Effect of tracheal gas insufflation on demand valve triggering and total work during continuous positive airway pressure ventilation.

Tracheal gas insufflation (TGI) improves CO2 clearance and may reduce work of breathing by lowering the required minute ventilation (VE). However, TGI might also impair the ability to trigger the ventilator, because to lower external circuit pressures, inspiratory effort must outstrip catheter flow rate (Vc) and overcome the dynamic hyperinflation caused by TGI. We studied these effects using a two-chamber lung model of the respiratory muscles (RM) and lungs (L). The RM-chamber was ventilated using a sinusoidal flow pattern with a tidal volume (VT) of 0.5 L at various peak inspiratory flow rates (Vpk) to simulate differences in effort intensity. The L-chamber was connected to a 60-L/min continuous flow circuit with a 10 cm H2O positive end-expiratory pressure valve and to 3 different ventilatory demand valve circuits, each set at continuous positive airway pressure (CPAP) of 10 cm H2O. We used continuous TGI at 0, 2.5, 5, 10, and 15 L/min. The work of triggering (W-trig) increased with increasing Vc and decreased with increasing Vpk. The L-ventilator failed to trigger when Vc was 15 L/min and Vpk was 20 L/min. At a fixed VE, the effect of TGI on total mechanical inspiratory work (W-tot) was relatively small and varied among the different CPAP systems used. We conclude that weak patients may fail to open the demand valve of the CPAP system during TGI at high catheter flow rates. The net effect of TGI on the effort made by ventilated patients would depend not only on the interactions between TGI and the ventilator, but also on the efficiency of TGI in decreasing dead-space and lowering the VE requirement.