Control of the duration of expiration

Abstract

We recorded tidal volume, inspiratory and expiratory durations, upper airway resistance, abdominal pressure and diaphragmatic electromyograms in resting, unanesthetized cats. During air breathing expiratory duration (spontaneous t e) exceeded the time required for passive collapse of the intubated respiratory system (passive t e). This difference between spontaneous and passive t e values resulted from the influence of two factors that retard expiratory flow: post-inspiratory activity of the diaphragm and the resistance of the upper airway. Hypercapnia consistently led to a decrease in spontaneous t e, accompanied by a decrease in upper airway resistance, an increase in abdominal expiratory muscle activity and, in some animals, a decrease in post-inspiratory activity of the diaphragm. In cats studied 1–4 weeks after bilateral cervical vagotomy, t e was generally increased, a change that is consistent with the operation of a volume-related feedback system controlling t e in the intact animal. Our analysis of the factors that regulate expiratory flow indicates the importance of variations in upper airway resistance brought about by movements of the vocal cords. We propose that by this mechanism, via the operation of volume-related feedback, the larynx is a significant determinant of t e and the respiratory frequency.