Pulsatile electrical impedance response from cerebrally dead adultpatients is not a reliable tool for detecting cerebral perfusion changes

Abstract

The original objective of this work was to verify the possibility of usingelectrical pulsatile cerebral impedance measurements as a diagnostic aid forassessing the brain-death condition in adults; a subordinate target was tovalidate a simple method for detecting perfusional changes in the brain. Tothis end, impedance signals were recorded, for a comparative study, from bothlive subjects and brain-dead patients, using a simple four-electrodearrangement. Rather unexpectedly, pulsatile transcephalic impedance waveformsexhibiting a temporal dependance similar to those of live subjects weredetected in artificially ventilated, cerebrally dead, adult subjects;distributions of the time delays between impedance peaks and ECG peaks werealso recorded for the two groups (dead and live subjects). These data providedno evidence, at the 1% significance level, against the hypothesis that thetwo sample groups are drawn from identical populations. The detection ofimpedance variations from brain-dead patients can be explained by the residualpersistence of blood flow through the scalp, by mechanical variationssynchronous with the heart beat and by the presence of the oscillating flowand the systolic spikes that precede the final blood flow arrest.The fact that impedance variations can be traced back to a multiplicity ofcauses, unrelated to the normal unidirectional flow, renders the transcephalicimpedance method inappropriate for detecting cerebral perfusion changes inadults.This conclusion is also strengthened by some theoretical results recentlyderived from a multilayer model of the head.