The drainage of CSF has emerged as an accepted intervention for reducing the risk of ischemic paralysis in patients undergoing thoracoabdominal aortic aneurysm repair. An important determinant of oxygen supply to the spinal cord is the spinal cord perfusion pressure (SCPP), defined as the gradient between mean distal aortic arterial pressure (MDAP) and CSF pressure (CSFP) (WADA et al., 2001). Conceptually, the drainage of CSF reduces CSF pressure and thus increases the perfusion pressure and oxygen delivery to the spinal cord. When cross clamping of the aorta occurs, the distal aortic pressure decreases and the gradient between arterial and CSF pressure narrows. Additionally, a significant increase in CSF pressure has been demonstrated to occur with aortic cross-clamping itself (WADA et al., 2001), which would serve to further reduce the spinal cord perfusion pressure. The reasons for this increase in CSF pressure with aortic cross-clamping are unclear, but may be related to changes in venous capacitance causing sequestration of venous blood within the dura and bony neural axis (Piano and Gewertz, 1990). Seminal animal experiments performed decades ago demonstrated that following descending aortic occlusion, a decrease of arterial pressure to or below that of CSF pressure (decreasing SCPP) almost universally resulted in paraplegia, while the drainage of CSF to reduce CSF pressure (and thus increase SCPP) was effective in preventing paraplegia. (MIYAMOTO et al., 1960; Oka and Miyamoto, 1987).