The Minnesota impedance cardiograph - theory and applications

This system has evolved into a noninvasive device to derive information relating to the function of various parts of the cardiovascular system. The only contact with the body is via 4 aluminium coated Mylar electrode strips, 2 around the neck and 2 around the abdomen. The outer 2 electrodes are connected to a constant current oscillator that delivers a small current at a high frequency. The other 2 electrodes are information electrodes, connected to a high impendance input amplifier and detecting circuits. During the cardiac cycle a small impedance change occurs and a differentiator extracts the first time derivative of this impendance change, dZ/dt. In flowmeter studies on animals it was found that a linear relation obtained between peak dZ/dt and peak ejection rate. This formed the basis of extracting the stroke volume and cardiac contractility indices from thoracic impedance changes.