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To determine a) whether packed red blood cell transfusion (RBCT) increases local brain tissue oxygen partial pressure (Pbto2) in a neurocritical care population; and b) what (if any) demographic, clinical, or physiologic variables mediate the assumed change.Prospective observational study.A neurosurgical intensive care unit at a university-based level I trauma center and tertiary care hospital.Thirty-five consecutive volume-resuscitated patients with subarachnoid hemorrhage or traumatic brain injury, without cardiac disease, requiring Pbto2 monitoring and receiving RBCT were studied between October 2001 and December 2003.None.The following physiologic variables were measured and compared 1 hr before and after RBCT: Pbto2, intracranial pressure, cerebral perfusion pressure, hemoglobin oxygen saturation (Sao2), Fio2, hemoglobin, and hematocrit. An increase in Pbto2 was observed in 26 of the 35 patients (74%). In nine patients, Pbto2 decreased after RBCT. The mean (±sd) increase in Pbto2 for all patients was 3.2 ± 8.8 mm Hg (p = .02), a 15% change from baseline (1 hr before RCBT). This Pbto2 increase was associated with a significant mean increase in hemoglobin and hematocrit after RBCT (1.4 ± 1.1 g/dL and 4.2% ± 3.3%, respectively; both p < .001). Cerebral perfusion pressure, Sao2, and Fio2 were similar before and after RBCT. Among the 26 patients whose Pbto2 increased, the mean increase in Pbto2 was 5.1 ± 9.4 mm Hg or a 49% mean increase (p < .01).RBCT is associated with an increase in Pbto2 in most patients with subarachnoid hemorrhage or traumatic brain injury. This mean increase appears to be independent of cerebral perfusion pressure, Sao2, and Fio2. Further study is required to determine why Pbto2 decreases in some patients after RBCT.