Evaluating Monitoring Techniques for Postoperative Spinal Cord Ischemia

During open surgery of a thoraco-abdominal aortic aneurysma (TAAA), diminished blood flow to the myelum can result in hypoxia, compromising proper function of the spinal cord. Intraoperatively, motor evoked potentials (MEP) are elicited to measure the functional integrity of the spinal cord. MEPs have proven to be a reliable marker of spinal cord ischemia. Moreover, these potentials react within minutes, which facilitates interventions to restore the blood flow. Monitoring intraoperatively with this ancillary test has reduced the rate of paraparesis to \< 5%. Unfortunately, in the early postoperative period, spinal cord vulnerability is high. Therefore, some patients develop paraparesis, not during the surgical procedure, but after the surgical procedure. Postoperatively, suboptimal blood flow may lead to critical loss of function. This inadequate perfusion results in "delayed paraparesis". In the postoperative patient, it is not possible to measure MEPs when sedation is decreased, due to the high intensity of the electrical stimulus, which is unacceptably painful in the unanesthetized or partially anesthetized patient. Therefore ancillary tests are needed which can detect spinal cord ischemia postoperatively early, thus preceding the phase with clinically overt paraparesis. The test should be reliable and easy to perform for an extended period of time (up to several days). The purpose of this study is to explore the usefulness of various neurophysiological tests regarding accuracy and feasibility for the detection of spinal cord ischemia. In particular, to find a diagnostic test which is acceptable for the unanesthetized or partially anesthetized patient and therefore can also be performed postoperatively. These tests will be examined in fully sedated as well as partially sedated patients. The following candidate tests will be examined: 1. Long loop reflexes (LLR) consisting of F-waves. 2. Oxygenation measurements of the paraspinal muscles using Near-infrared spectroscopy (NIRS).