Assessing gas exchange

In clinical practice there are two sorts of measurements, a) arterial oxygen and carbon dioxide partial pressure (PaO2, PaCO2) or arterial oxygen saturation (SaO2), 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 (PaO2 falls and PaCO2 rises), the body compensates by increasing ventilation and lowering PaCO2). Therefore, a high PaCO2 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 PaCO2 falls and PaCO2 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.