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Healthy individuals are able to tolerate profound, short-term decreases in hemoglobin levels and oxygen saturation without serious consequences, but critically ill patients in respiratory failure lack the necessary reserve capacity to preserve tissue oxygenation. The development of progressive anemia in ICU patients has led to much interest and debate about transfusion practices, yet optimal hemoglobin levels and how they should be achieved remain unclear. Animal and human studies regarding critical oxygen delivery provide the rationale for optimizing hemoglobin levels and supporting cardiovascular function during respiratory failure. Theoretically, the oxygen-carrying benefit of RBCs should hasten recovery from respiratory failure, and transfusions would therefore be expected to shorten the duration of mechanical ventilation. However, evidence to the contrary has been reported. Controversies related to transfusions and their inability to improve outcomes suggest that further research regarding transfusion alternatives is needed, especially in anemic patients with respiratory failure. (CHEST 2005; 128:576S-582S)
Key words: anemia; oxygen delivery; respiratory failure; transfusion; intensive care
Abbreviations: CABG = coronary artery bypass graft; CO = cardiac output; DO^sub 2^ = oxygen delivery; SaO^sub 2^ = arterial oxygen saturation; VO^sub 2^ = oxygen consumption
Learning Objectives: 1. To review the basic determinants and alterations observed in oxygen delivery during respiratory failure. 2. To summarize clinical studies that have examined factors such as anemia and transfusions and their effect on outcomes in patients with respiratory failure.
Normal homeostasis requires the interdependent actions of the respiratory and circulatory systems to provide a flow of oxygenated blood to the tissues in order to maintain aerobic metabolism.1 Respiratory failure, circulatory collapse, or profound decrements in the oxygen-carrying capacity of the blood may lead to life-threatening consequences. Moderate degrees of dysfunction in the respiratory, circulatory, or hematologic systems may not be life threatening if there is physiologic reserve capacity in the other two limbs of this aerobic support system triad.2 Survival of patients with respiratory failure requires not only treatment of the primary condition but also attention to deficits or potential deficits in relevant associated organ systems. Anemia may have serious consequences in the patient with respiratory failure if oxygen delivery (DO^sub 2^) to the tissues is impaired and aerobic metabolism is not maintained.3-5
DETERMINANTS OF DO^sub 2^
Tissue hypoxia in the critically ill patient is a serious...