Implications and management of anemia in cardiac surgery: Current state of knowledge

Abstract

Decreasing oxygen delivery during and after cardiopulmonary bypass (CPB) can adversely affect patient outcomes. Balancing morbidities associated with anemia, red blood cell (RBC) transfusion, pharmaceuticals to prevent fibrinolysis, and reoperations for bleeding poses a dilemma. One would optimally balance these factors to minimize patient risk. Uncertainty in defining an optimal transfusion threshold, a trigger for reoperation, and a strategy for use of pharmaceuticals is evidenced by the considerable variability in clinical practice worldwide. 1,2 Therefore, to gain insight to inform surgical practice, we have brought together current knowledge of the association of preoperative anemia, intraoperative anemia, blood transfusion, and measures to address blood loss with adverse outcomes after cardiac surgery. PREOPERATIVE ANEMIA AND OUTCOMES The World Health Organization defines anemia as a hemoglobin level less than 13 g/dL in men and less than 12 g/dL in women. By that measure, prevalence of preoperative anemia in patients undergoing cardiac surgery ranges from 16% to 54%, with severe anemia (hemoglobin < 10 g/dL) in 5.5%. 3-5 Patients presenting with low hemoglobin are more susceptible to end-organ ischemia; however, oxygen demands on CPB vary, and it is unclear what hemoglobin level is adequate. Nonetheless, several large observational studies suggest that preoperative anemia is associated with increased noncardiac morbidity and mortality after CPB (Table 1). Zindrou and colleagues 6 performed an observational study of 2059 patients undergoing isolated coronary artery bypassgrafting(CABG)thatshowedthatpatientswithapreoperative hemoglobin of 10 g/dL or less had a 5-fold higher in-hospital mortality than those with a greater hemoglobin concentration.Thiswasconfirmedbymultivariableanalysis. However, this study may have been confounded by RBC transfusion, which was not considered in the analysis. Likewise, van Straten and colleagues 5 from the Dutch Central Bureau for Statistics performed an observational study of 10,626 patients who underwent CABG. Preoperative anemia, as defined by the World Health Organization, was an independent risk factor for increased early (<30 days) mortality and higher degree of preoperative anemia for higher late mortality. Preoperative hemoglobin of 14.5 g/dL or greater in men and 13.5 g/dL in women was associated with an 88% survival at 8 years, whereas a value less than 12 g/dL in men and less than 11 g/dL in women was associated with a 55% survival at 8 years. This study did not factor intraoperative anemia or use of RBC transfusion into the analysis. To account for transfusion as a potential confounder, Ranucci and colleagues 7 reviewed 3003 consecutive patients undergoing isolated CABG without perioperative RBC transfusion and found no association between preoperative anemia and postoperative mortality. However, they reported a 5-fold greater unadjusted occurrence of major morbidity in patients with a preoperative hematocrit (HCT) of 33% or less versus 42% or greater. After risk adjustment, preoperative HCT and lowest HCT on CPB were associated with an increased postoperative occurrence of prolonged ventilation, renal insufficiency, stroke, and reoperation. Several additional studies have identified an association between preoperative anemia and postoperative morbidity. A multicenter observational study by Kulier and colleagues 4 demonstrated that preoperative anemia (hemoglobin <13 g/dL) was associated with increased neurologic, renal, and gastrointestinal complications. Anemia was not associated with more cardiac complications, although these events were related to perioperative RBC transfusion. De Santo and colleagues 8 studied a co