[The noninvasive determination of cardiac output by means of impedance cardiography. Comparative evaluation with a thermal dilution technique (author's transl)].

When a high frequency alternating current is passed through the thorax by means of tape-electrodes, the electrical impedance Z is changing synchronously with the cardiac cycle. The relation between the first derivative dZ/dt and the stroke volume is the basis for the application of impedance cardiography (IK) Determinations of cardiac output (CO) at rest during ergometer exercise using this noninvasive technique were compared with CO values obtained by the thermal dilution method. In 57 subjects simultaneous determinations using both methods were performed at resting conditions. The correlation coefficient was r = 0.63. In addition, in 48 of the subjects CO was measured with both methods during supine ergometer exercise. Comparison of the exercise values resulted in a correlation coefficient of r = 0.63. In all 48 cases there was a concordant change of the IK and thermodilution CO values. It is concluded that relative changes of CO are reliably detected by IK. The absolute values can be utilized only approximately.