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Relationship of bioimpedance to thermodilution and echocardiographic measurements of cardiac function

Bioimpedance cardiography has been suggested as a noninvasive means to monitor cardiac function. However, this method has not been compared to more conventional techniques such as echocardiography. This study compared simultaneously obtained thermodilution cardiac output and right ventricular ejection fraction (RVEF), and echocardiographic left ventricular fractional shortening (LVFS), to bioimpedance cardiac output and the maximum first derivative of the bioimpedance signal (dZ/dtmax) during positive inotropic stimulation and preload reduction. Eight pigs were instrumented with a rapid response thermistor (positioned in the pulmonary artery) and bioimpedance electrodes. Simultaneous thermodilution, echocardiographic, and bioimpedance measurements were performed at baseline and after 5,10, and 15 min of isoproterenol infusion (0.5 μg/kg.min). In six pigs, measurements were also performed after balloon occlusion of the inferior vena cava. A significant correlation was observed between LVFS and dZ/dtmax (r = .88, n = 35) over all time points. Thermodilution and bioimpedance cardiac output were in close agreement (r = .92, n = 35). However, bioimpedance overestimated cardiac output in the very low and high output states. The mean difference between thermodilution and bioimpedance cardiac outputs was — 0.02 ± 0.37 L/min. There was a positive relationship between RVEF and dZ/dtmax (r = .54, n = 35). In summary, bioimpedance was significantly correlated with thermodilution cardiac output over a wide hemodynamic range. The peak first derivative of the bioimpedance signal dZ/dtmax may provide a noninvasive index of ventricular pump performance. Further studies are required to evaluate the diagnostic value of bioimpedance cardiography in the clinical setting. (Crit Care Med 1990; 18:414)