A device that records the variations in thoracic electrical impedance caused by the activity of the heart muscle, usually detected at the body surface. A current is passed through the body and the product of this current and the body impedance is the electrical potential difference between two sensing electrodes. The graphic representation of the variation of the impedance with time is recorded in a diagram (impedance cardiogram). It is used to study the action of the heart muscle, for diagnosis of specific cardiac abnormalities, and to reveal trends or changes in heart function, especially cardiac output. Some types may include additional measurements (e.g., pulse oximetry, blood pressure).