B.: Measurement of central blood volume by external monitoring. Circulation Research 9: 626

nique for measurement of central blood vol-ume (CBV), it has become apparent that the method may be insensitive to small but significant changes in volume. Reasons for this, presumably, rest with the rather large volume measured and the inherent errors of the technique.1 Using a peripheral vein in-jection, the volume measured may be as high as 60 per cent of the total blood volume.2 Only by using pulmonary-artery injection can it be reduced to 20 per cent of total blood volume.3 It would appear that, with these large vol-umes, small but significant changes (on the order of 100 to 200 cc.) could occur in various disease states or acute situations without being detected.1 ' 3 Use of radioactive materials and external monitoring appears to offer a method for measuring just the blood in the heart and lungs without the need for cardiac catheteri-zation. By the use of a sensitive counter over the precordium and a gamma-emitting iso-tope, separate peaks for right and left ventric-ular filling can be measured. The difference in time between the two peaks multiplied by the cardiac output gives a measure of pulmo-nary blood volume (PBV). Although the initial work with this technique was done by Prinzmetal in 1949,4 the central blood volume has not been studied in a large series of normal subjects. This investigation was under-taken to evaluate the technique in such a series. Methods Hospitalized ambulatory patients without evi-dence of cardiovascular disease were used in th