Eleven adult patients who underwent cardiac surgery with extracorporeal circulation were studied with serial blood-volume determinations by radioactive-tag methods. Deficits were demonstrated postoperatively in ten, despite precise measurement and replacement of lost blood. As the usual clinical observations may not accurately reflect hypovolemia, direct measurement, when properly done, is a more reliable method for the estimation of blood volume. Of the two methods employed for measurement of blood volume, the use of radioactive chromium to tag red blood cells appears to be the more reliable when a single sampling technique is used.
McClenahan JB, Yamauchi H, Roe BB. Blood-Volume Studies in Cardiac-Surgery Patients. JAMA. 1966;195(5):356-361. doi:10.1001/jama.1966.03100050064020