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 s pontaneous breathing trial, and in 20 stable, spontaneously breathing patie nts matched for age and disease. After 40 +/- 14 min of spontaneous breathi ng, 20 of 28 patients with COPD and all 11 PCS patients were judged ventila tor-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), ne uromuscular drive was high (P-0.1: 5.6 +/- 1.6 and 3.9 +/- 1.9 cm H2O), ins piratory 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/capaci ty balance was altered (Pdi /Pdi(max) and Ppl /Ppl(max) > 0.4) and the effe ctive 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 breath s/min/L and 56% of patients with COPD with f/VT < 80 breaths/min/L. Those w ho failed despite a low f/VT ( < 80 breaths/min/L) either showed ineffectiv e 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 hi gh 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 o f a high drive to breathe, the imbalance between increased work load and re duced inspiratory muscle strength causes respiratory distress and CO2 reten tion. Noninvasive measurements (breathing pattern, P-0.1, P-0.1/ VT/TI) may give better insight into weaning failure useful in clinical decision-makin g, particularly in patients with COPD not showing rapid shallow breathing ( 56% in this study).