Ventilation-perfusion mismatching in chronic obstructive pulmonary disease during ventilator weaning.

Using the multiple inert gas elimination technique, we studied ventilation-perfusion (VA/Q) relationships in eight patients with chronic obstructive pulmonary disease (COPD) during mechanical ventilation (MV) and again during weaning (spontaneous ventilation [SV] through an endotracheal tube) from MV needed for acute respiratory failure. The patients, seven men and one woman with a mean age of 63 +/- 2.8 (SEM) yr (FEV1 33 +/- 5.2% of predicted), required MV for 9.0 +/- 2.4 days prior to the study. The patients were studied at maintenance FIO2 (0.28 to 0.40) while breathing 100% O2, both during MV and SV. After 30 min of SV, PaCO2 increased from 48.9 +/- 3.4 to 58.3 +/- 3.1 mm Hg (p = 0.003) and pH decreased from 7.42 +/- 0.01 to 7.36 +/- 0.01 (p = 0.001) without significant changes in PaO2. Despite a decrease in tidal volume (VT) from 700.0 +/- 41.1 during MV to 313.0 +/- 39.6 ml during SV (p = 0.001), minute ventilation remained unchanged (from 8.2 +/- 0.7 during MV to 7.4 +/- 0.6 L/min during SV). Furthermore, cardiac output (QT), oxygen delivery (QO2), and mixed venous PO2 (PVO2) significantly rose during SV when compared with the MV (QT: from 4.7 +/- 0.4 to 6.7 +/- 0.7 L/min, p = 0.011; QO2: from 857.3 +/- 113.0 to 1078.5 +/- 158.9 ml/min, p = 0.0074; PVO2: from 36.7 +/- 1.1 to 42.3 +/- 2.2 mm Hg, p = 0.041). Overall VA/Q inequality worsened as blood flow was redistributed to low VA/Q areas (from 9.4 +/- 4.4 to 19.6 +/- 5.3% of QT, p = 0.05). The dispersion of the ventilation distribution (log SDV) significantly worsened during SV (from 1.0 +/- 0.08 during MV to 1.2 +/- 0.08 during SV, p = 0.044). No changes were observed in either series dead space or ventilation of high VA/Q ratio units.(ABSTRACT TRUNCATED AT 250 WORDS)