99m Tc-labelled serum albumin in cardiac output and blood volume studies.

The use of human serum albumin labelled with iodine-131 (RIHSA) as the radioactive indicator in the measurement of cardiac output by the external counting technique is not ideal since the emission characteristics, physical half-life, and physiological fate restrict the amount that can be administered. A more suitable material, especially because of the short physical half-life of the isotope involved, is albumin labelled with technetium-99m. This communication describes a simple three-stage process for the preparation of such a complex which was then tested in subjects with no impairment of cardiac performance for its suitability in assays of cardiac output by the external counting method. Values were within the same range as those obtained with RIHSA in comparable subjects. Blood volumes estimated with 9"9Tc-albumin were within the limits of physiological variation of values derived with RIHSA in the same subjects. Serial blood samplings and urine collection during a period of 24 hours after administration showed that the preparation was lost continuously from the circulation at a more rapid rate than RIHSA. The greatest loss was in the first few hours when most of the urinary excretion of the isotope occurred. Comparison of the present data with published results using an alternative preparation suggests that our product may be more stable. Of the various methods available for the measurement of cardiac output, the external counting technique using a gamma-emitting isotope (Huff, Feller, Judd, and Bogardus, 1955; Veall and Vetter, 1958) is probably the least traumatic as far as the patient is concerned. Disturbance is minimal and, in particular, there is no requirement for catheterization of the heart or great vessels as is the case with the Fick and dye dilution methods. In general, '311-labelled human serum albumin (RIHSA) is used as the indicator but this material is not ideal since the emission characteristics restrict the amount that can be administered and the physical half-life and physiological fate leave a high circulating residuum in serial assays. Technetium-99m is potentially a better isotope as the emission characteristics permit more activity to be given and the short physical half-life contributes to a lower residuum. However, a prerequisite of the external counting method is the need for the indicator to remain intact in the circulation for at least 10 to 15 minutes after administration so that samples may be withdrawn for blood volume assay. To meet this condition the isotope must be combined to a relatively stable …