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Objective: In order to determine whether critical intercostal artery is present in the aneurysm during descending thoracic or thoracoabdominal aortic surgery, changes of transcranial motor-evoked potentials (Tc-MEPs) were monitored following infusion of cold blood into the aorta as an adjunct 'on-site assessment'. Accuracy of this method was evaluated. Methods: Fourteen patients were examined for Tc-MEPs changes following infusion of cold blood (4 °C, 300-450 ml) into the aneurysm. The intercostal arteries in the aneurysm were reconstructed when the Tc-MEPs amplitude decreased to below 50% of the baseline within 3 min after cold blood infusion. When the amplitude did not decrease, every intercostal artery in the aneurysm was ligated. Results: The Tc-MEPs amplitude did not decrease in eight cases (57%), while it decreased in six cases (43%). In the former, no case presented with paraplegia despite every intercostal artery being ligated. In the latter, the amplitude recovered after reconstruction in four patients, who had no paraplegia postoperatively. In the remaining two cases, however, the amplitude did not recover: one died of multiple organ failure with postoperative assessment unfeasible; the other developed paraplegia following surgery. Except one case with operative death, both sensitivity and specificity of our criteria with cold blood infusion was 100% in this series. Conclusions: Cold blood infusion into the clamped segment of aorta accelerates Tc-MEPs changes and can possibly reduce ischemic insults of spinal cord during diagnostic process, while it accurately detects presence of critical intercostal artery in the segment. This method appears to be promising adjunct on-site assessment.