Selective antegrade cerebral perfusion (SACP) is a commonly used technique for maintaining cerebral blood flow (CBF) during the use of hypothermic cardiac arrest (HCA) for aortic arch reconstruction. However, even with an intact Circle of Willis, asymmetric CBF is a common occurrence during HCA when SACP is used. The investigators have previously shown that ultrasound guided extrinsic compression of the left carotid artery can increase left cerebral oxygen saturation, and improved symmetry of CBF; however, this has not been investigated formally. In this study, the investigators will firstly formally assess the incidence and severity of asymmetrical cerebral flow between the left and right hemispheres in patients undergoing aortic arch repair (n=20). CBF will be assessed indirectly through regional cerebral oxygen saturation (SrO2) measured via near-infrared spectroscopy-derived cerebral oximetry. In addition, a number of clinical outcome parameters (up to 30 days post-op) will be evaluated. The investigators expect that significant left-side cerebral hypoperfusion will consistently be observed in patients undergoing aortic arch repair using SACP.