CPAP REDUCES INSPIRATORY WORK MORE THAN DYSPNEA DURING HYPERINFLATIONWITH INTRINSIC PEEP

Hyperinflation with intrinsic positive end-expiratory pressure (PEEPi) loads the respiratory muscles and causes dyspnea in obstructive lung disease. Continuous positive airway pressure (CPAP) has shown some eff icacy in reducing inspiratory work and dyspnea. However, in obstructiv e lung disease, inspiratory work and dyspnea may be increased by addit ional factors that may not be affected by CPAP. Therefore, to study th e effects of hyperinflation with intrinsic PEEP and CPAP in isolation, we used a mechanical analog of airway closure to increase end-expirat ory lung volume in normal subjects. In five subjects in whom inspirato ry work was measured, increasing end-expiratory lung volume by 1 and 2 L increased inspiratory work per breath from 0.42+/-0.04 J to 1.17+/- 0.15 J (p<0.05 compared with baseline) and 1.58+/-0.22 J (p<0.05 compa red with baseline and to the lesser level of hyperinflation), Although CPAP reduced work per breath and per minute to levels not significant ly different from baseline, it had little effect on dyspnea. In ten su bjects hyperinflated to 2.4+/-0.12 L above FlRC, breathing could be su stained 19.5+/-4.5 min before quitting the load This was increased to 26.7+/-5.2 minby 10 cm H2O CPAP (p=0.052). Inspiratory dyspnea was mod estly reduced by CPAP during these endurance trials, We conclude that CPAP can substantially ameliorate the respiratory work load induced by hyperinflation with intrinsic PEEP. However, the effects of CPAP on d yspnea and endurance are more limited. This suggests that the limits t o breathing at high lung volumes are related to factors in addition to respiratory muscle work, and that CPAP may be of more value in reduci ng the work than in relieving the distress of obstructive lung disease .