Anemia is associated with brain tissue hypoxia and metabolic crisis after severe brain injury

Introduction

After severe brain injury, anemia can adversely affect cerebral oxygen delivery and brain tissue oxygen (PbtO₂). However, it is unclear whether low hemoglobin (Hb) contributes to brain tissue hypoxia or compromises oxidative metabolism.

Methods

We studied 28 consecutive patients with severe brain injury (15 with subarachnoid hemorrhage, 8 with intracerebral hemorrhage, and 5 with traumatic brain injury) who underwent multimodality intracranial pressure, PbtO₂ and microdialysis monitoring. The relationship between Hb levels and the risk of brain tissue hypoxia (BTH), defined as PbtO₂ ≤ 15 mmHg, and metabolic crisis (MC), defined as lactate/pyruvate ratio ≥ 40 and glucose < 0.7 mmol/l, was analyzed with general linear models of logistic function for dichotomized outcomes utilizing generalized estimating equations for model estimation.

Results

The mean age was 53 (20 to 83) years, 18 (64%) patients were female, median Glasgow coma scale was 6 (IQR 4 to 8) and 32% were dead at discharge. A total of 3,209 hours of monitoring and 297 Hb measurements were collected and analyzed. The Brain sinoarterial S100B concentration differences over time. Mean/SE. *P < 0.05. mean Hb was 10.1 ± 1.5 mg/dl. Hb values were categorized into four ranges: ≤ 8 mg/dl, 8.1 to 9 mg/dl, 9.1 to 10 mg/dl and >10 mg/dl. The range with lowest frequency of MC and BTH (9.1 to 10 mg/dl) was defined as the reference. For every reduction in Hb range below the reference, there was an increased risk of MC (adjOR = 1.7 (95% CI = 1 to 2.9), P = 0.048 for 1 to 9 mg/dl; and adjOR = 4.2 (1.6 to 11.4), P < 0.01 for ≤ 8 mg/dl) (Figure 1). A reduction in Hb below 8 mg/dl was also associated with increased risk of BTH (adjOR = 2.5 (1 to 6.5), P = 0.04) (Figure 2). The number of hours per day monitored spent in MC, but not with BTH, was associated with mortality at discharge (adjOR = 1.6 (1.1 to 2.4), P = 0.02).

Figure 1

Relative frequency of metabolic crisis across hemoglobin ranges.

Figure 2

Relative frequency of brain tissue hypoxia across hemoglobin ranges.

Conclusion

Mild reductions in blood Hb below 9 mg/dl are associated with an increased risk of MC and BTH after severe brain injury. MC had a more pronounced relationship with anemia and was associated with a greater risk of death.