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

The objective of this project was to develop a non-invasive and portable brain imager, based on electrical impedance tomography, for use by anaesthetists. fEITER gives the 100 frames per second resolution needed to study sub-second mechanisms underlying consciousness1. fEITER met the requirements of the Medical Device Directive, received MHRA “no objection”, and a favourable ethical opinion from S.Manchester LREC (ISRCTN 93596854).All subjects gave written, informed consent. fEITER injected a sinusoidal current (1mA pk-pk, 10kHz) between opposite electrode pairs (ZipPrep, Covidien,UK), pseudo-randomly selected from 32 electrodes positioned using the 10-20 montage. Non-current injection electrodes were used for >500 voltage measurements during each 10ms measurement frame. Flash stimuli were presented using fEITER. Stage 1 of the study (awake, completed) requiredthe recruitment of twenty ASA I volunteers. Twenty ASA I or II patients scheduled for elective surgery with BIS monitoring are being recruited now for stage 2 (anaesthetised).The rheoencephalograph2 was time-locked to the ECG but with different latencies depending on the position of the recording on the head. Sub-second responses to single flashes were of sufficient magnitude to reconstruct as 3D maps of conductivity change that highlighted the visual cortex and frontal lobes in temporal ranges expected for visual processing. Large, subsecond changes in trans-cerebral impedance were noted during propofol induction (figure 1).References: 1. Koch C, Greenfield S. Scientific American 2007; 297: 76-83; 2. Bodo M et al. J. Physics 2010; doi:10.1088/1742-6596/224/1/012088.Acknowledgements: This study was supported by the NIHR Manchester Biomedical Research Centre, and funded by the Wellcome Trust.