A critical comparison of radiocardiograms with
99mTc-and
131I-albumins was made to determine whether or not
99mTc-albumin can be used as a tracer for measuring blood volume (BV) and cardiac output (CO). Simultaneous injection of both isotopes was given to twenty-two patients to obtain radiocardiograms for each by means of a single scintillation counter with two pulse height analyzers. Such method allows for the determination of the energy levels for both
99mTc and
131I. The radiocardiographic pattern was somewhat different between
99mTc-and
131I-radiocardiograms : The ratio of left to right peak height was 16% on the average lower in the
99mTc-radiocardiogram than in the
131I-one. The reason for the lower ratio is explained by greater absorption of
99mTc-radioactivity within the soft tissue interposing between the scintillation probe and heart. The values for
99mTc-derived blood volume were 7.4% on the average larger than those for
131I-derived blood volume (p less than 0.01), although the correlation was excellent (r= 0.98, p less than 0.01). There was no systemic difference between cardiac output determined by
99mTc- and
131I-albumins since the overestimation of blood volume can compensate for the underestimation of CO/BV by
99mTc-study. In view of its rapid extravasation,
99mTc-albumin is not so appropriate tracer for accurate measurements of BV and CO/BV, although it may be used for measurement of cardiac output.
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