Ventilation-Perfusion Mismatching in Chronic Obstructive Pulmonary Disease during Ventilator Weaning

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Abstract
Using the multiple inert gas elimination technique, we studied ventilation-perfusion (a/) 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 (t), oxygen delivery (O2), and mixed venous PO2 (PO2) significantly rose during SV when compared with the MV (t: from 4.7 ± 0.4 to 6.7 ± 0.7 L/min, p = 0.011; O2: from 857.3 ± 113.0 to 1078.5 ± 158.9 ml/min, p = 0.007; PO2: from 36.7 ± 1.1 to 42.3 ± 2.2 mm Hg, p = 0.041). Overall a/ inequality worsened as blood flow was redistributed to low a/ areas (from 9.4 ± 4.4 to 19.6 ± 5.3% of t, 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 a/ ratio units. During both MV and SV, administration of 100% O2 did not increase pulmonary shunting although the dispersion of the pulmonary blood flow distribution (log SDQ) increased significantly (from 1.40 ± 0.1 to 1.56 ± 0.2 during MV and from 1.47 ± 0.2 to 1.63 ± 0.2 during SV, p = 0.014 each; normal range, 0.3 to 0.6), suggesting release of hypoxic pulmonary vasoconstriction. Our results show that discontinuing MV in these patients leads to increased a/ mismatch associated with both a less efficient breathing pattern and changes in t. The increase of cardiac output prevented a drop in PaO2 when patients were removed from MV. Thus, both intrapulmonary and extrapulmonary factors change acutely during weaning from MV. Despite the impairment in a/ inequality observed during SV, all patients could be satisfactorily weaned.