Gas analysis of blood samples, obtained from different sites in the heart and great vessels, permits demonstration of admixture of oxygenated blood in the venous side or admixture of venous blood in the arterial side of the circulation.
1951 March, J. H. Lehmann, A. D. Johnson, W. C. Bridges, J. Michel, D. M. Green, “Cardiac Catheterization—A Diagnostic Aid in Congenital Heart Disease”, in Northwest Medicine, volume 50, number 3, Portland, Ore.: Northwest Medical Publishing Association, page 170