Mild hypothermia protects the spinal cord from ischemic injury in a chronic porcine model

Objectives: During thoracoabdominal aortic aneurysm repair, prolonged compromise of spinal cord blood supply can result in irreversible spinal cord injury. This study investigated the impact of mild hypothermia during aortic cross-clamping on postoperative paraplegia in a chronic porcine model. Methods: The thoracic aorta was exposed and cross-clamped in 30 juvenile pigs (20-22 kg) for different intervals at normothermia (36.5°C), and during mild hypothermia (32.0°C). Three pigs were evaluated at each time and temperature. Myogenic motor-evoked potentials (MEPs) were monitored, and postoperative recovery evaluated using a modified Tarlov score. Results: There were no significant hemodynamic or metabolic differences between individual animals, and the groups had equivalent arterial pressures (mean 64.3±3.6 mmHg). Time to recovery of MEPs correlated with severity of injury; all animals with irreversible MEP loss suffered postoperative paraplegia. At normothermia, animals with 20 min of aortic cross-clamping emerged with normal motor function, but those cross-clamped for 30 min suffered paraplegia. With mild hypothermia, animals tolerated 50 min of aortic cross-clamping without evidence of neurologic injury, but were all paraplegic after 70 min of ischemia. Animals appeared to recover normal motor function after 60 min of aortic cross-clamping at hypothermia initially, but exhibited delayed-onset paraplegia 36 h postoperatively. Conclusions: Our observations indicate that mild hypothermia dramatically increases the tolerance of the spinal cord to ischemia in the pig, and therefore suggests that cooling to 32.0°C should be encouraged during surgery which may compromise spinal cord blood supply. An ischemic insult of borderline severity may result in delayed paraplegia.