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

The original objective of this work was to verify the possibility of using electrical pulsatile cerebral impedance measurements as a diagnostic aid fo r assessing the brain-death condition in adults; a subordinate target was t o validate a simple method for detecting perfusional changes in the brain. To this end, impedance signals were recorded, for a comparative study, from both live subjects and blain-dead patients, using a simple four-electrode arrangement. Rather unexpectedly, pulsatile transcephalic impedance wavefor ms exhibiting a temporal dependance similar to those of live subjects were detected in artificially ventilated, cerebrally dead, adult subjects, distr ibutions of the time delays between impedance peaks and ECO peaks were also recorded for the two groups (dead and live subjects). These data provided no evidence, at the 1% significance level, against the hypothesis that the two sample groups are drawn from identical populations. The detection of im pedance variations from brain-dead patients can be explained by the residua l persistence of blood flow through the scalp, by mechanical variations syn chronous with the heart beat and by the presence of the oscillating flow an d the systolic spikes that precede the final blood flow attest. The fact that impedance variations can be traced back to a multiplicity of causes, unrelated to the normal unidirectional flow, renders the transcepha lic impedance method inappropriate for detecting cerebral perfusion changes in adults. This conclusion is also strengthened by some theoretical results recently d erived from a multilayer model of the head.