Analysis of Interactions Between Central and Vagal Respiratory Control Mechanisms in Cats

In anesthetized cats, regulation of the duration of inspiration (Ti) and expiration (Te) has been investigated by analyzing the operation of central inspiratory on-and off-switch mechanisms. Using the tracheal pressure (P) generated during airway occlusion at FRC as a measure of central inspiratory activity, and the end-inspiratory pressure (P0ax) as the setting of the off-switch threshold, we considered two indices in the central and lung volume-related control of Ti: (a) P/P0max, an index of central inspiratory contribution to inspiratory off-switching, and (b) 1¿P/P0 max´ an index of the additional vagal input needed to terminate inspiration. Both indices were found to be independent of PaCO2, varying solely as a function of time from the onset of inspiration. Changes in the relationship between tidal volume (VT) and Ti with variation in body temperature and unilateral vagotomy were attributed to separate effects on the 1¿P/P0max vs Ti and 1¿P/P0max vs VT relations, respectively. The rapid shallow breathing pattern observed with continuous negative pressure breathing has been explained on the basis of early activation of an inspiratory on-switch mechanism by lung deflation receptors.