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OBJECTIVES Neuroprotection during aortic arch surgery involves selective antegrade cerebral perfusion. The parameters of cerebral perfusion, e.g. flow rate, are inconsistent across centres and are subject of debate. The aim of this study was to determine the cerebral perfusion flow rate during hypothermic circulatory arrest required to meet preoperative awake baseline regional cerebral oxygen saturation (rSO2). METHODS Patients scheduled for aortic arch surgery with hypothermic circulatory arrest were enrolled in this prospective observational study. After initiation of hypothermic circulatory arrest, bilateral selective antegrade cerebral perfusion was established and cerebral flow rate was continuously increased. The primary end point was the difference of cerebral saturation from baseline during cerebral perfusion flow rates of 6, 8 and 10 ml/kg/min. RESULTS A total of 40 patients were included. During antegrade cerebral perfusion, rSO2 was significantly lower than the baseline at 6 ml/kg/min [−7.3, 95% confidence interval (CI): −1.7, −12.9; P = 0.0015]. In contrast, flow rates of 8 and 10 ml/kg/min resulted in rSO2 that did not significantly differ from the baseline (−2; 95% CI: −4.3, 8.3; P > 0.99 and 1.8; 95% CI: −8.5%, 4.8%; P > 0.99). Cerebral saturation was significantly more likely to meet baseline values during selective antegrade cerebral perfusion with 8 ml/kg/min than at 6 ml/kg/min (44.1%; 95% CI: 27.4%, 60.8% vs 11.8%; 95% CI: 0.9%, 22.6%; P = 0.0001). CONCLUSIONS At 8 ml/kg/min cerebral flow rate during selective antegrade cerebral perfusion, regional cerebral oximetry baseline values are significantly more likely to be achieved than at 6 ml/kg/min. Further increasing the cerebral flow rate to 10 ml/kg/min does not significantly improve rSO2.