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The critical importance of PaCO2 during intracranial aneurysm surgery. Case report.

In a patient undergoing craniotomy for clipping of an anterior communicating artery aneurysm, sodium nitroprusside was used to lower systemic arterial blood pressure to 80/35 mm Hg (mean arterial blood pressure (MABP) = 50 mm Hg), at which time the electroencephalogram (EEG) changed abruptly from normal to a burst suppression pattern. At the onset of burst suppression, PaCO2 was 18 mm Hg. After PaCO2 had been increased to 28 mm Hg, the patient tolerated a blood pressure of 45/25 mm Hg (MABP = 32 mm Hg) during aneurysm clipping without EEG change. The observations reported here support the conclusion that, with moderate hypotension, hypocarbia may cause brain ischemia and that the level of PaCO2 may influence the degree of hypotension that may be safely used during aneurysm surgery. The importance of repeated arterial blood gas measurements when induced hypotension is employed for berry aneurysm surgery is stressed. Intraoperative EEG monitoring may be employed to help judge safe levels of intraoperative hypotension during intracranial aneurysm surgery.