Intraoperative assessment of cerebral ischemia during carotid surgery. One of the problems in carotid surgery is the intraoperative detection of brain ischemia. None of the methods (EEG; stump pressure) applied so far have been successful. Branston et al. (1974) found a threshold relationship between cortical cerebral blood flow and cortical somatosensory evoked potential (SEP). As the local blood flow fell below about 16 ml/100 g/min a progressive reduction occurred in the amplitude of the cortical evoked potential (N20/P25), implying a fundamental failure of neuronal function in the somatosensory cortex. We have monitored cortical SEP (somatosensory evoked potential) during 734 CEA's (carotid endarterectomies) in order to find an index of risk of incipient cerebral ischemia during carotid cross-clamping, to determine the need for shunting and the causes of early irreversible neurologic deficits. In 59 cases evaluation of SEP was not possible because of technical difficulties. During 586 CEA's no alteration of SEP occurred. However, 4 patients had an immediate postoperative neurologic deficit, while the SEP remained normal. Abnormal SEP occurred in 89 cases and in 6 of these an irreversible loss of SEP was seen. These patients awoke with a new neurologic deficit. We found a reversible abnormal SEP in 83 cases. Reversible changes of SEP occurred mainly during carotid clamping. The diagnostic sensitivity of intraoperative SEP monitoring in predicting neurologic outcome was 60% with a specificity of 100%. 