Mechanical determinants of early acute ventilatory failure in COPD patients: A physiologic study

Objective: The purpose of this study is to investigate the respiratory mechanics, breathing pattern, and pressure-generating capacity of respiratory muscles during the early phases of an acute exacerbation of COPD. Design: Prospective study. Setting: Division of Emergency Critical Care and Chronic Ventilator Unit. Patients: A total of 24 COPD patients: nine patients requiring ventilatory support because of acute respiratory acidosis due to COPD exacerbation (NPPV group, pH 7.28 ± 0.02); seven patients successfully managed with medical therapy only (SB group, pH 7.39 ± 0.04); eight clinically stable, long term mechanically ventilated, COPD patients (IPPV group). Measurements: Respiratory mechanics during a period of unsupported breathing. Results: A rapid shallow breathing, in the presence of a high drive to breath and a high diaphragmatic tension-time index (TT_di), was found in NPPV and IPPV groups compared to the SB group (f/V _T ratio: 118 ± 43 and 137 ± 65, respectively, versus 37 ± 12 breaths/min/L; P _0.1: 5.0 ± 1.0 and 5.4 ± 1.4, respectively, versus 2.2 ± 0.2 cmH₂O, TT_di: 0.168 ± 0.035 and 0.161 ± 0.039, respectively, versus 0.057 ± 0.033); at variance, PEEPi_dyn was greater in IPPV compared to the other two groups. A significant relationship was observed between TT _di ratio and f/V _T (Rho 0.756). Conclusion: During the early phases of an acute exacerbation, patients with COPD and acute respiratory failure had an imbalance between the decreased capacity of the respiratory muscles to generate pressure and the increased respiratory load. This imbalance was similar to that recorded in patients with COPD and chronic ventilatory failure. In both groups, the imbalance was associated with rapid shallow breathing. Among the mechanical constraints to ventilation, only PEEPi,dyn was different between acute and chronic patients with ventilatory failure.