During open abdominal aortic aneurism (AAA) repair cerebral blood flow is challenged. Clamping of the aorta may lead to unintended hyperventilation as metabolism is reduced by perfusion of a smaller part of the body and reperfusion of the aorta releases vasodilatory substances including CO2. We intend to adjust ventilation according end-tidal CO2 tension (EtCO2) and here evaluated to what extent that strategy maintains frontal lobe oxygenation (ScO2) as determined by near infrared spectroscopy. For 44 patients [5 women, aged 70 (48-83) years] ScO2, mean arterial pressure (MAP), EtCO2, and ventilation were obtained retrospectively from the anesthetic charts. By clamping the aorta, ScO2 and EtCO2 were kept stable by reducing ventilation (median, -0.8lmin(-1); interquartile range, -1.1 to -0.4; P<0.001). During reperfusion of the aorta a reduction in MAP by 8mmHg (-15 to -1; P<0.001) did not prevent an increase in ScO2 by 2% (-1 to 4; P<0.001) as EtCO2 increased 0.5kPa (0.1-1.0; P<0.001) despite an increase in ventilation by 1.8 l min(-1) (0.9-2.7; P<0.001). Changes in ScO2 related to those in EtCO2 (r=0.41; P=0.0001) and cerebral deoxygenation (-15%) was noted in three patients while cerebral hyperoxygenation (+15%) manifests in one patient. Thus changes in ScO2 were kept within acceptable limits (±15%) in 91% of the patients. For the majority of the patients undergoing AAA repair ScO2 was kept within reasonable limits by reducing ventilation by approximately 1lmin(-1) upon clamping of the aorta and increasing ventilation by approximately 2lmin(-1) when the lower body is reperfused.