In healthy young subjects, the PAo2Pao2 is less than 10 mm Hg, and it increases to 10 to 20 mm Hg in the elderly. The ideal alveolar-arterial difference in partial pressure of oxygen (PAo2Pao2) can be used to distinguish hypoventilation from other causes of hypoxemia, 3 but it cannot be used to differentiate among diffusion abnormality, ventilation-perfusion ratio (V/Q) inequalities, or shunt. The ideal index is theoretically unaffected by changes in the fraction of inspired oxygen (FIo2) and does not require the sampling of mixed venous blood. 2 Therefore, several indices derived from ABG measurements have been developed to improve the accuracy of estimates of gas-exchange efficiency. Recent gas analyzers have transformed Pao2 measurement into a simple, rapid procedure with an acceptable risk of error, but Pao2 measurement alone is inadequate for estimating the efficiency of gas exchange. The direct measurement of ABGs is fundamental for clinical purposes. Hypoxemia is a major problem in respiratory medicine, both because it is very common and because its rapid evaluation and treatment are necessary to prevent irreversible organ damage. Practical aspects of ABG analysis are covered elsewhere, 1 but it is also important to understand derived ABG parameters and additional testing. Basic measurements obtained from an arterial blood sample may be insufficient, however, to give a complete picture of physiological impairment and to help clinicians determine appropriate treatment.

The measurement of arterial blood gases (ABGs) is an indispensable component of patient evaluation and follow-up in severe respiratory and renal diseases. Disease can profoundly impair lung function and may expose patients to increased risk for cell death.

Shunt vs dead space 100 oxygen