A quasi-power theorem for bulk conductors: comments on rheoencephalography

A special case of Tellegen´s theorem, the quasi-power theorem, is developed for bulk conductors, and a bulk conductor analog of Cohn´s theorem is developed that can be used to predict the sensitivity of impedance plethysmographic measurements to tissue impedance changes in a system of bulk conductors. An example addressing the effects of scalp and cranium on the inference of regional blood flow in brain tissue raises questions about the origins of the REG (rheoencephalogram). While the author feels this work suggests further investigation into the origins of the REG is warranted, its clinical usefulness, based on validated correlations with cerebral blood flow, remains undisputed.