INSPIRATORY MUSCLE WORK OF BREATHING DURING FLOW-BY, DEMAND-FLOW, AND CONTINUOUS-FLOW SYSTEMS IN PATIENTS WITH CHRONIC OBSTRUCTIVE PULMONARY-DISEASE

The effect of flow-by continuous positive airway pressure (CPAP) in comparison with continuous-flow (CF) CPAP on inspiratory muscle work of breathing (WI) in intubated patients is not known. We hypothesized that WI during flow-by CPAP would be comparable with that during CF CPAR. In nine intubated male patients recovering from acute respiratory failure related to chronic obstructive pulmonary disease, we compared the effects of flow-by, demand-flow, and CF CPAP on WI. We also evaluated the extent to which the addition of 5 cm H2O of pressure support to demand-flow CPAP (DF-PS5) decreases WI. At CPAP levels of zero and 8 cm H2O, flow-by, demand-flow, DF-PS5 were applied in random order followed by CF CPAP for 15 min each. WI (expressed as Joules per liter and Joules per minute), maximal airway pressure drop during Inspiration (DELTA-Paw), total lung resistance (RL), lung compliance (CL), and ventilatory variables were measured. At CPAP of zero cm H2O, WI with flow-by was comparable with CF CPAP, and significantly less than with demand-flow CPAP, At both CPAP of zero and 8 cm H2O, the addition of 5 cm H2O PS to demand-flow CPAP reduced WI significantly to a level comparable with that of flow-by CPAP. At both CPAP levels, DELTA-Paw was the largest, with demand-flow CPAP. RL and CL were not significantly different between the different CPAP systems. We conclude that WI with flow-by CPAP is comparable with that with CF CPAP, and significantly less than with demand-flow CPAP. The addition of 5 cm H2O PS to demand-flow CPAP reduces WI to a level comparable with that of flow-by CPAP. The large DELTA-Paw, a result of the inability of the ventilator to meet the patient's early ventilatory demand, accounts for the greater WI during demand-flow CPAP.