Blood warms as it flows retrograde from a femoral cannulation site to the carotid artery during cardiopulmonary bypass.

Hypothermia protects the brain from injury during cardiopulmonary bypass and hypothermic circulatory arrest. However, when the arterial cannula is at sites other than the ascending aorta, brain cooling before circulatory arrest may be incomplete, contributing to poor neurological outcome. The authors performed an experiment using a rabbit model of cardiopulmonary bypass with femoral arterial perfusion. Arterial perfusate and carotid artery temperatures were simultaneously measured in six rabbits undergoing cardiopulmonary bypass, three with femoral artery cannulation, and three with subclavian artery cannulation. The subclavian artery site was chosen as a surrogate for ascending aortic cannulation. Carotid blood temperature was 3.7 degrees C to 0.3 degree C warmer than arterial perfusate temperature during femoral arterial perfusion. The temperature difference decreased over time (p = 0.0007). The temperature gradient was significantly less (p = 0.002) when subclavian arterial perfusion was used. Carotid blood temperature was 1.1 degrees C to 0.0 degree C warmer than arterial perfusate temperature during subclavian cannulation. The authors conclude that when femoral perfusion cooling is used, blood is warmed as it flows in a retrograde direction in the descending aorta to the carotid artery.