Impedance cardiography: the failure of sternal electrodes to predict changes in stroke volume

A study was conducted on 26 healthy male adults comparing the stroke volume (SV) and cardiac output (CO) measured with green dye and impedance cardiography in supine, sitting and during 50 W bicycle exercise. The impedance voltage pick-up electrodes were modified from the standard band location and placed at the suprasternal notch and 10 cm below. The percentage change of SV going from supine to sitting was -27.2 +/- 7.4% and -16.9 +/- 22% for the dye and impedance values respectively. The percentage change of SV going from sitting to exercise was 20.9 +/- 11.9% and 36.7 +/- 32.9% for the dye and impedance values respectively. The percentage change in SV determined by impedance shows poor correlation (supine to sitting R = 0.16 and sitting to exercise R = 0.42) with dye values. The individual impedance values for a given change in position or exercise showed considerably more variation compared with the dye. Impedance determination of SV using electrodes on the sternum does not appear to be a valid method for SV and CO measurements.