Reflex control of expiratory airflow and duration.

Unanesthetized, unrestrained cats were studied repeatedly after placement of a permanent tracheostomy, catheters for respiratory pressure measurements, and respiratory muscle EMG electrodes. The tracheostomy was opened or closed by a remote mechanism. Opening the tracheostomy reduced tracheal pressure to zero and diverted flow from the upper airway; closing the tracheostomy reestablished the normal pathway for airflow. Opening the tracheostomy during expiration evoked reflex responses in the diaphragm, in the laryngeal abductor and in abdominal expiratory muscles. These were sustained compensatory ("tracking") responses which, in each case, acted to offset loss of expiratory braking by the upper airway. Occlusion of the tracheostomy during expiration produced the opposite responses. Responses to tracheostomy opening usually overcompensated for the loss of upper airway resistance, suggesting that extrathoracic tracheal receptors may participate in tracking. Changes in the expiratory time course of lung volume were accompanied by changes in the duration of expiration. These "triggering" responses were shown to operate independently of the tracking responses though both were eliminated by vagotomy.