Invited Commentary

Abstract

Does the likeness in means and ranges of hemoglobin (Hgb) from birds to mammals suggest that this physiological equilibrium has emerged more than once by natural selection? Is it by chance that the “ideal hematocrit” across the animal kingdom, when plotted against viscosity, usually follows a similar concave curve, peaking around a value of 40? Are events that evolved to maximize survival and reproductive fitness hundreds of millions of years ago relevant to perioperative management of patients? And should the anemia of a child with a moderately sized ventricular septal defect be corrected to avoid surgery? Cholette and colleagues [1Cholette J.M. Swartz M.F. Rubenstein J. et al.Outcomes using a conservative versus liberal red blood cell transfusion strategy in infants requiring cardiac operation.Ann Thorac Surg. 2017; 103: 206-215Abstract Full Text Full Text PDF PubMed Scopus (39) Google Scholar] do not address these questions. I congratulate them, however, for their perseverance (this is a follow-up study) and for completing likely a difficult study. Further, they clearly show that one can avoid red blood cell (RBC) transfusion in most children after biventricular repair and in a reasonable fraction of those palliated with cavopulmonary or systemic-to-pulmonary artery shunting. They randomized 134 patients after biventricular and 57 patients after palliative surgery (including 12 s/p Norwood), to either a “restrictive” (Hgb of 7.0) or “liberal” (Hgb of 9.0 g/dL) threshold for transfusion. They report similar lactate, avO2 differences, and clinical results between the two groups and, therefore, infer that clinical judgment should guide transfusion decisions. But whose judgment? Once a critical care colleague said that just because you can stand in a canoe in the middle of the lake, it does not mean you should. Therefore, without specific guidelines, one is left with the judgment of one clinician versus another. Most would agree that RBC transfusions are not without risk; for example, many have shown the associated increased risk of infection. The typical reason (tachycardia, low blood pressure, low urine output, low mixed venous saturations) for treating anemia are practical starting points, but some patients have much less reserve than others do, and clinical circumstances may mask or decrease the potential for applicability of such features. The baby with pulmonary atresia/intact ventricular septum and RV dependent coronary circulation and coronary atresia may not tolerate heart rates in the 150s or 160s, let alone the 170s or 180s. Lactate, as used by the authors, is a late marker of tissue hypoxia, only to rise with anaerobic glycolysis. Other notable findings from the study include the difference in transfusion rates between the two groups: one transfusion. Authors provide no information on number of laboratory tests, which is perhaps the primary driving force for blood loss after surgery (all patients started with same baseline Hgb). It is tempting to speculate that the restrictive approach perhaps influenced the perioperative management. If true, adopting a restrictive strategy would be a laudable goal in lessening the number of such tests. It is also notable that 4 of the 6 babies for whom the restrictive strategy was violated were newborns who had undergone one form or another of arch reconstruction. Over 80 years ago, Robin Fahraeus remarked on the fall of hematocrit in blood flowing through smaller-caliber tubes. Future observations confirmed the differential segregation of RBCs and plasma in divisions of microvasculature, whether by default or because of vasoactive agents. These observations and other experiments of nature (e.g., the Antarctic Icefish compensates for no Hgb by having a much larger heart, high stroke volume, and dense capillary network) highlight that while an arbitrary Hgb value is not a signal for transfusion, great caution should be exercised when deciding whether a patient should or should not receive a transfusion. The decision not to transfuse could be just as dangerous as the one to transfuse. Outcomes Using a Conservative Versus Liberal Red Blood Cell Transfusion Strategy in Infants Requiring Cardiac OperationThe Annals of Thoracic SurgeryVol. 103Issue 1PreviewThe optimal hemoglobin for infants after cardiac operation is unknown. Red blood cells (RBCs) are commonly transfused to maintain high hemoglobin concentrations in the absence of a clinical indication. We hypothesized that infants can be managed with a postoperative conservative RBC transfusion strategy, resulting in lower daily hemoglobin concentrations, without evidence of impaired oxygen delivery (ie, lactate, arteriovenous oxygen difference [avO2diff]), or adverse clinical outcomes. Full-Text PDF