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

Abstract

The original objective of this work was to verify the possibility of using electrical pulsatile cerebral impedance measurements as a diagnostic aid for assessing the brain-death condition in adults; a subordinate target was to 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 brain-dead patients, using a simple four-electrode arrangement. Rather unexpectedly, pulsatile transcephalic impedance waveforms exhibiting a temporal dependance similar to those of live subjects were detected in artificially ventilated, cerebrally dead, adult subjects; distributions of the time delays between impedance peaks and ECG 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 impedance variations from brain-dead patients can be explained by the residual persistence of blood flow through the scalp, by mechanical variations synchronous with the heart beat and by the presence of the oscillating flow and the systolic spikes that precede the final blood flow arrest. The fact that impedance variations can be traced back to a multiplicity of causes, unrelated to the normal unidirectional flow, renders the transcephalic impedance method inappropriate for detecting cerebral perfusion changes in adults. This conclusion is also strengthened by some theoretical results recently derived from a multilayer model of the head.

Intracranial microprobe for evaluating neuro-hemodynamic coupling in unanesthetized human neocortex

“…An average (Fig. 4b) of 500 EKG pulses elucidates a 12% peak increase in blood flow, as well as a latency of 150ms between the R-peak and rCBF increase, consistent with previous studies (Montgomery et al, 1995; Basano et al, 2001). …”

“…As predicted, the heartbeat strongly modulated the rCBF inferred from laser Doppler, with the main rCBF peak occurring 150ms following the R-peak in the QRS wave of the EKG, consistent with prior estimates of the vascular delay from the heart to the brain (Montgomery et al, 1995; Basano et al, 2001; Kucewicz et al, 2007). While latency time periods have been relatively consistent across studies, the direction and absolute size of the peak change from baseline is highly variable due to a number of reasons including measurement modality and location.…”

“…The pulsatile cerebrovascular response, synchronized to the EKG, is a strong and reliable signal that has been used to validate various novel non-invasive and invasive hemodynamic measurement techniques (Montgomery et al, 1995; Basano et al, 2001; Zhang et al, 2006; Kucewicz et al, 2007). In each of these studies, an increase in rCBF was observed after each cardiac pulse, with a latency of 100–200ms measured from the R-peak of the QRS wave to the initial increase in rCBF.…”

Influence of the scalp thickness on the intracranial contribution to rheoencephalography

“…With regard to REG II, most authors support the hypothesis that the impedance signals obtained in their experimental studies could be due (to a greater or lesser extent) to the scalp blood flow (Jacquy et al 1974, Weindling et al 1982, Jevning et al 1989. In this sense, the results obtained by Basano et al (2001) are significant, who found no significant differences between the REG II waveforms obtained from healthy subjects and those obtained from braindead patients, whose blood flow in the internal carotid artery was shown to be completely stopped with transcranial Doppler sonography. However, it must be noted that several authors have measured true brain pulsatile impedance using intracerebral electrodes both in humans (Laitinen 1968) and in animals (Bodo et al 2003).…”

To what extent is the bipolar rheoencephalographic signal contaminated by scalp blood flow? A clinical study to quantify its extra and non-extracranial components

“…This controversy led REG II configurations to be preferred to REG I, even though Basano et al [12] did not find any differences between the REG II signals registered from healthy subjects and those from patients with brain death diagnosis.…”