Effect of assisted ventilation on respiratory drive of normal anesthetized dogs

We measured the diaphragm electromyogram (EMGd) and airway occlusion pressure (P100) of anesthetized dogs at the ends of 30-min of alternating spontaneous breathing and assisted mechanical ventilation (inspiratory positive pressure triggered by a minimal inspiratory effort; IPPB). Peak airway pressures (Paw) of 5, 10, 15, and 20 cm H2O were applied in random order during IPPB. The initial rate of rise of EMGd (4 experiments) and P100 (9 experiments) decreased during IPPB, both in proportion to the Paw. At Paw of 20 cm H2O, PaCO2, EMGd and P100 decreased on the average 5.2 Torr, 58% and 56%, respectively. When PaCO2 was kept the same during IPPB as during spontaneous breathing (8 experiments), IPPB reduced EMGd and P100 15% and 24% respectively, both statistically significant. During progressive cooling of both cervical vagi, the effect of IPPB (Paw=20 cm H2O) on EMGd and P100 decreased progressively, near maximal at 6--4 degrees C and after vagotomy. However, the decrease in PaCO2 during IPPB was not affected. These data suggest that increasing tidal volume during IPPB depresses respiratory drive (EMGd and P100), partially from decreased PaCO2 and partially from stimulating pulmonary stretch receptors. The drop in PaCO2 during IPPB does not appear to be vagally mediated.