Gas Exchange in COPD

Abstract

For many years investigators, using measurements of respiratory gases and radioactive techniques, have suggested that considerable mismatching of ventilation and blood flow was likely to account for the major degree of the abnormal gas exchange seen in patients with advanced chronic obstructive pulmonary disease (COPD). Combining measurements of arterial blood respiratory gases and spirometry, as well as certain clinical features, Burrows et al. [1] 30 years ago classified COPD patients into two distinct categories. Type B patients, or “blue bloaters”, presented with marked cough and sputum production, recurrent cor pulmonale, and who were more likely to have hypoxia and hypercapnia. By contrast, the type A patients, or “pink puffers” were found, at autopsy, to have significant anatomic emphysema. Their gas exchange was characterized by a normal or low PaCO2 and only a mild decrease of the PaO2, including a low diffusing capacity. Unfortunately, it was soon realized that estimates based on traditional tools used for the measurement of gas exchange, such as the respiratory gases, are too insensitive for accurate assessment of the degree of ventilation-perfusion (\({{\overset{\cdot }{\mathop{V}}\,}_{A}}/\overset{\cdot }{\mathop{Q}}\,\)) mismatch actually present, hence leading to underestimation of the \({{\overset{\cdot }{\mathop{V}}\,}_{A}}/\overset{\cdot }{\mathop{Q}}\,\) inequality. Moreover, the information provided by such measurements is limited because values of respiratory gases are partly dependent on factors additional to \({{\overset{\cdot }{\mathop{V}}\,}_{A}}/\overset{\cdot }{\mathop{Q}}\,\) inequality, such as minute ventilation, cardiac output and oxygen uptake. By contrast, the multiple inert gas exchange technique gives a detailed picture of \({{\overset{\cdot }{\mathop{V}}\,}_{A}}/\overset{\cdot }{\mathop{Q}}\,\) relationship without inflicting by itself perturbation to pulmonary gas exchange [2, 3].