Determination of Total Blood Volume from Measurements of Total Red Blood Cell Mass and Plasma Volume, using Simultaneously Injected Isotopes

Abstract

IN both clinical practice and, experimental research, isotope dilution methods^1–7 are being used routinely for the determination of total blood volume. The isotopes most commonly used are chromium.-51 (labelled red blood cells) and iodine-131 (serum albumin), both γ-emitting isotopes. Chromium-51-labelled red blood cells are used to obtain the total red blood cell circulating mass, which, in conjunction with the haematocrit, allows for an indirect determination of the total blood volume. Conversely, radioactive iodinated serum albumin gives a measure of total plasma volume and, with the hæmatocrit, an indirect measurement of the total blood volume. The use of either of these methods alone is subject to criticism, since the final calculations for total blood volume are based on the assumption that the peripheral hæmatocrit is the same as, or represents, a constant percentage of the total body hæmatocrit. It has been demonstrated that these assumptions are not valid^5–14. The ideal method for excluding hæmatocrit error would be to measure the red cell portion and the plasma portion of the vascular compartment separately, the total blood volume then being the sum of these two independent measurements. Several methods^3,9,15,16 have appeared utilizing chromium-51-tagged red cells and radioactive iodinated serum albumin in the same subject for measurements of total circulating red cell mass and total plasma volume, respectively. Some methods^5,19 have combined repetitive injections of the respective isotopes; whereas others, the simultaneous injection of both. In those methods¹⁹ where the isotopes are injected separately, it is necessary to obtain repeated samplings of the subject's blood, which in clinical practice may be only a slight inconvenience to the patient, but is, with small animals, a significant handicap. Repeated bleeding and intravenous injecting of small laboratory animals are not only difficult but also may create stress in the animals which could influence the results in the final determinations.