Pulmonary gas exchange response to weaning with pressure-support ventilation in exacerbated chronic obstructive pulmonary disease patients.

OBJECTIVE: To assess if pressure-support ventilation (PSV) can improve ventilation-perfusion (V(A)/Q) imbalance observed during the transition from positive-pressure ventilation to spontaneous breathing in intubated chronic obstructive pulmonary disease (COPD) patients during weaning.
DESIGN: Prospective study.
SETTING: Respiratory intensive care unit of a tertiary university hospital. PATIENTS: Seven mechanically ventilated COPD male patients (age 68+/-6 (SD) years; FEV(1) 26+/-6% predicted) during weaning.
INTERVENTIONS: Patients were studied during three ventilatory modalities: (1) assist-control ventilation (ACV), tidal volume (V(T)), 8-10 ml. kg(-1); (2) PSV aimed to match V(T)in ACV, 15+/-1 cmH(2)O and (3) spontaneous breathing. MEASUREMENTS AND
RESULTS: Arterial and mixed venous respiratory blood gases, V(A)/Q distributions, hemodynamics and breathing pattern were measured. Compared with both ACV and PSV, during spontaneous breathing patients exhibited decreases in V(T) (of 43%, p<0.001) and increases in respiratory rate (of 79%, p<0.001), PaCO(2) (of 8.5 mmHg, p=0.001), cardiac output (of 27%, p<0.001) and mixed venous oxygen tension (of 3.4 mmHg, p=0.003), while PaO(2) remained unchanged throughout the study. Except for a shift of the pulmonary blood flow distribution to areas with lower V(A)/Q ratios (p=0.044) and an increase of dead space (of 25%, p=0.004) during spontaneous breathing, no other changes in V(A)/Q distributions occurred. No differences were shown between ACV and PSV modalities.
CONCLUSION: In COPD patients during weaning, PSV avoided V(A)/Q worsening during the transition from positive-pressure ventilation to spontaneous breathing. Hemodynamics, blood gases or V(A)/Q mismatch were no different between ACV and PSV when both modalities provided similar levels of ventilatory assistance.