COMPARISON OF THE EFFECTS OF ISCHEMIA ON EARLY COMPONENTS OF THE SOMATOSENSORY EVOKED-POTENTIAL IN BRAIN-STEM, THALAMUS, AND CEREBRAL-CORTEX

In 14 ventilated, normocapnic baboons anesthetized with .alpha.-chloralose, local CBF [cerebral blood flow] (hydrogen clearance) and the amplitude and latency of local components of the somatosensory evoked potential (SEP, median nerve stimulation) were measured bilaterally in ventrobasal thalamus (VPL), medial lemniscus (ML) and cerebral cortex before and during progressive ischemia, produced by occlusion of the right middle cerebral artery and subsequent controlled reductions in mean systemic blood pressure (MSBP). The 1st significant reduction from control of the left cortical SEP amplitude occurred in the range of 30-40 mm Hg MSBP, but those of the VPL and ML responses only below 30 mm Hg; in the range of 20-30 mm Hg, the average SEP amplitudes in cortex, VPL, and ML were 8.6, 72.6 and 90.7% of control, respectively. In terms of local CBF, the cortical SEP threshold was in the range of 15-20 ml/100 g per min (as in previous work), that of VPL in the range of 10-15 ml/100 g per min, but the ML response was only markedly reduced below 10 ml/100 g per min. Thus, the differential ischemic sensitivity of the SEP between the 3 regions was clearly demonstrated. As the neuraxis is descended, there apparently is an increasing resistance of electrophysiological function to systemic hypotension, together with a decreasing threshold for local ischemia.