Invited Commentary

Abstract

Hemoglobin (Hgb) concentrations and hematocrit (HCT) percents are traditional assays in the operating room and intensive care unit used to monitor oxygen carrying capacity, to monitor blood loss, and to make transfusion decisions. However, it has long been recognized that as concentration measurements, Hgb and HCT are not directly proportional to the total oxygen carrying capacity because of multiple factors that include perioperative changes in blood volume [1Jones J.G. Holland B.M. Hudson I.R. Wardrop C.A. Total circulating red cells versus haematocrit as the primary descriptor of oxygen transport by the blood.Br J Haematol. 1990; 76: 288-294Crossref PubMed Scopus (49) Google Scholar]. This recognition has led to searches for other, more direct assays that reflect anemia-induced tissue hypoxia, such as oxygen extraction ratios [2Goodnough L.T. Despotis G.J. Hogue Jr., C.W. Ferguson Jr, T.B. On the need for improved transfusion indicators in cardiac surgery.Ann Thorac Surg. 1995; 60: 473-480Abstract Full Text PDF PubMed Scopus (81) Google Scholar], methemoglobin levels [3Tsui A.K. Dattani N.D. Marsden P.A. et al.Reassessing the risk of hemodilutional anemia: some new pieces to an old puzzle.Can J Anaesth. 2010; 57: 779-791Crossref PubMed Scopus (33) Google Scholar], and sublingual microcirculatory oximetry [4Yuruk K. Almac E. Bezemer R. Goedhart P. de Mol B. Ince C. Blood transfusions recruit the microcirculation during cardiac surgery.Transfusion. 2011; 51: 961-967Crossref PubMed Scopus (66) Google Scholar]. Despite currently available technology, clinicians still use Hgb and HCT as their guides to red blood cell (RBC) transfusions. In this article, George and colleagues [5George T.J. Beaty C.A. Kilic A. et al.Hemoglobin drift after cardiac surgery.Ann Thorac Surg. 2012; 94: 703-709Abstract Full Text Full Text PDF PubMed Scopus (16) Google Scholar] and the critical care team retrospectively evaluated 200 adult patients undergoing cardiac surgery to define the natural progression of postoperative Hgb levels. This report illustrates the problem of using Hgb levels to evaluate anemia. Cardiac surgical patients have multiple blood volume and RBC volume changes because of a multitude of factors, including blood shed from the surgical site, blood salvaged, crystalloid/colloid infusion, and other volume shifts caused by diuresis, chest tube drainage, and third spacing. Another consideration is frequent blood drawing in the intensive care unit because of blood gas determinations and laboratory tests that also may potentially contribute to anemia. Postoperatively, the authors have identified an average decline, or “hemoglobin drift,” of 1.1 ± 1.4 g/dL, or the equivalent of one unit of blood, followed by a mean recovery of Hgb level 0.7 ± 0.7 g/dL. These effects on Hgb concentration need to be taken into consideration when physicians make transfusion decisions based on Hgb concentration measurements. Alternative indicators for blood transfusion therapy are needed, such as quantitative measurements of RBC volume losses. Decisions to transfuse should be made on the basis of the percentage of a patient's baseline RBC volume that is lost [6Goodnough L.T. Verbrugge D. Vizmeg K. Riddell J.T. Identifying elective orthopedic surgical patients transfused with amounts of blood in excess of need: the transfusion trigger revisited.Transfusion. 1992; 32: 648-653Crossref PubMed Scopus (67) Google Scholar], inasmuch as the very definition of anemia is a loss of RBC volume. Finally, new technologies with noninvasive capabilities for determining tissue oxygen saturation and tissue Hgb index (an index of the amount of Hgb in a near-infrared spectroscopy volume) have recently shown promise [7Yuruk K. Bartels S.A. Milstein D.M. Bezemer R. Biemond B.J. Ince C. Red blood cell transfusions and tissue oxygenation in anemic hematology outpatients.Transfusion. 2012; 52: 641-646Crossref PubMed Scopus (39) Google Scholar]. Additional studies are needed to better understand fluctuations in Hgb concentrations that occur postoperatively and whether we should continue to use this kind of information to define transfusion decisions. Hemoglobin Drift After Cardiac SurgeryThe Annals of Thoracic SurgeryVol. 94Issue 3PreviewRecent literature suggests that a restrictive approach to red blood cell transfusions is associated with improved outcomes in cardiac surgery patients. Even in the absence of bleeding, intravascular fluid shifts cause hemoglobin levels to drift postoperatively, possibly confounding the decision to transfuse. The purpose of this study was to define the natural progression of hemoglobin levels in postoperative cardiac surgery patients. Full-Text PDF