Vagally mediated modification of inspiratory activity by changes in airway pressure

The impact of varying the end-expiratory transpulmonary pressure (PE'tp) on phrenic (Phr) and parasternal intercostal (IC) activity early in inspiration was studied in 12 spontaneously breathing, anaesthetized cats. PE'tp was varied in a stepwise fashion by rapidly changing the tracheal pressure (Ptr) from ambient pressure to a pressure in the range between -1.2 and +1.2 kPa relative to ambient pressure, and vice versa. The amplitudes of integrated Phr and IC activity were determined in each breath at three particular points in time (t), corresponding to 0.1, 0.2 and 0.3 sec after the onset of Phr activity, respectively. For all three values of t, Phr and IC activity increased with a decrease in Ptr and decreased with an increase in Ptr. The relationships between Phr activity and PE'tp and between IC activity and PE'tp could be described satisfactorily by mono-exponential functions. An increase in PE'tp of 0.1 kPa resulted in a decrease in Phr activity of 16%, 13% and 12% for the 3 aforementioned values of t, respectively. For IC activity the corresponding values were 42%, 42% and 36%, respectively. After bilateral vagotomy no changes in Phr activity in response to changes in Ptr were observed any more, whereas the changes in IC activity had decreased to about one tenth of the corresponding values obtained before vagotomy. Apparently, extra-vagal mechanisms contribute for a small part to the effect of changes in PE'tp on IC activity. Our results indicate that relatively small changes in PE'tp may have a large impact on central inspiratory output.