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This study examines how the recording of the lumbar and subcortical components of the posterior tibial nerve (PTN) SEPs may usefully replace that of cortical components in situations in which these components cannot be reliably obtained (infants, high concentrations of halogenated gasses). Lumbar, brain-stem, and cortical PTN SEPs were intraoperatively monitored in 7 patients undergoing repair of aortic coarctation under variable isoflurane concentration (up to 1.2%). Four patients were less than 1 year old. Two distinct activities were evidenced at the lumbar level in all of the patients: the dorsal root component (DRC) and the dorsal horn negativity (DHN). The equivalent of the adult P30 (lemniscal positivity; LP) was also present in all of the patients, whatever their age or the concentration of isoflurane. By contrast, the parietal activities were absent intraoperatively in the youngest patients. Spinal-cord ischemia consecutive to aortic cross-clamping gave rise to early DHN changes and later alterations of the LP in the two patients in which it occurred, while the DRC and the peripheral nerve activities remained unchanged. This elective sensitivity of the DHN is likely due to it being dependent on the gray matter of the spinal cord, the basal metabolism of which is greater than that of the white matter and to the situation of the DHN generator in a watershed zone of the spinal cord. This study emphasizes the interest of PTN SEPs for spinal-cord monitoring in vascular surgery and the importance of combining the recording of parietal activities with that of the lumbar spinal components.

Median nerve somatosensory evoked potentials (SEPs) were recorded in 9 patients undergoing profound hypothermia for surgical repair of the aortic arch. In addition to the known increase in peak latencies, hypothermia gave rise to the appearance of peaks ('P13,' 'N14') inconsistently recognized at normothermia; moreover, profound hypothermia is associated with the disappearance of cortical activities around 20 degrees, of subcortical waves at lower temperatures. The practical implications of the results are 3-fold: firstly, they suggest that the 'P13' and P14 should both be intracranially generated, at a pre- and postsynaptic level with respect to the cuneate nucleus, respectively; secondly, they show that some discrepancies between previous papers dealing with SEPs and hypothermia can be explained by differences in the choice of the reference; thirdly, they bring some suggestions on a better use of SEPs to monitor patients undergoing aortic arch surgery.