BACKGROUND: The transfusion of relatively older red blood cells (RBCs) has been associated with both morbidity and mortality in trauma patients in observational studies. Although the mechanisms responsible for this phenomenon remain unclear, alterations in the microcirculation as a result of the transfusion of relatively older blood may be a causative factor. To assess this hypothesis, we evaluated microvascular perfusion in trauma patients during RBC transfusion. METHODS: Anemic but otherwise stable trauma intensive care unit patients with orders for transfusion were identified. Thenar muscle tissue oxygen saturation (StO2) was measured continuously by near-infrared spectroscopy during the course of transfusion of one RBC unit. Sublingual microcirculation was observed by sidestream dark-field illumination microscopy before and after transfusion of one RBC unit. Thenar muscle StO2 was recorded during the course of transfusion. Pretransfusion and posttransfusion perfused capillary vascular density (PCD) was determined by semiquantitative image analysis. Changes in StO2 and PCD relative to age of RBC unit were evaluated using mixed models that adjusted for baseline StO2 and Spearman correlation, respectively. RESULTS: Overall, 93 patients were recruited for study participation, 69% were male, and average Injury Severity Score (ISS) was 26.4. The average pretransfusion hemoglobin was 7.5 mg/dL, and the average age of RBC unit transfused was 29.4 days. The average peritransfusion StO2 was negatively associated with increasing RBC age (slope, j0.11; p = 0.0014). Change in PCD from pretransfusion to posttransfusion period was found to correlate negatively with RBC storage age (Spearman correlation, j0.27; p = 0.037). CONCLUSION: The transfusion of relatively older RBC units was associated with a decline in both StO2 and PCD. Collectively, these observations demonstrate that transfusions of older RBC units are associated with the inhibition of regional microvascular perfusion. In patients requiring multiple units of RBCs, alteration of the microcirculation by relatively older units could potentially contribute to adverse outcomes. Copyright © 2013 by Lippincott Williams & Wilkins.