<qa>  What will be used to measure the effect of the vasopressor on cardiac function?This will be a prospective, randomized, double-blinded, cross-over clinical trial to compare the effects of phenylephrine or norepinephrine on cardiac function during carotid artery surgery in patients under general anesthesia. The effect of the vasopressor on cardiac function will be measured using electrocardiography, transesophageal echocardiography and cardiac troponin I intraoperatively and postoperatively. We will use a cross-over pattern since both drugs exhibit rapid onset and short duration of action. We hypothesize that there will be less myocardial ischemia associated with the use of norepinephrine when compared with phenylephrine during carotid artery surgery. 