Diaphragm inhibition with positive pressure ventilation: quantification of mechanical effects

To quantify any mechanical inhibitory effect of nasal intermittent positive pressure ventilation (IPPV) on inspiratory activity of the diaphragm we ventilated five conscious relaxed subjects on two occasions at respiratory rates similar to quiet breathing (QB) and at three levels of applied pressure (Pappl)--6, 9 and 12 cmH2O, each during hypocapnia (P(CO2) allowed to decrease) and eucapnia (CO2 added to inspired gas). Diaphragm activity was assessed from transdiaphragmatic pressure (esophageal and gastric balloons) and diaphragm EMG (surface electrodes) both integrated with time (integral(Pdi x dt) and integral(EMGdi x dt), respectively). Neural inspiratory time (Tin) was measured as onset to peak of the integral(EMGdi x dt) signal. Relative to QB, integral(Pdi x dt) was 50-69% less during eucapnic IPPV 6-12 cmH2O (P < 0.005) and 67-85% less during hypocapnic IPPV (P < 0.005). Tin decreased (P < 0.05) with IPPV and, on ceasing IPPV, there was apnoea (prolonged expiratory time) on 23 of 27 occasions; these changes were independent of P(CO2). Integral(EMGdi x dt) decreased (P < 0.05) at Pappl 12 cmH2O during eucapnia and at all Pappl during hypocapnia. The repeatability of integral(EMGdi x dt) was substantially less than integral(Pdi x dt) (F = 42, P << 0.01). We conclude that, during non-invasive IPPV in awake healthy subjects mechanical factors are of major importance in inhibiting inspiratory activity of the diaphragm.