Effects of the components of positive airway pressure on work of breathing during bronchospasm

INTRODUCTION Partial assist ventilation reduces work of breathing in patients with bronchospasm; however, it is not clear which components of the ventilatory cycle contribute to this process. Theoretically, expiratory positive airway pressure (EPAP), by reducing expiratory breaking, may be as important as inspiratory positive airway pressure (IPAP) in reducing work of breathing during acute bronchospasm. METHOD We compared the effects of 10 cmH2O of IPAP, EPAP, and continuous positive airwaypressure (CPAP) on inspiratory work of breathing and end-expiratory lung volume (EELV) in a canine model of methacholine-induced bronchospasm. RESULTS Methacholine infusion increased airway resistance and work of breathing. During bronchospasm IPAP and CPAP reduced work of breathing primarily through reductions in transdiaphragmatic pressure per tidal volume (from 69.4 +/- 10.8 cmH2O/l to 45.6 +/- 5.9 cmH2O/l and to 36.9 +/- 4.6 cmH2O/l, respectively; P < 0.05) and in diaphragmatic pressure-time product (from 306 +/- 31 to 268 +/- 25 and to 224 +/- 23, respectively; P < 0.05). Pleural pressure indices of work of breathing were not reduced by IPAP and CPAP. EPAP significantly increased all pleural and transdiaphragmatic work of breathing indices. CPAP and EPAP similarly increased EELV above control by 93 +/- 16 ml and 69 +/- 12 ml, respectively. The increase in EELV by IPAP of 48 +/- 8 ml (P < 0.01) was significantly less than that by CPAP and EPAP. CONCLUSION The reduction in work of breathing during bronchospasm is primarily induced by the IPAP component, and that for the same reduction in work of breathing by CPAP, EELV increases more.