VARIABILITY OF INDEXES OF HYPOXEMIA IN ADULT-RESPIRATORY-DISTRESS-SYNDROME

Objective: To determine the usefulness of indices of hyperemia in asse ssing patients with the adult respiratory distress syndrome (ARDS). De sign: Retrospective analysis of previously published data that describ e the distributions of ventilation and pulmonary blood flow in ARDS. S etting: University research laboratory. Patients: Sixteen patients wit h ARDS. Interventions: The FIO2 was varied between 0.21 and 1.0 in a c omputer model of gas exchange, based on a 50 compartment model of vent ilation/perfusion inhomogeneity plus true shunt and deadspace. The ind ices of hypoxemia that were calculated as a function of inspired oxyge n concentration included Pao(2)/Flo(2), arterial/alveolar ratio (Pao(2 )/alveolar Po-2), the alveolar-arterial Po-2 difference (P[A-a]o(2)), respiratory index (P[A-a]o(2)/Pao(2)), and venous admixture. Measureme nts and Main Results: The Pao(2)/FIO2 ratio in patients with moderate shunts (<30%) varied considerably with alteration in FIO2. At both ext remes of FIO2, the Pao(2)/FIO2 in these patients was substantially gre ater than at intermediate FIO2. Patients with larger shunts (>30%) had greater Pao(2)/FIO2, ratios at low FIO2, but the Pao(2)/FIO2, ratios decreased to relatively stable values at FIO2 values of >0.5. In all p atients, Pao(2)/FIO2, remained relatively stable at FIO2 values of gre ater than or equal to 0.5 and Pao(2) values of less than or equal to 1 00 torr (less than or equal to 13.3 kPa). Other Po-2-based indices exh ibited less stability as Flo(2) was varied. If hypoxemia resulted from true shunting, venous admixture was found to be stable at all FIO2 va lues. However, approximately one half of patients had clinically impor tant hypoxemia resulting from mismatching of ventilation and blood flo w. In these patients, venous admixture varied substantially with chang e in FIO2, and the degree of variation was proportional to the fractio n of cardiac output perfusing gas exchange units with ventilation/perf usion ratios of <0.1. Conclusions: All indices of hypoxemia are affect ed by changes in Flo(2) in patients with ARDS. Pao(2)FIO(2), ratio exh ibits the most stability at FIO2 values of greater than or equal to 0. 5 and Pao(2) values of less than or equal to 100 torr (less than or eq ual to 13.3 kPa), and is a useful estimation of the degree of gas exch ange abnormality under usual clinical conditions. Venous admixture var ies substantially with alteration of FIO2 in patients who have clinica lly important ventilation/perfusion abnormalities. Under these circums tances, venous admixture is a poor indicator of the efficiency of pulm onary oxygen exchange, even if venous admixture is calculated from mea sured arterial and venous oxygen content values. Estimated venous admi xture, based on an assumed arterial-venous oxygen content difference, is even more unreliable.