Objectives: Proportional-assist ventilation (PAV) is able to unload respiratory muscles in proportion to the subject's inspiratory effort. However, leak-related alterations in the flow signal, effort-induced modifications in respiratory mechanics, or approximate adjustment of PAV could jeopardize such a theory. The aim of this study was to compare noninvasive PAV and pressure-support ventilation (PSV) in healthy volunteers with external thoracic restriction at rest and during exercise.
Design: Prospective, crossover, randomized study.
Setting: Investigation unit in a nonteaching hospital.
Patients: Seven volunteers with external thoracic restriction.
Intervention: After external thoracic restriction to increase elastance (9.00 +/- 1.63 cm H2O/L estimated from the level of elastic assistance), PAV and PSV were compared at rest and during exercise (90 W for 10 mins).
Measurements and main results: Flow, airway pressure, and changes in esophageal pressure were measured, and the tidal volume (Vt) and inspiratory muscle effort indexes were calculated. At rest, all variables were comparable during PSV and PAV. Exercise produced a 200% increased in Vt with no change in the breathing frequency and a 400% increased in inspiratory muscle effort indexes. During exercise, peak inspiratory airway pressure was significantly higher with PAV than with PSV (24 +/- 5 vs. 10 +/- 2 cm H2O, p <.05). The Vt and breathing frequency (23 +/- 4 vs. 24 +/- 3 breaths/min) were similar, but the inspiratory muscle effort indexes were significantly lower with PAV than with PSV. A significant linear correlation was found between changes in esophageal pressure and the peak inspiratory airway pressure during PAV (r =.94, p =.0001), whereas, as expected, it was not the case during PSV (r =.27, p =.34).
Conclusion: In volunteers with external thoracic restriction mimicking a patient with increased elastic work of breathing, the breathing pattern at rest and during exercise were comparable with PSV and PAV, whereas inspiratory muscle effort was lower with PAV during exercise because of the significant automatic increase in assistance with PAV.