Patients will undergo intraoperative TCD (Transcranial Doppler) The TCD examination will assess the brain's ability to increase cerebral blood flow in response to a pharmacological challenge (CO2 inhalation). TCD measures the degree of cerebral vasodilation, identified as an increase in flow velocity on TCD. This "cerebrovascular reserve" we hypothesize will be able to predict performance on postoperative neuropsychometric tests