J Davidson

Functional electrical impedance tomography by evoked response: a new device for the study of human brain function during anaesthesia

fEITER is a novel imaging device which monitors changes in cerebral impedanceacross the whole brain. We aimed to evaluate haemodynamic and electrophysiological changes across the brain in response to volunteers performing the Valsalva manoeuvre (VM). Each volunteer had 32 ZipPrepTM (Covidien, UK) electrodes placed on the scalp using the 10-20 system. The VM was initiated at 10 s and released at 25 s. Sinusoidal current of 1 mA pk-pk was injected at 10 kHz; continuous voltage data were recorded from all electrodes excluding the current injection pair, for 60 s at a temporal resolution of 10ms. The VM was performed by 15 volunteers at CMFT.Figure 1. Voltage data from a single volunteer for injection pair FPZ_OZ; voltagemeasurement pair C4_F4 (upper trace) and simultaneous chest recording of ECG(lower trace). Phases I ? IV of the VM are shown across both traces.Figure 1 (upper trace) shows sub-second cerebral transimpedance changes in response to the VM. Pooled voltage measurements beginning at 5 s were compared to measurements at 8 s for an epoch of 40 ms; no significant differences were observed during this reference period prior to the VM being initiated. Voltage measurements during the VM at 20 s were significantly different to those taken at 5 s for an epoch of 40 ms (Wilcoxon Signed Ranks test, p < 0.05).Sub-second transimpedance changes signify neural mechanisms involved in cerebral auto-regulation during the most distinct phases of the VM (I and III). The cerebral transimpedance waveform (Figure 1) is similar to those obtained from middle cerebral artery recordings previously performed using transcranial Doppler1. These findings show fEITER has the potential to monitor haemodynamic regulation across the brain.References: 1. Zhang R et al. Stroke 2004;35:843-847Acknowledgement: This study was funded by the Wellcome Trust.