The influence of extravascular lung water on cardiac output measurements using thoracic impedance cardiography

The purpose of this study was to investigate the influence of pulmonary oed ema as measured with the double indicator dilution technique on the accurac y of cardiac output (CO) measurement using thoracic impedance cardiography (TIC) compared with thermodilution in thirteen sepsis patients. Differences in the Kubicek and Sramek-Bernstein equation with respect to pulmonary oed ema were explored theoretically and experimentally. From a parallel two cyl inder model a hypothesis can be derived that CO determined with the Kubicek equation is oedema independent, whereas CO determined using the Sramek-Ber nstein equation is oedema dl:pendent. Experimentally, CO determined using K ubicek's equation correlated better with thermodilution CO (r = 0.75) than CO determined with the Sramek-Bernstein equation (r = 0.25). The effect of oedema on the accuracy of TIC was investigated by comparing the differences in the CO of impedance and thermodilution to the extravascular lung water index. For the Kubicek equation the difference was not influenced by oedema (r = 0.04, p = 0.84), whereas for the Sramek-Bernstein equation the differ ence was affected by oedema (r = 0.39, p = 0.05). Thus, the effects of pulm onary oedema on the accuracy of TIC measurements can better be understood w ith the parallel cylinder model. Moreover, the Kubicek equation still holds when pulmonary oedema is present, in contrast to the Sramek-Bernstein equa tion.