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Abstract
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.