ObjectiveTo determine the usefulness of indices of hypoxemia in assessing patients with the adult respiratory distress syndrome (ARDS).DesignRetrospective analysis of previously published data that describe the distributions of ventilation and pulmonary blood flow in ARDS.SettingUniversity research laboratory.PatientsSixteen patients with ARDS.InterventionsThe FIO2was varied between 0.21 and 1.0 in a computer model of gas exchange, based on a 50-compartment model of ventilation/perfusion inhomogeneity plus true shunt and deadspace. The indices of hypoxemia that were calculated as a function of inspired oxygen concentration included PaO2/FIO2, arterial/alveolar ratio (PaO2/alveolar PO2), the alveolar-arterial PO2difference (P[A-a]O sub 2), respiratory index (P[A-a]O2/PaO2), and venous admixture.Measurements and Main ResultsThe PaO2/FIO2ratio in patients with moderate shunts (<30%) varied considerably with alteration in FIO2. At both extremes of FIO2, the PaO2/FIO2in these patients was substantially greater than at intermediate FIO230%) had greater PaO2/FIO2ratios at low FIO2, but the PaO2/FIO2ratios decreased to relatively stable values at FIO20.5. In all patients, PaO2/FIO2remained relatively stable at FIO2or=to0.5 and PaO2values of <or=to100 torr (<or=to13.3 kPa). Other PO2-based indices exhibited less stability as FIO2was varied. If hypoxemia resulted from true shunting, venous admixture was found to be stable at all FIO2values. However, approximately one half of patients had clinically important hypoxemia resulting from mismatching of ventilation and blood flow. In these patients, venous admixture varied substantially with change in FIO2, and the degree of variation was proportional to the fraction of cardiac output perfusing gas exchange units with ventilation/perfusion ratios of <0.1.ConclusionsAll indices of hypoxemia are affected by changes in FIO2in patients with ARDS. PaO2/FIO2or=to0.5 and PaO2values of <or=to100 torr (<or=to13.3 kPa), and is a useful estimation of the degree of gas exchange abnormality under usual clinical conditions. Venous admixture varies substantially with alteration of FIO sub 2 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.(Crit Care Med 1997; 25:41-45)