Anemia and brain oxygen after severe traumatic brain injury

<i>Purpose</i> To investigate the relationship between hemoglobin (Hgb) and brain tissue oxygen tension (PbtO2) after severe traumatic brain injury (TBI) and to examine its impact on outcome. <br> <i>Methods</i> This was a retrospective analysis of a prospective cohort of severe TBI patients whose PbtO₂ was monitored. The relationship between Hgb—categorized into four quartiles (≤9; 9–10; 10.1–11; >11 g/dl)—and PbtO₂ was analyzed using mixed-effects models. Anemia with compromised PbtO₂ was defined as episodes of Hgb ≤ 9 g/dl with simultaneous PbtO₂ < 20 mmHg. Outcome was assessed at 30 days using the Glasgow outcome score (GOS), dichotomized as favorable (GOS 4–5) vs. unfavorable (GOS 1–3). <br> <i>Results</i> We analyzed 474 simultaneous Hgb and PbtO₂ samples from 80 patients (mean age 44 ± 20 years, median GCS 4 (3–7)). Using Hgb > 11 g/dl as the reference level, and controlling for important physiologic covariates (CPP, PaO₂, PaCO₂), Hgb ≤ 9 g/dl was the only Hgb level that was associated with lower PbtO2 (coefficient −6.53 (95 % CI −9.13; −3.94), <i>p</i> < 0.001). Anemia with simultaneous PbtO₂ < 20 mmHg, but not anemia alone, increased the risk of unfavorable outcome (odds ratio 6.24 (95 % CI 1.61; 24.22), <i>p</i> = 0.008), controlling for age, GCS, Marshall CT grade, and APACHE II score. <br> <i>Conclusion</i> In this cohort of severe TBI patients whose PbtO₂ was monitored, a Hgb level no greater than 9 g/dl was associated with compromised PbtO₂. Anemia with simultaneous compromised PbtO2, but not anemia alone, was a risk factor for unfavorable outcome, irrespective of injury severity.