The Pulmonary Blood

The measurement of blood volume in the pulmonary vascular bed during life became theoretically feasible with the introduction of indicator dilution methods by Stewart (1) and Hamilton, Moore, Kinsman and Spurling (2). The principles and formulas set forth by these investigators have since undergone extensive scrutiny by theoretical analysis, in circulation models, and in vivo. As a result of these studies, there exists now general agreement that the introduction of an indicator substance into the central circulation, either directly or by peripheral venous injection, and downstream recording of its concentration change with time during the first transit permits the determination of three important circulatory parameters: 1) the cardiac output, 2) the mean transit time from injection to sampling site, and 3) by multiplication of cardiac output and mean transit time, the circulating volume of blood between injection and sampling sites, including all temporally equidistant points in the vascular bed. Practically speaking, then, the measurement of pulmonary blood volume in vivo requires determination of the mean transit time of an indicator from the pulmonary artery to the left atrium together with cardiac output. The development of right and left heart catheterization has made the pulmonary artery and left atrium accessible as injection and sampling sites in man. Injection of an indicator through a