When ready, patients will be transported into the operating room and anesthesia will be induced. Induction will consist of propofol (1 - 5 mg/kg single intravenous bolus), fentanyl and vecuronium for neuromuscular blockade. Other drugs and dosages of opioids and neuromuscular blockers are at the discretion of the anesthesiologist. After induction and tracheal intubation, patients will receive maintenance anesthesia of 0.5 minimal alveolar concentration (MAC) isoflurane in a 50% air/oxygen balanced mixture supplemented with fentanyl at the discretion of the anesthesiologist. At the onset of cardiopulmonary bypass (CPB), study drug (nicardipine or equivalent volume of placebo - 0.9% saline) infusion at 5 mg/hr will be initiated, and patients will receive 0.5 MAC isoflurane in the CPB circuit sweep gas. Bolus doses of 100mcg phenylephrine will be administered to both groups in order to maintain a constant mean arterial pressure of at least 50 mmHg. Cooling will occur primarily through the CPB machine. When the patient's brain temperature reaches 28o C, isoflurane (via the pump) will be reduced to 0.25 MAC. When ECS on EEG and ablation of cortical responses on SSEP have both occurred, CPB and study drug infusion will be halted, and thoracic aortic surgery will be commenced. After aortic repair has occurred, CPB and study drug infusion at 5 mg/hr will be reinstated, anesthesia administration resumed, and the patient actively rewarmed. When the patient's brain temperature reaches 28o C (as recorded by nasopharyngeal temperature), patients will receive 0.5 MAC isoflurane. After the patient has been fully re-warmed and is ready for separation from CPB, study drug infusion will be halted. At this point, but not before, commercially available nicardipine may be infused if so desired. 10 ml blood samples will be drawn from the pump at baseline and 15 minute intervals thereafter until HCA is achieved. When the pump is restarted, further samples will be drawn at 15 minute intervals until the patient separates from CPB after which no further samples will be taken. One sample of 10 ml will be drawn from the retrograde cardioplegia line immediately after placement (baseline) and one sample will be drawn immediately prior to separation from CPB. In total, approximately 100 ml of blood will be drawn from the patient for research purposes. This volume represents a tiny percentage of the excess volume associated with the pump prime, and is insignificant in terms of its effect on hemodynamics.