Effect of respiratory loading on the relationship between occlusion pressure and diaphragm EMG during hypoxia and hypercapnia.

In 7 conscious, unsedated goats with chronically implanted diaphragm electrodes, the effect of respiratory loading on the relationship between occlusion pressure and diaphragm EMG was studied. Diaphragm electrical activity (ED) quantified by the moving average technique was measured in separate trials during progressive hyperoxic hypercapnia and progressive isocapnic hypoxia, both before and after inspiratory resistance had been increased by externally applied loads. Airway occlusion was performed during inspiration on random breaths at functional residual capacity, and the maximum negative pressure (Pmax) was measured. In all 7 goats, occlusion pressure was greater with external loads (EL) than control. The peak ED of unoccluded breaths was greater with EL than control in all animals during hypoxia but in only 3 of the animals during hypercapnia. In half of the trials, the ratio of Pmax to ED of occluded or unoccluded breaths was higher with EL than control. In the remaining studies, the ratio was the same. FRC was unaffected by EL. In the one goat in which it was measured, intercostal EMG was also greater during EL than control. These results suggest that the inspiratory muscles other than the diaphragm are recruited during EL causing the Pmax to ED ratio to rise even if neuromechanical coupling remains unchanged. It is speculated that in conscious, unsedated animals, acute increases in the resistance to air flow affect the distribution of respiratory motor activity.