PROCEDURE 33 • Arterial partial pressure of oxygen (PaO2) is primarily determined by the concentration of inspired oxygen and the amount of carbon dioxide in the alveolus.1–3 • In healthy lungs, alveolar oxygen diffuses rapidly into the pulmonary capillaries, and arterial oxygenation approximates that of the alveolus. The normal alveolar-arterial oxygen difference (A-aDO2) in a patient breathing 21% oxygen is 10 to 20 mm Hg (i.e., PAO2 [100], PaO2 [80]). When 100% oxygen is inspired, the normal gradient is 50 to 70 mm Hg.1–3 • Trends in alveolar-arterial (A-a) gradient are evaluated most accurately when the PAO2 and PaO2 are measured on room air or after inspiration of 100% oxygen for 15 minutes.1–3 • Other clinical indices of oxygenation that are commonly used include the PaO2😛AO2 (a:A) ratio and the ratio of PaO2 to fraction of inspired oxygen (FiO2; P:F). These indices all are relatively easy to use and are helpful in estimates of trends in hypoxemia.1,4,5 • The advantage of the a:A and P:F ratios is that a more constant value, despite changes in FiO2, can be calculated. A normal a:A ratio is 0.8 to 1. The smaller the number, the higher the degree of shunt. The normal value for P:F ratio is greater than 300. A smaller P:F ratio reflects a higher degree of shunt. A P:F ratio of 200 to 300 is used to define acute lung injury (ALI), whereas a P:F ratio of less than 200 is associated with acute respiratory distress syndrome (ARDS).4,5 • In patients with shunt (perfusion to unventilated lung units), venous blood is shunted past the closed alveoli without becoming oxygenated. Although PaO2 may be “normal” because of an increase in FiO2, a shunt exists. The A-a gradient increases. A-a gradient is considered a useful, albeit crude, clinical estimate of shunt. The A-a gradient value is helpful to trend changes in oxygenation status, the effect of therapies and other interventions. • Concepts related to shunt and the refractory nature of shunt to increasing FiO2 are inherent in all the indices (i.e., shunt is not responsive to oxygen). Although other reasons for hypoxemia exist in addition to shunt (i.e., hypoventilation on room air, diffusion block, ventilation to perfusion [V/Q] mismatch), the indices are generally most often used in the most severe pulmonary conditions that affect oxygenation. In these conditions (e.g., ARDS), quantification of the degree of shunt is helpful to trend the progress of the disease, determine the efficacy of therapies, and aid in prognosis.1–3,5
Indices of Oxygenation
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33: Indices of Oxygenation
