Physiologic aspects of anemia☆
Section snippets
Overview of oxygen transport
Hemoglobin is a complex molecule consisting of four globin moieties, each incorporating an iron-containing heme ring where oxygen may bind. The O2 carrying capacity of hemoglobin or binding affinity to O2, is represented graphically by a sinusoidal relationship between the hemoglobin saturation and the partial pressure of oxygen (pO2). This relationship, referred to as the oxyhemoglobin dissociation curve, enables efficient loading in the lungs at high pO2s and efficient unloading in the
Adaptation to anemia and transfusions
In anemia, O2 carrying capacity is decreased, but tissue oxygenation is preserved at hemoglobin levels well below 100 g/L. Following the development of anemia, adaptive changes include a shift in the oxyhemoglobin dissociation curve and hemodynamic and microcirculatory alterations. The shift to the right of the oxyhemoglobin dissociation curve in anemia is primarily the result of increased synthesis of 2,3-DPG in red cells [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28],
Microcirculatory effects of anemia and red cell transfusions
At the level of the microcirculation, different mechanisms potentially could increase the amount of O2 supplied to tissues within capillary networks. In a model of the microcirculation proposed by Krogh [89], [90], O2 supply to the tissues may be enhanced through recruitment of previously closed capillaries, increased capillary flow, and increased O2 extraction from existing capillaries. The degree of anemia, the specific tissue bed, and a variety of disease processes all may impact on
Interaction between pathophysiologic processes and anemia
Several disease processes affecting either the entire body or specific organs potentially limit adaptive responses, thereby making patients more vulnerable to the effects of anemia. Specifically, heart, lung, and cerebrovascular diseases have been proposed to increase the risk of adverse consequences from anemia [35], [120], [121]. Age, severity of illness, and therapeutic interventions also may affect adaptive mechanisms.
The heart, and more specifically the left ventricle, may be particularly
Red cell transfusions and oxygen kinetics
The authors identified 18 studies (Table 2) evaluating the impact of red cell transfusions on O2 kinetics. Although hemoglobin concentration increased significantly in all the studies, DO2 did not increase in four of them. Despite the administration of 1 to 3 packed red blood cell units, cardiac output did not change or even tended to decrease. In the 14 studies that showed an increase in systemic DO2, only five reported an increase in systemic O2 consumption. Blood lactate levels were not
Summary
The most important adaptive responses from a physiological stance involved the cardiovascular system, consisting in particular of elevation of the cardiac output and its redistribution to favor the coronary and cerebral circulations, at the expense of the splanchnic vascular beds. The evidence regarding these physiological responses, especially in experimental studies that permit the control of many variables, is particularly powerful and convincing. On the other hand, there is a remarkable
Acknowledgements
We are grateful to the Canadian Medical Association Expert Working Group for Guidelines on Transfusion of Red Cells and Plasma in Adults and Children for their support and assistance in preparing the original manuscript and to Dr. Claudio Martin for his meticulous review of the original manuscript published in the Canadian Medical Association Journal.
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Dr. Hébert is an Ontario Ministry of Health Career Scientist.