Measurement of brain electrical impedance: animal studies in rheoencephalography

In spite of the known importance of cerebral blood flow (CBF) monitoring for aviation, spaceflight, military and emergency medicine, and neurosurgical intra- and postoperative monitoring, there is no standard noninvasive technique for continuous CBF monitoring. One potential method for this purpose is the electrical impedance technique, called rheoencephalography (REG). The development of improved electronics and computation tools has done much to overcome the difficulties of REG measurement. REG technology now has possibilities for application to the fields mentioned above.

Our hypothesis was that REG would reflect CBF changes.

Three experimental studies were undertaken to further define in vivo (rat, pig) CBF measurements by analysis of REG pulse waves. CO2 inhalation (4-20%), brain electrical stimulation, and aorta compression (5 min) were the applied CBF manipulations. In the case of aorta compression, global CBF was measured by REG, and local CBF by the laser Doppler method. Data were digitized and processed off-line.

During CO2 inhalation and electrical stimulation of the brain, REG amplitude increased, indicating increased cerebral fluid volume. A linear relationship was established between CO2 concentration and REG peak amplitude (correlation coefficient: 0.88, p = 0.05), and the ascending portion of the curve (0.88, p = 0.05). During aorta compression, systemic arterial pressure increased (p = 0.008), and REG amplitude decreased (-23.75%, p = 0.01).

These studies have confirmed the REG amplitude changes during known CBF manipulations. The difference between local and global CBF response demonstrated CBF autoregulation and heterogeneity. Together, these studies indicate the usefulness and potential benefit of computerized REG monitoring for the above-mentioned fields.