Effect of tracheal gas insufflation 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 (V over dot E). However, TGI might also impair the ability to trigger the ventilatory, because to lower external circuit pressures, inspiratory effort must outstrip catheter flow rate (V over dot c) 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 now pattern with a tidal volume (VT) Of 0.5 L at various peak inspiratory flow rates (V over dot pk) to simulate differences in effort intensity. The L-chamber was connected to a 60-L/min continuous now circuit with a 10 cm H2O positive end-expiratory pressure valve and to 3 different ventilator 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 V over dot c and decreased with increasing V over dot pk. The L-ventilator failed to trigger when V over dot c was 15 L/min and V over dot pk was 20 L/min. At a fixed V over dot E, 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 V over dot E requirement.