BACKGROUND The integrity of the blood-cerebrospinal fluid (CSF) barrier during cardiopulmonary bypass (CPB) with hypothermic circulatory arrest (HCA) has not been systematically studied, especially in children. We tested the hypothesis that the blood-CSF barrier is disrupted by CPB. METHODS The study randomized 25 piglets (mean weight, 11 kg) to five groups (5 per group): anesthesia alone (control); CPB at 37 degrees C with full-flow (FF); CPB at 25 degrees C with very low flow (LF); and HCA at 15 degrees C and 25 degrees C. pH-stat strategy was applied during CPB. An epidural catheter was inserted into the cisterna magna for collection of CSF. CSF and blood samples were collected at seven points: after induction of anesthesia (baseline), at 10, 50 and 115 minutes after start of CPB, just before the end of CPB, and at 30 and 120 minutes after CPB. Albumin levels in CSF and plasma were measured to assess blood-CSF barrier integrity and the albumin ratio (CSF/plasma) was calculated (Q(Alb)). RESULTS In both HCA groups, the Q(Alb) was significantly higher than in the control and 37 degrees C FF groups (all p < 0.05), whereas Q(Alb) in the 37 degrees C group was not significantly different vs control. CONCLUSIONS The blood-CSF barrier is impaired by CPB with 1 hour of 15 degrees C or 25 degrees C HCA. Further investigations are needed to understand the behavior of the blood-CSF barrier during CPB and its role in neuroprotection.