Am. J. Respir. Crit. Care Med., Volume 161, Number 4, April 2000, 1115-1123

Physiologic Determinants of Ventilator Dependence in Long-term Mechanically Ventilated Patients

ANDREA PURRO, LORENZO APPENDINI, ANDREA DE GAETANO, MARTA GUDJONSDOTTIR, CLAUDIO F. DONNER, and ANDREA ROSSI

"Salvatore Maugeri" Foundation, IRCCS Rehabilitation Institute of Veruno, Veruno; Division of Pulmonary Disease, Ospedale Maggiore di Borgo Trento, Verona, Italy

To investigate the pathophysiologic mechanisms of ventilator dependence, we took physiologic measurements in 28 patients with COPD and 11 postcardiac surgery (PCS) patients receiving long-term 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_max and /Ppl_max > 0.4) and the effective inspiratory impedance was high (P_0.1/VT/TI  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).

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