Clinical evaluation of impedance cardiography

We tested the validity of thoracic impedance cardiography for measuring cardiac output in man by comparing absolute values obtained using the non-invasive impedance method with values obtained using the invasive thermodilution method. We also compared per cent changes in cardiac output using impedance cardiography and thermodilution in response to environmental manipulations including cardiac pacing and intravenous administration of ergonovine, dipyridamole, or isoproterenol. Among 19 patients, absolute values for cardiac output, using the impedance and the thermodilution techniques, agreed well (r = 0.85, P less than 0.001). The per cent change in cardiac output by impedance cardiography was positively correlated with the per cent change by thermodilution for the several manipulations (overall r = 0.87, P less than 0.001). Impedance cardiography does appear, in general, to measure cardiac output and stroke volume validly in man, even in situations where heart rate and stroke volume change in opposite directions.