Gas Exchange in Obstructive Lung Disease

Abstract

Structural and functional changes in the lungs in severe obstructive lung disease produce gas exchange abnormalities. Consequently, one might expect to be able to use gas exchange measurements, in addition with clinical examination, as a guide to progress in patients management, and this is generally attempted through the use of arterial blood gases. The problem is that arterial PO₂ is determined not only by pulmonary factors, but also by extrapulmonary factors which cause changes in mixed venous PO₂. Table 1 shows the pulmonary and extrapulmonary factors affecting arterial PO₂. Among the pulmonary factors, inspired oxygen concentration and alveolar ventilation can be manipulated to some extent by the physician. Ventilation-perfusion inequality, shunt and diffusion disequilibrium, on the other hand, are more intrinsic functions reflecting the state of health of the lungs and only indirectly improved by therapeutical interventions.