Upper and lower extremity somatosensory evoked potential recording during surgery for aneurysms of the descending thoracic aorta

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Abstract
Since tibial nerve somatosensory evoked potentials (SEPs) recording isinfluenced by hemodynamic changes and anesthetics, alterations cannotalways be attributed to spinal cord ischemia, so causing false positiveresults. Additional recording of median nerve SEPs facilitatesinterpretation. From January 1988 to July 1993, 60 consecutive patients (44men, 16 women, mean age 66 years, ranging from 26 to 83 years) underwentsurgery for an aneurysm of the descending thoracic aorta using anon-heparinized left heart bypass (Biomedicus pump). In 40 patientsrecording of the tibial and median nerve SEPs was performedintraoperatively by stimulating both nerves alternately. In 32 patients(80%) both recordings were uneventful. In three patients (7.5%) the tibialnerve SEP temporarily disappeared due to peripheral ischemia on terminationof the bypass for the creation of an open distal anastomosis. In threepatients (7.5%) near loss of both tibial and median SEP recordings wascaused by low blood pressure and/or anesthetics. In two patients (5%)isolated loss of the tibial nerve SEP was due to ischemia in the spinalpathway of the tibial nerve. The tibial nerve SEP signal returned tonormal: in one patient after reperfusion of intercostal arteries localizedwithin the aneurysm, in the other patient after drainage of cerebrospinalfluid (CSF). Continuous recording of both tibial and median nerve SEPsgives consistent information on spinal cord ischemia, reducing the falsepositive rate of the lower extremity SEP to 7.5%.

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