Effects of inspiratory flow on diaphragmatic motor output in normal subjects

Increasing inspiratory flow (V) over dot has been shown to shorten neural i nspiratory time (TIn) in normal subjects breathing on a mechanical ventilat or, but the effect of (V) over dot on respiratory motor output before inspi ratory termination has not previously been studied in humans. While breathi ng spontaneously on a mechanical ventilator, eight normal subjects were int ermittently exposed to 200-ms-duration positive pressure pulses of differen t amplitudes at the onset of inspiration. Based on the increase in (V) over dot above control breaths (Delta(V) over dot), trials were grouped into sm all, medium, and large groups (mean Delta(V) over dot: 0.51, 1.11, and 1.65 l/s, respectively). We measured TIn, transdiaphragmatic pressure (Pdi), an d electrical activity (electromyogram) of the diaphragm (EMGdi). Transient increases in (V) over dot caused shortening of TIn from 1.34 to 1.10 (not s ignificant), 1.55 to 1.11 (P < 0.005), and 1.58 to 1.17 s (P < 0.005) in th e small, medium, and large Delta(V) over dot groups, respectively. EMGdi me asured at end TIn of the pulse breaths was 131 (P < 0.05), 142, and 155% (P < 0.05) of the EMGdi of the control breaths at an identical time point in the small, medium, and large trials, respectively. The latency of the excit ation was 126 +/- 42 (SD) ms, consistent with a reflex effect. Increasing ( V) over dot had two countervailing effects on Pdi: 1) a depressant mechanic al effect due primarily to the force-length (11.2 cmH(2)O/l) relation of th e diaphragm, and 2) an increase in diaphragm activation. For the eight subj ects, mean peak Pdi did not change significantly, but there was significant intersubject variability, reflecting variability in the strength of the ex citation reflex. We conclude that increasing inspiratory (V) over dot cause s a graded facilitation of EMGdi, which serves to counteract the negative e ffect of the force-length relation on Pdi.