Review Series: Lung function made easy: Assessing gas exchange

Abstract

In clinical practice there are two sorts of measurements, a) arterial oxygen and carbon dioxide partial pressure (PaO 2 , PaCO 2 ) or arterial oxygen saturation (SaO 2 ), and b) the transfer capacity for carbon monoxide (TLCO). The former measures the output or performance of the lung as a gas exchanger, and the latter estimates the available surface area or potential for gas exchange. As gas exchange deteriorates (PaO 2 falls and PaCO 2 rises), the body compensates by increasing ventilation and lowering PaCO 2 . Therefore, a high PaCO 2 represents chronic respiratory or “compensation” failure, either chemo-insensitivity (“won't breathe”) or neuromuscular weakness/increased work of breathing (“cannot breathe”). Chronic respiratory failure may progress to acute failure in which PaCO 2 falls and PaCO 2 rises progressively, assisted ventilation is usually required. The TLCO is a laboratory test which measures the integrity of the blood-gas barrier, it is particularly useful in the assessment of emphysema, interstitial disease and pulmonary vascular disease. Chronic Respiratory Disease 2007; 4: 205—214