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J Thorac Cardiovasc Surg 2005;129:1200-1201
© 2005 The American Association for Thoracic Surgery
Letters to the Editor |
Department of Surgery University of Illinois at Chicago Chicago, IL 60612
To the Editor:
Hemoglobin dilution is an expected physiologic response during cardiopulmonary bypass (CPB) surgery. Current controversy, however, centers around this question: what is a safe hematocrit level during CPB before the patient sustains less than an expected outcome? The main reason for the lack of consensus regarding blood transfusion may stem from the lack of a direct cause (hematocrit level) and effect (morbidity and mortality) relationship or an association or both causality and association. Although it has been found that low preoperative hemoglobin levels are correlated with poorer outcome,1 it does not mean that correcting this number will result in improved outcome. This same argument holds true for intraoperative hematocrits with the understanding that new-onset intraoperative anemia is reversible and mainly caused by dilution, whereas preoperative anemia is pathologic and mainly caused by nondilutional processes. Also, despite understanding the reversibility concept, most decisions of intraoperative transfusion stem from personal and institutional experience, with no defined dimensions. In May 2004, the National Heart, Lung, and Blood Institute working group published an executive summary regarding future directions in cardiac surgery.2 Creating a cardiovascular surgery clinical research network was one of the pillars, and I hope that the working group and the National Institutes of Health–sponsored workshop for neurocognitive changes after cardiac surgery will consider this trial an important direction toward filling an existing critical gap.
Why is a hematocrit trial timely?
The patient’s physiologic status must be the underlying cause for a transfusion, and the outcome of the transfusion (effect) must also be considered. The practicing surgeon, including those in training, is currently confused with the paradigm of cause and effect that seems to argue that mortality is higher among patients with low hematocrit (<25% in women and <23% in men3,4) and high hematocrit (>34%).5 Randomization is lacking in the adult cardiac surgery group thus far, despite a clear benefit of increased hematocrit for neurologic outcome in pediatric heart surgery (mean intraoperative hematocrit: 27.8% vs 21.5%).6
Moreover, despite the benefit of increased oxygen-carrying capacity with increased hematocrit, we must be aware that blood transfusions expose patients to a variety of potential cellular and humoral antigens. Conservation of blood during surgery will always be important because of the shortages of donor blood, the risks associated with the use of allogenic blood products, and the costs of these products.
At this stage, the outcome measured needs to be better studied in a larger group of patients to determine whether a beneficial effect is achieved when the cause is simply a new-onset decrease in hematocrit during CPB that requires the action of blood transfusion.
How can we define the dimensions for safe intraoperative hematocrit level?
In the context of a study trial, all variables in a scientific study must be controlled except for a single variable that is being studied. This is an incredible task, but it can be solved with a national task force among many regional centers and coordinated in the most organized fashion. Recruitment of centers that are indifferent to institutional experience is key for eliminating bias. The center with established excellence in surgical outcome among The Society of Thoracic Surgeons risk stratification subgroups will minimize technically dependent outcome variables and maximize the weighted effect of a single variable—hematocrit. The patient population description should include elective surgical patients who require primary or redo CPB coronary, valve, or both types of surgery; who lack hematologic causes for anemia; who have a well-defined etiology of disease process; and who lack prior blood transfusions in the last 3 months, including a well-defined level of comorbidity.
Selection of hematocrits
Given the already available data in the literature, it is important to chose 2 pathways, each performed randomly at each collaborating surgical unit, given that neither the perfusionist nor the surgeon will be blinded. This will mathematically accommodate a range of preoperative hematocrits to better define absolute and relative risk or benefit to the population sample after transfusion or not. The 2 pathways are (1) transfusing for hematocrits below a discrete value (20%, 25%, and 30%) and (2) transfusing for hematocrits when an absolute difference in CPB hematocrits (5, 10, 15, and 20 integers) is documented during surgery.
The volume of blood transfused should be held constant, and 1 unit of allogeneic blood should be sufficient except in rare circumstances. Should the next hematocrit level respond, no further blood transfusion should be given, yet further transfusion will be given if levels keep decreasing. Time from acknowledging the hematocrit level after the intervention (blood transfusion) will need to be further defined at discussion committees, among other variables, especially regarding triggers for preoperative CPB, postoperative CPB, and intensive care unit periods with respect to 4 important groups of patients—those with left ventricular dysfunction, recent myocardial infarction, congestive heart failure, and incomplete revascularization—because such groups require a higher hematocrit level.
A host of other blood product transfusion, perfusion, and operative variables—including age of blood transfused; priming volume; length of operation; amount of blood transfusion; type of blood transfused (eg, leukocyte depleted, cytomegalovirus status, and allogeneic vs predonated autologous blood); pump run; blood-primed pump; use of preoperative antiplatelets, anticoagulants, and antifibrinolytics; duration of CPB; temperature control (hypothermia and normothermia); volume of crystalloids and colloids infused and reason for infusion; and volume of other blood products infused—will need to be adjusted after consultation with a consortium of perfusionists and hematologists who are experienced in this field.
Also, a similar policy for transfusion should be adopted after surgery to avoid the potential influence of postoperative transfusion on clinical outcome. Outcome, including neurological outcome and survival, should be temporally and qualitatively documented particularly early after surgery and several years later.
Overall, there must be a safe range of hematocrits for each patient in adult and pediatric patients. Available studies remain largely observational and cannot adjust for the effects of all the confounders. In the interim, conservative use of blood products in heart surgery is warranted because of the infectious and noninfectious complications of allogeneic blood transfusion.
References
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