In biomedical engineering, bioimpedance may be a measure of changes in the electrical conductivity of the thorax or heart. In that case, the measure is sometimes based on pulsatile blood volume changes in the aorta. Bioimpedance is relevant to the development of devices to measure cardiac output and circulating blood volume. Electrical conductivity can vary as a result of breathing. Because of that and other variability, the reliability of bioimpedance for obtaining accurate data has been called into question.

source:Wikipedia