Inter-relations of the volume and timing components of ventilation during carbon dioxide rebreathing in awake and anaesthetized subjects.

We studied the relations between the volume and timing components of ventilation during carbon dioxide rebreathing in conscious subjects and patients anaesthetized with propofol. In conscious responses, breath-by-breath minute ventilation (VI) generally correlated better with end-tidal carbon dioxide than did tidal volume (VT), but VT correlated better than VI in the anaesthetized responses. The source of this difference was that, whereas VT and the inspiratory period were both smaller and less variable when subjects were anaesthetized rather than conscious, the expiratory period was no less variable, and this disturbed the usual inverse relation between VT and the duration of the ventilatory cycle. Anaesthesia stabilized the switch from inspiration to expiration, but not that from expiration to inspiration. In some patients, it produced a disturbance pronounced enough to suggest bimodality of the timing of expiration.