Monitoring of somatosensory evoked potentials during surgical procedures on the thoracoabdominal aorta. IV. Clinical observations and results.

Thirty-three patients undergoing operations on the descending thoracic or thoracoabdominal aorta were monitored to evaluate causes and effects of spinal cord ischemia as manifested by changes in somatosensory evoked potentials. Maintenance of distal aortic perfusion pressure (greater than 60 mm Hg) by either shunt or bypass techniques in 17 patients resulted in preservation of somatosensory evoked potentials and a normal postoperative neurologic status, irrespective of the interval of thoracic cross-clamping (range 23 to 105 minutes). In 16 other patients in whom cross-clamp time ranged from 16 to 124 minutes, evoked potential loss was observed because of failure to provide distal perfusion (n = 8), inadequate maintenance of distal perfusion pressure (less than 60 mm Hg) despite shunt/bypass (n = 6), or interruption of critical intercostal arteries (n = 2). Incidence of paraplegia in the entire group was 15.1% (5/33) and was limited to only those patients in whom evoked potential loss occurred (5/16, 31.2%) (p = 0.02). Loss of somatosensory evoked potentials for more than 30 minutes resulted in a 71.2% (5/7) incidence of paraplegia, whereas no neurologic deficit was noted in patients (0/26) in whom evoked potential loss was either prevented or limited in duration to 30 minutes (p less than 0.001 versus loss for more than 30 minutes). Intraoperative monitoring of somatosensory evoked potentials is a sensitive indicator of spinal cord ischemia. Simple aortic cross-clamping, failure to maintain distal perfusion pressure above 60 mm Hg, and inability to reimplant critical intercostals in a timely fashion result in a high rate of paraplegia if duration of spinal cord ischemia as measured by somatosensory evoked potentials exceeds 30 minutes. Routine evoked potential monitoring during thoracoabdominal procedures appears useful in assessing the adequacy of spinal cord perfusion. Furthermore, it can alert the surgeon to the necessity for critical intercostal artery reimplantation as well as the need for adjustment or regulation of distal aortic perfusion.