Electrical impedance plethysmography: a critical analysis.

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Abstract
In the course of calibrating an electrical impedance plethysmograph for blood flow monitoring, it was found that It was not only uncallbratable, but that the theory of this type of plethysmography was also invalid. The widely held concept of electrical current flow in blood-containing tissues as a function of blood shunt pathway impedance and volume conductance was found to be in error because of the extremely small (immeasurable) electrical impedance alteration of the tissue segment under measurement. On the other hand, very large impedance changes occur at the electrode-tissue interface because of a variety of (actors, including the primary one of simple outward mechanical pressure which is produced by the systolic volume filling. This produces a good volume-impedance "curve fit," but it has been misinterpreted as arising from conductance changes within the blood/tissue volume ratio. Laboratory experiments with bovine tissues and with simulated blood volume changes in a synthetic system showed that electrical impedance plethysmo-graphs of all types, including the so-called impedance spirometer and the so-called rheoencephalograph, produce totally uncallbratable strain-gauge-type signals.