Objective: To elucidate the effects of continuous positive airway pressure (CPAP) on breathing pattern, gas exchange and the ability to sustain spontaneous breathing (SB) in chronic obstructive pulmonary disease (COPD) patients with dynamic hyperinflation.
Design: Prospective study with two randomised trials of SB without and with CPAP in each patient.
Setting: Medical intensive care units (ICUs) in two university hospitals.
Patients: Nine dynamically hyperinflated, intubated COPD patients recuperating from acute exacerbation.
Interventions: One SB trial with CPAP (5-7.5 cmH2O), one without (control) in each patient.
Measurements: airway opening pressure, gas flow and thus breathing pattern, oxygen uptake, carbon dioxide excretion, arterial blood gases, dyspnoea and respiratory drive (P100).
Results: With CPAP, intrinsic positive end-expiratory pressure (PEEPi) fell from 11.4 to 6.3 cm H2O (p < 0.05). Eight patients sustained SB with CPAP for the maximum time planned (30 min), one failed after 18 min. In contrast, only four patients successfully completed the control trial, the others failing after 5-18 min (p < 0.05). Dyspnoea-gauged on a visual analogue scale by five patients--was less severe or occurred later with CPAP. Breathing with CPAP tended to be slower (18.9 vs 22.2 min(-1), p < 0.05) and deeper (tidal volume 370 vs 323 ml). At the end of the control run, PaCO2 was higher (60 vs 55 mmHg, p < 0.05) and still rising while being stable at the end of the CPAP trial.
Conclusion: CPAP helps severely ill COPD patients sustain SB. Apparently it does so by promoting slower, deeper breathing and thus facilitating carbon dioxide elimination.