Physiologic determinants of ventilator dependence in long-term mechanically ventilated patients

To investigate the pathophysiologic mechanisms of ventilator dependence, we took physiologic measurements in 28 patients with COPD and 11 postcardiac surgery (PCS) patients receiving longterm mechanical ventilation during a spontaneous breathing trial, and in 20 stable, spontaneously breathing patients matched for age and disease. After 40 +/- 14 min of spontaneous breathing, 20 of 28 patients with COPD and all 11 PCS patients were judged ventilator-dependent (VD) We found that in the 31 VD patients tidal volume was low (VT: 0.36 +/- 0.12 and 0.31 +/- 0.08 L for COPD and PCS, respectively), neuromuscular drive was high (P-0.1: 5.6 +/- 1.6 and 3.9 +/- 1.9 cm H2O), inspiratory muscle strength was reduced (Pdi(max): 42 +/- 12 and 28 +/- 15 cm H2O), and lung mechanics were abnormal, particularly PEEPi (5.9 +/- 3.0 cm H2O) and lung resistance (22.2 +/- 9.2 cm H2O/L/s) in COPD. The load/capacity balance was altered (Pdi /Pdi(max) and Ppl /Ppl(max) > 0.4) and the effective inspiratory impedance was high (P-0.1/VT/TI greater than or equal to 10 cm H2O/L/s). Failure to wean occurred in patients with f/VT > 105 breaths/min/L and 56% of patients with COPD with f/VT < 80 breaths/min/L. Those who failed despite a low f/VT ( < 80 breaths/min/L) either showed ineffective inspiratory efforts, which artificially lowered f/ VT (n = 8), or did not increase breathing frequency (n = 5), but P-0.1 and P-0.1/VT/TI were as high as in other VD patients. In the 31 VD patients, Pa-CO2 increased during the weaning trial (+12.3 +/- 8.0 mm Hg). We conclude that in the presence of a high drive to breathe, the imbalance between increased work load and reduced inspiratory muscle strength causes respiratory distress and CO2 retention. Noninvasive measurements (breathing pattern, P-0.1, P-0.1/ VT/TI) may give better insight into weaning failure useful in clinical decision-making, particularly in patients with COPD not showing rapid shallow breathing (56% in this study).