Ventilation following induction of general anaesthesia by thiopentone

We measured occlusion pressure, tidal volume, frequency, and the duration of the ventilatory phases in twenty-eight patients before and immediately after induction of anaesthesia by thiopentone. From these measurements we derived mean inspiratory flow rate and "effective" impedance to flow. Occlusion pressure was generally, but not universally, less after induction. Mean inspiratory flow rate was reduced due to a combination of smaller tidal volume and shorter duration of inspiration. This was noted in all twenty-eight patients, as was a clearcut increase in "effective" impedance to flow. These results suggest an important mechanical component to the reduced ventilation noted following induction of general anaesthesia.