Adverse Effects of RBC Transfusions: A Unifying Hypothesis

Healthy Volunteers

Blood Transfusions

Transfusion of red blood cells is often used in critically ill patients with low red blood cell counts to prevent disease progression and death. Recent studies suggest that the use of aged versus fresh red blood cells are associated with worse clinical outcomes. There is evidence that red blood cells work with the cells lining our blood vessels to produce a variety of substances that normally cause arteries to relax and increase blood supply. Two of these substances are called nitric oxide (NO) and endothelium-derived hyperpolarizing factor (EDHF). The investigators are trying to determine the nature of these substances in human beings when they are transfused aged versus fresh red blood cells. It is their thought that aged red blood cells have less of the substances (NO and EDHF) that naturally relax our arteries and further changes the blood supply. One way to determine this is to transfuse a subject's own aged and fresh red blood cells and inject substances such as L-NMMA (L-NG monomethyl arginine) and TEA (tetraethylammonium chloride), which block the production of NO and EDHF respectively, and then, study what happens to the blood flow. There is evidence that red blood cells produce NO, which normally causes arteries to relax and increase blood supply. The investigators will try to determine the nature of NO in red blood cells and whether the amount of this substance is altered because of different blood processing and storage techniques. It is their thought that aged red blood cells have less NO that naturally relaxes our arteries and further changes the blood supply. This study is designed to determine the most ideal way of storing and processing blood.

Transfusion of red blood cells (RBCs) is often effective at preventing morbidity and mortality in anemic patients. In contrast, recent studies indicate that some RBC components may have functional defects (RBC storage lesions) that actually cause morbidity and mortality when transfused. For example, patients transfused with RBCs stored \>14 days have statistically worse outcomes than those receiving fresher RBC units. In addition to the age of stored RBCs, the volume transfused may be important. The TRICC study showed that specific patients whose transfusions were limited by a restrictive trigger (RBCs transfusions only when hemoglobin \[Hb\] \< 7 g/dL) had significantly better outcomes than those transfused with a more liberal trigger (\[Hb\] \< 10 g/dL Hb). This finding has been particularly difficult to understand since conventional wisdom suggests that an elevated \[Hb\] should be beneficial because it supports increased O2 delivery. Recipient-specific factors may also contribute to the occurrence of these adverse events. Unfortunately, these events have been difficult to investigate because up to now they have existed only as statistical occurrences of increased morbidity and mortality in large data sets. There are currently no clinical or laboratory methods to detect or study them in individual patients. The microcirculation is composed of a continuum of small vessels including small arterioles, capillaries, and post-capillary venules. The microcirculation represents an actively-adjusting vascular circuit that matches blood flow (and O2 delivery) to local tissue oxygen demands. While the physiologic mechanisms that match O2 delivery to local requirements are incompletely understood, endothelium-derived nitric oxide (NO) clearly plays an important role. Interestingly, recent work has revealed that in addition to transporting O2 and CO2, the RBC also controls local NO concentrations and thus may also play a surprisingly important role in regulating blood flow in the microcirculation. Herein, the investigators bring together previously unconnected data to propose a unifying hypothesis, centered on insufficient NO bioavailability (INOBA), to explain the increased morbidity and mortality observed in some patients following RBC transfusion. In this model, variables associated with RBC units (storage time; 2,3-DPG concentration) and transfusion recipients (endothelial dysfunction; hematocrit \[Hct\]) collectively lead to changes in NO levels in vascular beds. Under certain circumstances, these variables are aligned such that NO concentrations are markedly reduced, leading to vasoconstriction, decreased local blood flow and insufficient O2 delivery to end organs. Under these circumstances, the likelihood of morbidity and mortality escalates. The INOBA hypothesis is attractive because of its explanatory power and because it leads to a number of readily testable predictions, which will be investigated in the following aims: Aim 1: To investigate the effects of blood processing and storage (using standard FDA-approved conditions) on NO production and scavenging by human RBCs/Hb in vitro. Using sensitive biochemical assays (electron spin resonance \[ESR\]) and a rat aortic ring in vitro bioassay, the investigators will test the effects of RBC storage time, leukoreduction, and irradiation on NO synthesis and/or scavenging by intact RBCs and free Hb. Modifications such as washing and rejuvenation will be investigated as possible approaches to correct abnormalities in NO bioavailability. Aim 2: To transfuse healthy volunteers and investigate the effects of storage-related RBC changes on blood flow, tissue oxygenation, and biomarkers of cardiovascular function. The investigators will determine whether RBCs prepared and stored under conditions that alter NO bioavailability in vitro (Aim 1) inhibit NO-mediated vasodilation, reduce tissue perfusion, and decrease tissue O2 delivery in healthy transfusion recipients in vivo. The role of 2,3-DPG depletion as well as exercise-induced O2 demand will also be investigated with these specialized experimental systems.

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