Effects of active and passive sighs in normoxia and hyperoxia on the breathing of patients anaesthetized with infusions of propofol.

Spontaneous augmented breaths (active sighs) reduced the tidal volume and inspiratory time of succeeding breaths; manual lung inflations (passive sighs) reduced the tidal volume but had little effect on inspiratory time. Sighs in air, whether active or passive, reduced tidal volume more than sighs in hyperoxia (100% oxygen or 33% oxygen in nitrous oxide) after both active and passive sighs (overall difference about 10%); the reduction in inspiratory time after a sigh was less affected by gas mixture. Calculated mean inspiratory flow was reduced after passive sighs, but active sighs were more likely to cause arousal, which complicated the analysis. Tidal volume was reduced after a sigh partly because of reduced peripheral chemoreceptor input, the main effect of which was to reduce inspiratory flow, and partly because inspiratory time was shorter. Although the chemoreceptors may affect inspiratory time after a sigh, the greater effect of active sighs compared with passive sighs makes it likely that the shortening was either part of the neural output that causes spontaneous sighs, or was caused by mechanoreceptor input not mimicked by manual lung inflation.