Source:http://linkedlifedata.com/resource/umls/id/C1322951
UMD: Cardiographs used for recording the variations of the thoracic electrical impedance caused by the activity of the heart muscle, usually detected at the body surface. A high-frequency (usually between 20 and 200 kHz), low intensity current is passed longitudinally across the body segment, and, according to Ohm's law, the product of this current multiplied by the body impedance generates a voltage between two sensing electrodes. The graphic representation of the variation of the impedance (sometimes and/or its first derivative also) with time is recorded in a diagram (impedance cardiogram). These cardiographs are used to study the action of the heart muscle, for diagnosis of specific cardiac abnormalities and to reveal trends or changes in heart function. Impedance cardiograph data may be also use to calculate the stroke volume and cardiac output.