Monitoring of somatosensory evoked potentials: a primer on the intraoperative detection of spinal cord ischemia during aortic reconstructive surgery.

The acute interruption of blood supply to the spinal cord during thoracic and thoracoabdominal aortic reconstructions, if unabated, inevitably causes neurological injury secondary to regional hypoxia. Techniques that address the multifactorial nature of spinal cord ischemic injury have evolved to preserve neuromotor function. However, the overall incongruity of the spinal cord's vascular anatomy makes it virtually impossible to predict, with any degree of certainty, the duration of aortic cross-clamping (AXC) that can safely be endured. The sensitivity of evoked potential monitoring to the disruption of spinal cord perfusion has led to the emergence of this modality as an effective tool at the surgeon's disposal for the intraoperative assessment of distal aortic perfusion and cord viability during proximal AXC. Somatosensory evoked potentials (SSEP) provide invaluable diagnostic data as to the status of cord function, through the continuous appraisal of signal amplitude and latency. A latency increase, as small as 10% of the pre-AXC value, is linked to a reduction of spinal cord perfusion pressure and thereby associated with a high incidence of neurological impairment. Four discrete types of SSEP responses have been identified to represent differing surgical scenarios during AXC. The Type I response (deterioration of SSEP within 3 to 5 minutes) is indicative of a failure to maintain a distal pressure of at least 60 mm Hg, whereas a Type II signifies adequate distal aortic perfusion. Sudden loss of signal as witnessed in a Type III SSEP implies compromised critical intercostal vessels and indicates their expeditious reimplantation. A gradual (30 to 50 minutes) SSEP "fadeout" corresponds to marginal distal perfusion, suggesting the presence of extensive pathology. Intraoperative evoked potential monitoring, in conjunction with distal aortic perfusion, permits rapid identification and correction of compromised spinal cord blood flow, permitting repair of aortic lesions without the added liability of time constraints.