Brain tissuepO2-monitoring in comatose patients: Implications for therapy

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Abstract
Monitoring of brain tissue partial pressure of O2 (ti-pO2) is a promising new technique that allows early detection of impending cerebral ischemia in brain-injured patients. The purpose of this study was to investigate the effects of standard therapeutic interventions .used in the treatment of intracranial hypertension in comatose patients on cerebral oxygenation. In the neurosurgical intensive care unit ti-p02 arterial blood pressure, intracranial pressure OCP), cerebral perfusion pressure (CPP) and jugula'r bulb oxygen saturation (SjV02) were prospectively studied (0.1 Hz acquisition rate) in 23 comatose patients (21 with sever~ traumatic brain injury, 2 with intracerebral hematoma) during various treatment modalities: elevation of CPP with dop~mine (n = 35), lowering of the head (n = 22), induced arterial hypocapnia (n = 13), mannitol infusion (n = 16), and decompressive craniotomy (n = 1). Ischemic episodes (‘IE’ = ti-p02< 10 mmHg for > 15 min) within the first week after the insult were always associated with unfavorable neurological outcome. Elevation of CPP from 32 ± 2 to 67 ± 4 mmHg significantly improved ti-p02 by 62% (13 ± 2 to 21 ± 1 mmHg) and reduced ICP indicating intact cerebral autoregulation. Further raising CPP from 68 ± 2 to 84 ± 2 mmHg did not alter ti-p02' Mannitol-induced ICP reduction from 23 ± 1 to 16 ± 2 mmHg did not affect ti-p02 nor did lowering of the head from 3rY to (Y. Hyperventilation from an endtidal pC02 of 29± 3 to 21 ± 3 mmHg normalized ICP and CPP, but significantly reduced ti-p02 from 31 ± 2 to 14 ± 3 nimHg. Decompressive craniotomy in· a 15-year old patient with refractory intracranial hypertension instantly restored ti-p02 Based on the present data, our understanding of many interventions previously believed to improve brain oxygenation might have to be. re-evaluated. A CPP > 60 mmHg emerges as the most important factor determining sufficient brain tissue p02' Any intervention used to further elevate CPP does-not improve ti-p02, to the contrary, hyperventilation even bears the risk of inducing brain ischemia. [Neural Res 1997; 19: 233-240]

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