Impedance cardiography (ICG) is a method which calculates Q from the measurement of changes in impedance across the chest over the cardiac cycle. Lower impedance indicates greater the intrathoracic fluid volume, and as the only fluid volume which changes beat to beat within the thorax is the blood, the change in impedance can be used to calculate the SV and, combined with HR, the Q. This technique has progressed clinically (often called BioZ, i.e. biologic impedance, as promoted by the leading manufacturer in the US) and allows non-invasive estimations of Q and total peripheral resistance using only 4 paired skin electrodes.
7Oceanz sells the BioZ and BioZ Dx monitor from CardioDynamics as well as other used medical equipment . These units are in like new condition and you can save a substantial amount of money by buying a used Bioz machine.