Simultaneous determinations of the total volume of red blood cells by use of carbon monoxide and chromium in healthy and diseased human subjects.

During recent years several methods have been introduced for estimating the total volume of circulating erythrocytes by measuring dilution in the blood stream of cells which have been tagged with radioactive phosphorus, iron, or chromium (1-6). Values for red cell volume thus obtained may be considered more reliable than indirect estimates based on hematocrit and "plasma volume" as measured with T-1824 or other substances which label plasma proteins, because no error is introduced by the discrepancy between venous (or arterial) and "body" hematocrits (7-9). For the same reason, a closer approximation to the true total blood volume is found by adding together red cell and "plasma" volumes, as measured independently, than is found by use of the plasma volume-hematocrit methods (10). Erythrocytes may also be labeled with carbon monoxide gas, and this is the basis of methods which have been employed widely to estimate blood volume and total circulating hemoglobin (11-17). It has long been suspected that the volume of distribution of carbon monoxide in the body (henceforth to be called "CO-available volume") may be larger than true blood volume, since the gas has affinity for pigments which are located extravascularly, such as myoglobin or non-circulating hemoglobin (11, 18, 19). Previous work has shown a discrepancy between CO-available volume and blood volume as measured with radioactively tagged cells, both in man (9) and in the dog (18).