Background: Impedance plethysmography applied to the head by using a pair of\nelectrodes attached to the scalp surface is known as bipolar Rheoencephalography\nor REG I and was originally proposed to measure changes in cerebral blood volume\nrelated to the heartbeat. REG I was soon discarded in favor of other REG\nconfigurations, since most of the signal was shown to be heavily contaminated by\nthe extracranial blood flow. The main goal of this study was to identify and compare\nthe part of the REG I signal caused by scalp blood flow with that originating from\nnon-extracranial sources.\nMethods: A clinical study involving thirty-six healthy volunteers was designed for\nthis purpose. REG I was first registered in each subject under normal conditions.\nA pneumatic cuff was then placed around the head and was inflated to arrest the\nscalp blood flow and a second REG I was recorded. Finally, a third REG I was taken\nimmediately after cuff deflation.\nResults: The REG I signal is attenuated, but not extinguished, during cuff inflation in\na wide subject-dependent range ratio from 0.12 to 0.68 (0.37 �± 0.15). The residual\nREG I signal has a waveform that is markedly different from that obtained before cuff\ninflation, which supports the hypothesis of the intracranial origin of the residual REG\nI signal. Additionally, an increase of 22% in REG I amplitude was observed when the\nhead cuff was deflated.\nConclusions: Waveform differences between extra and non-extracranial components\nare significant and these differences could be used in a method to distinguish\none from the other. However, a significant part of the REG I signal is caused by\na non-extracranial source and, therefore, it should not be used as a footprint of\nthe extracranial blood flow.
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