Non-invasive assessment of cardiac output during exercise in chronic obstructive pulmonary disease: comparison of the CO2-rebreathing method and electrical impedance cardiography

In exercise testing of patients with chronic obstructive pulmonary disease (COPD), non-invasive assessment of stroke volume (SV) and cardiac output (CO) would be valuable. Electrical impedance cardiography (EIC) has proved to be a valid and reliable instrument in healthy subjects. In this study it is investigated whether this also applies to patients with COPD. In 19 COPD patients simultaneous SV measurements were performed during steady-state exercise using the CO2-rebreathing method and EIC (using a fixed blood resistivity value (rho = 135 or 150 omega cm: EIC-135 and EIC-150) or a haematocrit based rho (EIC-ht)). Although close correlations were found (overall correlation between CO2-rebreathing and EIC-ht: R = 0.92 for CO, R = 0.79 for SV), SV and CO measured by means of EIC were significantly higher at low-intensity exercise and lower at high-intensity exercise. The mean differences between the CO2-rebreathing method and EIC-ht were 0.55 ml for SV and 0.01 l min-1 for CO (overall exercise data). The limits of agreement (2SD of the mean difference) were 24.7 ml for SV and 2.56 l min-1 for CO. These figures are comparable to what is found when healthy subjects are studied. CO was closely correlated to oxygen uptake using the CO2-rebreathing as well as the EIC method; the slope of the regression line was closer to what has been reported in the literature with EIC. Results were better with the EIC-ht than with the EIC-135 and EIC-150 methods. It is concluded that EIC is a reliable and valid method for measurements of SV and CO in COPD during exercise.