UPPER AND LOWER-EXTREMITY SOMATOSENSORY-EVOKED POTENTIAL RECORDING DURING SURGERY FOR ANEURYSMS OF THE DESCENDING THORACIC AORTA

Since tibial nerve somatosensory evoked potentials (SEPs) recording is influenced by hemodynamic changes and anesthetics, alterations cannot always be attributed to spinal cord ischemia, so causing false positi ve results. Additional recording of median nerve SEPs facilitates inte rpretation. From January 1988 to July 1993, 60 consecutive patients (4 4 men, 16 women, mean age 66 years, ranging from 26 to 83 years) under went surgery for an aneurysm of the descending thoracic aorta using a non-heparinized left heart bypass (Biomedicus pump). In 40 patients re cording of the tibial and median nerve SEPs was performed intraoperati vely by stimulating both nerves alternately. In 32 patients (80%) both recordings were uneventful. In three patients (7.5%) the tibial nerve SEP temporarily disappeared due to peripheral ischemia on termination of the bypass for the creation of an open distal anastomosis. In thre e patients (7.5%) near loss of both tibial and median SEP recordings w as caused by low blood pressure and/or anesthetics. In two patients (5 %) isolated loss of the tibial nerve SEP was due to ischemia in the sp inal pathway of the tibial nerve. The tibial nerve SEP signal returned to normal: in one patient after reperfusion of intercostal arteries l ocalized within the aneurysm, in the other patient after drainage of c erebrospinal fluid (CSF). Continuous recording of both tibial and medi an nerve SEPs gives consistent information on spinal cord ischemia, re ducing the false positive rate of the lower extremity SEP to 7.5%.