weight drop end point) in 90 s. Sevoflurane was given by progressively increasing the inhaled concentration to 8% so that induction should occur within 90-120 s. We measured time to change in breathing pattern, loss of voluntary finger tapping, loss of the lash reflex (tested at 15 s intervals), loss of postural tone in an outstretched arm, and loss of grip of a small metal cylinder held between finger and thumb. For methohexital and sevoflurane, the mean times for induction of anaesthesia occurred in the above order. With propofol, the lash reflex and tone were lost at the same time. The mean (SD) time to induction, by loss of arm tone was 64 (16) s for propofol, 83 (23) s for methohexital, and 94 (31) s for sevoflurane. The mean time to change in breathing pattern was 47 (20) s for propofol, 53 (14) s for methohexital, and 78 (29) s for sevoflurane. Although the time to achieve each end point was different, all the end points (except the lash reflex) appeared to provide similar measures of induction of anaesthesia. The pattern of breathing is an early sign of the onset of anaesthesia.