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from Section 6 - Hematology
Published online by Cambridge University Press: 05 September 2013
Introduction
The primary consideration in medical management of red cell disorders during surgery is to optimize hemoglobin concentration to provide for adequate oxygen delivery to tissues. Blood hemoglobin concentration is a primary direct and indirect determinant of tissue oxygenation. Blood oxygen content increases directly as hemoglobin concentration increases. Tissue oxygen delivery is a complex function of hemoglobin level, cardiac output, hemoglobin oxygen affinity, and tissue oxygen content. As hemoglobin level (more correctly red cell number) increases, blood viscosity increases and cardiac output may decrease so there is an optimal range of hemoglobin concentration that maximizes tissue oxygen delivery. The goal of preoperative and postoperative management is to maintain this optimal level at reasonable cost. Unfortunately, this optimal level is poorly defined in most clinical settings, varies between patients and within individual patients over time, and, even if well defined, cannot be maintained without unacceptable complication rates or costs. Perioperative management involves considering the optimal hemoglobin level for each clinical setting based on an informal cost–benefit analysis that usually is supported by incomplete outcome data.
A number of recent studies have shown that preoperative anemia is independently associated with increased morbidity and mortality in both cardiac and non-cardiac surgery. These studies are retrospective and cannot determine whether the anemia is causal of a worse outcome or a marker for severity of underlying disease. Until prospective controlled studies are completed, it seems prudent to try and identify the causes and correct the anemia before elective surgery. A large retrospective study done in the Veterans Affairs hospitals has shown that elevation in hematocrit above 54% is also associated with increased perioperative mortality and cardiac complications in an older male population.
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