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Hyperoxemia improves cerebral autoregulation in severe traumatic brain injury

The intrinsic autoregulation mechanisms of the cerebral vessels that normally maintain a constant cerebral blood flow (CBF) relatively independently from the cerebral perfusion pressure variations are frequently impaired in the severely traumatized. It has been shown that hyperventilation can restore the cerebral autoregulation and controlling intracranial pressure, but much less attention has been given to the effects of the hyperoxic state in the restoration of cerebral autoregulation. The purpose of the study was to compare the autoregulatory response to hyperventilation vs hyperoxia in severe traumatic brain injury.

We prospectively examined 186 (aged 34.4 ± 14.99 years) patients with severe traumatic brain injury (postresuscitation GCS 5.98 ± 3.0957) following admission to the neurosurgical ICU of a level one trauma hospital. Hyperventilation and hyperoxia studies were conducted, recording middle cerebral artery flow velocity and the autoregulation index (ARI) bilaterally and simultaneously at baseline and posthyperventilation (CO2 reactivity = %ΔCBF/ΔpCO2)/hyperoxia (O2 reactivity = ΔPbtO2/ΔpO2). Continuous multimodal neuromonitoring, intracranial pressure, mean arterial blood pressure, cerebral perfusion pressure, end-tidal CO2 (ETCO2), PbtiO2, and SjvO2, was recorded.

The ARI (normal 5 ± 1) in these head-injured patients averaged 2.2 ± 1.5 on day 1 and gradually improved over the 10 days of monitoring. The ARI significantly improved with hyperoxemia, during the first 6 days after injury when compared with the ARI measured at normoxemia. The mean left ARI difference during hyperoxia was 0.4069 ± 1.7948 while the right ARI difference was 0.4708 ± 1.8413. The average change in pCO2 during hyperoxia was 35.6468 ± 5.8778. These changes in the ARI during hyperoxia were smaller than those observed during hyperventilation. Hyperventilation increased the ARI by average 0.8519 ± 0.2310 on the left and 1.0833 ± 0.4654 on the right.

Pressure autoregulation was impaired in these head-injured patients. Hyperoxia significantly improved pressure autoregulation. The very small change in pCO2 induced by hyperoxia does not seem to explain this improvement in pressure autoregulation. Vasoconstriction induced by hyperoxia may partially contribute to the improved pressure autoregulation.

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Celis, R., Hlatky, R. & Robertson, C. Hyperoxemia improves cerebral autoregulation in severe traumatic brain injury. Crit Care 11 (Suppl 2), P357 (2007).

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