Cardiovascular and Hematologic Responses to Phlebotomy in Blood Donors*

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Abstract
A study was made of the cardiovascular and hematologic responses of 28 male and 13 female normal blood donors to withdrawal of about 500 ml. of blood. During the bleeding there was a predominant tendency toward a fall in systolic and a rise in diastolic blood pressure, a mild rise in net peripheral resistance, a slight fall in cardiac output and respiratory rate with little change in heart rate. The prothrombin time and partial thromboplastin time were unchanged but there was a significant fall in plasma clotting time. Immediately upon cessation of bleeding the hemoglobin and hematocrit values indicated a “dilution quotient” of 2 to 2.5 per cent but plasma fibrinogen, plasma prothrombin and leukocyte counts suggested a prompt and rapid over‐dilution of the plasma. In three donor reactions occurring in the course of the study, the primary changes were a sharp fall in blood pressure, peripheral resistance and pulse rate. It is suggested that the mechanism of post‐phlebotomy fainting may involve undue sensitivity of the intracranial circulation to neurohumoral agents, inadequacy in the functional “central blood volume” or deficient peripheral vascular reflexes.