Variability of indices of hypoxemia in adult respiratory distress syndrome

All indices of hypoxemia are affected by changes in FIO2 in patients with ARDS. PaO2/FIO2 ratio exhibits the most stability at FIO2 values of > or = 0.5 and PaO2 values of < or = 100 torr (< or = 13.3 kPa), and is a useful estimation of the degree of gas exchange abnormality under usual clinical conditions. Venous admixture varies substantially with alteration of FIO2 in patients who have clinically important ventilation/perfusion abnormalities. Under these circumstances, venous admixture is a poor indicator of the efficiency of pulmonary oxygen exchange, even if venous admixture is calculated from measured arterial and venous oxygen content values. Estimated venous admixture, based on an assumed arterial-venous oxygen content difference, is even more unreliable.