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Non-Invasive Impedance Cardiography applies a constant alternating electrical current to the chest and measures the corresponding voltage to detect changes in thoracic impedance.
Additional sensors are placed bilaterally on either side of the neck and thorax. The outer sensors transmit the alternating electrical current, and the inner sensors determine the thoracic impedance. Of these tissues, blood is the most electrically conductive. Since arterial blood flow is pulsatile and arterial vessel walls are compliant, pulsatile changes in blood volume occur in the thoracic arterial system, predominantly in the aorta, as a result of ventricular function. This change in blood volume results in a change in the electrical conductivity and thus the impedance of the thorax to electrical current. It is the measured, dynamic, beat-to-beat changes in impedance that are processed and applied to an algorithm to calculate stroke volume and cardiac output. [23]
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