Mechanisms of gas exchange response to lung volume reduction surgery in severe emphysema

Mechanisms of gas exchange response to lung volume reduction surgery in severe emphysema. J Appl Physiol 110: 1036-1045, 2011. First published January 13, 2011; doi:10.1152/japplphysiol.00404.2010.- Lung volume reduction surgery (LVRS) improves lung function, respiratory symptoms, and exercise tolerance in selected patients with chronic obstructive pulmonary disease, who have heterogeneous emphysema. However, the reported effects of LVRS on gas exchange are variable, even when lung function is improved. To clarify how LVRS affects gas exchange in chronic obstructive pulmonary disease, 23 patients were studied before LVRS, 14 of whom were again studied afterwards. We performed measurements of lung mechanics, pulmonary hemodynamics, and ventilation-perfusion (VA/Q ) inequality using the multiple inert-gas elimination technique. LVRS improved arterial PO₂ (PaO₂) by a mean of 6 Torr (P = 0.04), with no significant effect on arterial PCO₂ (PaCO₂), but with great variability in both. Lung mechanical properties improved considerably more than did gas exchange. Post-LVRS PaO₂ depended mostly on its pre-LVRS value, whereas improvement in PaO₂ was explained mostly by improved VA/Q inequality, with lesser contributions from both increased ventilation and higher mixed venous PO₂. However, no index of lung mechanical properties correlated with PaO₂. Conversely, post-LVRS PaCO₂ bore no relationship to its pre-LVRS value, whereas changes in PaCO₂ were tightly related (r² = 0.96) to variables, reflecting decrease in static lung hyperinflation (intrinsic positive end-expiratory pressure and residual volume/total lung capacity) and increase in airflow potential (tidal volume and maximal inspiratory pressure), but not to V A/Q distribution changes. Individual gas exchange responses to LVRS vary greatly, but can be explained by changes in combinations of determining variables that are different for oxygen and carbon dioxide.