Dynamic behavior during noninvasive ventilation - Chaotic support?

Acute noninvasive ventilation is generally applied via face mask, with modi fied pressure support used as the initial mode to assist ventilation. Altho ugh an adequate seal can usually be obtained, leaks frequently develop betw een the mask and the patient's face. This leakage presents a theoretical pr oblem, since the inspiratory phase of pressure support terminates when flow falls to a predetermined fraction of peak inspiratory flow. To explore the issue of mask leakage and machine performance, we used a mathematical mode l to investigate the dynamic behavior of pressure-supported noninvasive ven tilation, and confirmed the predicted behavior through use of a test lung. Our mathematical and laboratory analyses indicate that even when subject ef fort is unvarying, pressure-support ventilation applied in the presence of an inspiratory leak proximal to the airway opening can be accompanied by ma rked variations in duration of the inspiratory phase and in autoPEEP. The u nstable behavior was observed in the simplest plausible mathematical models , and occurred at impedance values and ventilator settings that are clinica lly realistic.