The importance of blood resistivity in the measurement of cardiac output by the thoracic impedance method

Twenty simultaneous pairs of cardiac output values from patients who did not have valvular abnormalities were obtained by the radioisotope method and the electrical-impedance method of W. G. Kubicek et al. If a standard value of 150 OMEGA -cm was assumed for the resistivity of each patient's blood, the mean value for the impedance cardiac output was 14. 5% high compared with the mean radioisotope value. In this study the patient's haematocrits ranged from 20 to 48%. Inserting the appropriate value of the resistivity for each patient into the stroke volume equation of Kubicek from the data of Geddes and Sadler (see Engineering Index 1973 Abstract No. 029661) made the mean impedance value 10. 3% low compared with the mean isotope value. The use the present authors' measured resistivity data made the mean impedance cardiac output value 21. 5% lower than the mean isotope value. The correlation coefficient between the impedance and isotope techniques was 0. 61 for the standard value of resistivity of 150 OMEGA -cm. Using the resisitivity data of Geddes and Sadler the correlation became 0. 87, and with the present authors' data it was 0. 88.