Prevention of paraplegia during aortic operations. Ischemic spinal cord injury after aortic cross-clamping may be produced by a steal phenomenon. The present study investigates this possibility by directly measuring the oxygen tension on the spinal cord surface in pigs. After simple clamping of the aorta, oxygen tension decreased significantly distal to the clamping site both after occlusion of the thoracic aorta at T3-4 (group 1) and after occlusion of the abdominal aorta at L-1 (group 2). Exclusion of the thoracic aorta by a second clamp at T-13 restored oxygen tension almost to the original level, whereas segmentation of the abdominal aorta up to S-1 hardly affected oxygen tension in the area of the artery of Adamkiewicz in most of the animals. We conclude that after aortic cross-clamping, blood tends to drain away from the spinal cord rather than supplying it longitudinally. Without knowledge of the position of the Adamkiewicz artery in humans as well as of the competence of the collateral circulation in the excluded segment, it is necessary to develop a new strategy for repair of the aorta. We describe and discuss two surgical techniques for the prevention of paraplegia after aortic cross-clamping: the counterocclusion technique and the bypass fractionated technique. 