Transcranial Doppler (TCD) enables real time detection of intraoperative emboli in the cerebral arteries seen as microembolic signals (MES), and is an essential neuromonitoring tool. Several studies demonstrated correlation between the number of MES and the occurrence as well as severity of postoperative neurological complications. However, the factors contributing to brain injury have not been elucidated in those studies. The investigators speculate that impairment of CVR is an important mechanism that persists and prolongs the duration of brain injury into postoperative period.