Electrical Impedance Plethysmography

Abstract

Variations of electrical impedance of various parts of human body with physiological activity are well known. Fluctuations in the electrical impedance of human thorax are synchronous with both respiratory activity and also with the cardiac cycle. Electrical impedance plethysmography is used to estimate the volume changes in different parts of the body due to pulsation of blood. Impedance pneumography is used to monitor volume changes in the thorax with ventilation. Impedance cardiography (ICG), a simple and noninvasive method, is used to estimate cardiac output based on the variation of thoracic impedance during the cardiac cycle. Clearly, changes in global impedance of the thorax, with cardiac activity, are produced not only by one source. Ventricular volume changes, blood volume in the great blood vessels, variation of blood flow rate and pulmonary circulation, changes in the geometry of the thorax are all contributors to the global impedance signal. But their relative contributions to the global impedance variation are not fully understood and still subject to continuing research.