Stroke volume measurement by impedance cardiography using a formula based on the delta z waveform

Abstract

In 12 patients with systemic hypertension the cardiac stroke volume was determined simultaneously by the dye dilution technique and by impedence cardiography. The impedance stroke volume was calculated using Kubicek's formula (using four different values of blood resistivity) and also by the formula sv=AL delta z/Z0 (from the transthoracic impedance change, the delta z waveform). Significant correlations were found between the stroke volume measured by the impedance and reference methods and there were significant differences between the correlation coefficients (range 0.81-0.92) irrespective of which formula was used or which value of blood resistivity was inserted in the Kubicek formula. Only the delta z formula gave no significant differences for both the mean difference and the regression coefficient. Measurement of the stroke volume with the delta z formula requires breath holding, but determination of the systolic ejection time or blood resistivity is not necessary, unlike the Kubicek formula. A low peak value of dz/dt indicated a poor correlation with the dye dilution method, and also indicated a reduced maximum rate of ejection of blood from the heart. The effective delta z formula affords a different approach for future investigations of the genesis of the cardiac impedance changes in the thorax.