Graded inspiratory inhibition: specific effects of flow rate.

The effect of inspiratory flow rate on graded inspiratory inhibition was examined in barbiturate-anesthetized paralyzed cats normally ventilated by a phrenic-driven servorespirator. Periodically, the animals were inflated, for a single breath, by a constant-flow volume ramp begun at one of several delays following phrenic onset. The phrenic activity during these volume-ramp test inflations was compared with phrenic activity in the absence of volume feedback. The relation between volume and phrenic inhibition was determined for two different flow rates. Inhibition of phrenic activity began when volume above functional residual capacity (FRC) exceeded a time-dependent threshold and, beyond this threshold, continued in a graded fashion. The threshold for the first detectable inhibition declined as inspiratory time progressed both in intact animals and in pneumotaxic-lesioned animals when phrenic activity reached an apneustic plateau. Inspiratory inhibition was associated with similar lung volume above FRC regardless of whether volume resulted from high or low flow rates, suggesting that the processing of the receptor feedback information offsets flow effects on receptor firing.