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THE pulmonary blood volume may be defined as the volume of blood interposed between the pulmonary artery, at the pulmonary valve, and the left atrium. The volume has been measured either on a relative or on an absolute basis, that is, by measurement of changes in pulmonary blood volume or by determination of the actual volume. The former method includes determination of lung volumes, or of pulmonary compliance, use of radioactive tracers to detect changes in radioactivity over specific areas of the lung, and use of a critically balanced teeter board.' Absolute values for pulmonary blood volume have generally been obtained by the indicatordilution technique in which the indicator is introduced into the right side of the heart and sampled from the systemic arterial tree.2 4 The volume measured varied with the injection site, that is, the peripheral venous tree, the right heart, or the pulmonary artery. The results have been analyzed by the StewartHamilton,2 3 the Bradley equilibration,5 and the Newman exponential downslope6 methods. Only the first of these approaches2 3 has proven to be applicable to the determination of pulmonary blood volume.' However, the blood volume so defined still varies with the exact injection and sampling sites employed. More recently Giuntini and associates7 described a precordial radioactive method for determining true pulmonary blood volume.The development of transseptal left heart catheterization has permitted more exact From tiie Section definition of the pulmonary blood volume by the indicator-dilution method. Three approaches have been utilized: In the first, indicator is injected into both the pulmonary artery and the left atrium and sampled from a systemic artery.8 9 The difference between the pulmonary-systemic artery volume and the left atrial-systemic artery volume is defined as the "true" pulmonary blood volume. In the second method the indicator is injected into the pulmonary artery and sampled from the left atrium.10 In the third method, indicator is injected into the superior vena cava or right atrium and sampled from the pulmonary artery and left atrium." The difference between the right atrial-left atrial volume and the right atrial-pulmonary artery volume is again defined as the "true" pulmonary blood volume. The purpose of this report is to compare the results of all three of these techniques for determination of pulmonary blood volume in 96 subjects. MethodsThe patients ranged from 17 to 64 years in age; they were studied in the fasting state. Rheumatic heart disease with mitral or aortic valve disease or both was the most common diagnosis. Congenital heart disease without a right-to-left or left-to-right shunt was present in some patients; a few were found to be free of heart disease after complete study. Indocyanine-green in doses of 2 to 3.5 mg together with saline flushes were utilized to inscribe the indicator-dilution curves from the various sites. A no. 5, 80-cm long Lehman catheter was passed into the superior vena cava or right atrium. A no. 6, 100-cm Lehman ...