Oxygen derived variables in acute respiratory failure.

The clinical course of 33 patients with acute respiratory distress syndrome (ARDS) was monitored by noninvasive oxygen derived variables and compared to data obtained by invasive monitoring. A total of 350 data points were used to compare the physiologic shunt fraction (Qsp/Qt) with the ratio of arterial oxygen to inspired oxygen concentration (PaO2/FIO2), the alveolar-arterial oxygen pressure difference [P(A-a)O2], the respiratory index (RI)-[P(A-a)O2/PaO2], and the ratio of arterial oxygen to alveolar oxygen (a/A). The PaO2/FIO2 ratio, the RI and the aA ratio correlated well with Qsp/Qt (r = 0.87 to 0.94). The P(A-a)O2 correlated less well (r = 0.68). Changes in the cardiac index (CI) and the arteriovenous oxygen content difference C(a-v)O2 had only a minimal effect on the correlation of the oxygen derived variables with Qsp/Qt, although a higher correlation resulted when these extrapulmonary factors were within normal range. We conclude that a number of oxygen derived variables may accurately reflect the degree of Qsp/Qt. The PaO2/FIO2 ratio is the easiest of these variables to calculate, yet accurately predicts the degree of Qsp/Qt throughout a course of acute respiratory failure.