Introduction Hypothermic circulatory arrest (HCA) in conjunction with selective antegrade cerebral perfusion (sACP) is frequently used for neuroprotection.(1) Nonetheless optimal sACP has only been investigated in animal studies.(2) The objective of this study was to determine sACP during HCA necessary to reach baseline tissue oxygenation as measured by near infrared spectroscopy (NIRS). Methods With local ethics committee approval, consecutive patients scheduled for elective aortic arch surgery in HCA were included in this prospective observational single center study. Bilateral sACP was institued by introducing perfusion catheters in the innominate and left carotid artery. The quality of sACP was monitored by bifrontal NIRS. Baseline NIRS were measured before anesthesia induction in the awake patient. NIRS values were recorded after initiation of HCA at sACP of 6, 8 and 10 milliliters (ml) per ideal body weight (kg) per minute (min). For comparison the mean of bifrontal NIRS in each patient was calculated for this interim analysis. Differences in NIRS between sACP flow rates were assessed by one-way ANOVA and Bonferroni t-test. A p Results Fifteen patients had complete datasets (4 female). NIRS was 62 (8) at baseline; 56 (11) at 6; 62 (11) at 8 and 63 (9) at 10 ml/kg/min sACP, respectively. ANOVA showed no significant differences between baseline and the three distinct sACP (p = 0.223). Bonferroni t-test indicated no differences between the different sACP and the baseline (all p>0.05). Duration of HCA was 12 (15) minutes. All patients were discharged after a length of hospital stay of 10 +/- 4 days. Three patients had new onset focal neurological deficits before discharge and three patients had postoperative delirium. Discussion SACP of 6-10ml is the range often used in centers of aortic arch surgery. (1) In an animal-model, the lower threshold of 6ml/kg/min was confirmed recently, since venous saturation decreased markedly when sACP was lowered below this sACP.(2) Our data support this finding in humans as sACP f of 6 ml/kg/min is sufficient to reach baseline NIRS values. A further increase, e.g., to 8 or 10 ml/kg/min, is not associated with an even better NIRS. The data suggest that sufficient frontal brain oxygenation can be achieved with a relatively low sACP flow rate in HCA.