Vagal influences on parasternal intercostal muscle inspiratory shortening during hypercapnia and airway occlusion.

To determine whether increases in electromyographic activity (EMG) are necessary for respiratory muscle shortening to occur during airway occlusion, respiratory changes in parasternal intercostal muscle length were measured using sonomicrometry in 11 anesthetized dogs during unoccluded (UB) and occluded (OB) breaths before and after vagotomy. During UB the extent of parasternal intercostal inspiratory shortening was greater after than before vagotomy both during oxygen breathing and during progressive hyperoxic hypercapnia. The relation between parasternal shortening, parasternal EMG, and tidal volume was not substantially affected by vagotomy. During OB parasternal intercostal EMG increased significantly compared to UB when the vagi were intact, but airway occlusion did not significantly change EMG activity post-vagotomy. However, both before and after vagotomy the parasternal intercostal shortened during OB in all animals. Parasternal intercostal inspiratory shortening during OB as a % of that during UB was significantly greater before compared to after vagotomy during oxygen breathing and moderate hypercapnia, but vagal integrity made no significant difference at high CO2. These results suggest that (1) pulmonary stretch receptor afferents inhibit parasternal intercostal inspiratory shortening but in proportion to their inhibitory effects on parasternal intercostal EMG and tidal volume, and (2) even when the EMG stays constant the parasternal intercostal muscle does not contract isometrically during occluded breaths.