Changes in venous admixture with alterations of inspired oxygen concentration

To assess the change in venous admixture during breathing of 100 per cent oxygen (FI(O₂) 1.0), shunt fraction (Q̇(s)/Q̇(t)) was calculated at a maintenance FI(O₂) (FI(O₂)(m): 0.27-0.70) and at FI(O₂) 1.0 in 40 studies of 34 patients with acute respiratory failure. At FI(O₂) 1.0 Q̇(s)/Q̇(t) increased in 26 studies, but did not increase in 14 studies. Patients in whom Q̇(s)/Q̇(t) increased during breathing of oxygen had mild respiratory failure, as indicated by low Q̇(s)/Q̇(t) values at FI(O₂)(m) and a low incidence of diffuse parenchymal infiltrates on chest roentgenograms. All patients who had recently had cardiovascular surgical procedures were in this group. Respiratory failure was more severe in those in whom Q̇(s)/Q̇(t) decreased with oxygen, as indicated by high Q̇(s)/Q̇(t) values at FI(O₂)(m), evidence of diffuse pulmonary disease by roentgenography, and signs of adult respiratory distress syndrome. The authors conclude that changes in Q̇(s)/Q̇(t) in response to FI(O₂) 1.0 in acute respiratory failure are related to the severity of respiratory insufficiency.