Effects of incremental doses of alfentanil and propofol on the breathing of anaesthetized patients.

Incremental doses of alfentanil and propofol were given to anaesthetized healthy patients undergoing routine orthopaedic surgery. Ventilation was recorded by respiratory inductance plethysmography and analysed by microcomputer. Alfentanil affected primarily expiratory time. The onset of effect lasted 82 s (range 25-173 s); offset was exponential, with a half-life of 146 s (range 62-260 s). Alfentanil also reduced tidal volume, but the effect was less obvious and less consistent. Propofol affected primarily tidal volume. The onset of effect lasted 34 s (range 17-69 s); offset was linear, with a time to 50% recovery of 92 s (range 47-161 s). Propofol had little effect on expiratory time. The drugs had little effect on inspiratory time. Three patients showed periods when the distribution of expiratory times was bimodal; the mechanism for this is unknown.