Measurement of Blood Loss in Cardiac Surgery: Still Too Much

Cardiac surgery is associated with a significant decrease in hematocrit. It is unclear whether that occurs from hemodilution, loss of red cells, or both. Hematocrit is a major determinant of transfusion decisions although transfusion is associated with increased morbidity and mortality. Physicians must determine whether this anemia is the result of hemodilution or red blood cell loss as the former would be treated with packed red blood cell transfusions and the latter by diuresis. We hypothesize that the decrease in hematocrit observed in cardiac surgery is due to hemodilution.

Blood volume (BV), plasma volume (PV), and red blood cell volume (RBCV) were measured in 54 patients undergoing coronary artery bypass graft surgery, valve surgery, or coronary artery bypass graft/valve surgery. Measurements were made preoperatively, immediately postoperatively, and 2 hours after surgery utilizing a dilution tracer method and hematocrit measurements.

Preoperative average BV was 6,094 mL (SD 1,904 mL), RBCV was 2,024 mL (SD 720 mL), and PV was 4,070 mL (SD 1,339 mL). Postoperative average BV was 4,834 mL (SD 1,432 mL), RBCV 1,226 mL (SD 527 mL), and PV 3,607 mL (SD 993 mL). Blood volume decreased 18% (p < 0.0001), RBCV decreased 38% (p < 0.001), and PV decreased 8% (p < 0.012). There were no significant changes between postoperative values and those 2 hours later in the cardiac surgery intensive care unit.

Decreases in hematocrit observed in cardiac surgery patients are due to significant red blood cell losses and not to hemodilution. Red blood cell losses averaged 38%. Plasma volume also decreased.