Contribution of the diaphragm and the other inspiratory muscles to different levels of tidal volume and static inspiratory effort in the rabbit.

1. The contribution of the diaphragm and that of the other inspiratory muscles at different levels of tidal volume and during static inspiratory efforts of various strength has been studied in supine rabbits by blocking phrenic conduction with an electrotonic current. The rabbits were lightly anaesthetized with urethane and pentobarbitone.2. The volume displaced by the extradiaphragmatic muscles (V(tEDM)) in vagotomized rabbits increases linearly with the tidal volume (V(t)), according to the function V(tEDM) = - 3.27 + 0.32 V(t). The relative contribution of extradiaphragmatic muscles (V(tEDM)/V(t) x 100) for resting ventilation is 12% and becomes ca. 25% for the maximum V(t) value attained during re-breathing.3. When the vagi are left intact, the V(tEDM) is always higher because of the compensatory hyperactivity of the extradiaphragmatic muscles due to Hering-Breuer reflexes during the phrenic block.4. The pressure exerted by the extradiaphragmatic muscles, during inspiratory efforts with closed airways, increases linearly with the strength of the effort, without any difference between intact and vagotomized rabbits. The relationship between the pressure exerted by the extradiaphragmatic muscles (P(EDM)) and the pressure exerted by all the inspiratory muscles (P) is expressed by the function P(EDM) = - 3.29 + 0.38P.5. These results indicate that the diaphragm is the main inspiratory muscle at all levels of inspiratory activity.