Reproducibility of the exponential rise technique of CO₂ rebreathing for measuring P_vCO₂ and C_vCO₂ to non-invasively estimate cardiac output during incremental, maximal treadmill exercise

The purpose of this study was to determine the reproducibility of the indirect Fick method for the measurement of mixed venous carbon dioxide partial pressure (P_vCO₂) and venous carbon dioxide content (C_vCO₂) for estimation of cardiac output (Q_c), using the exponential rise method of carbon dioxide rebreathing, during non-steady-state treadmill exercise. Ten healthy participants (eight female and two male) performed three incremental, maximal exercise treadmill tests to exhaustion within 1 week. Non-invasive Q_c measurements were evaluated at rest, during each 3-min stage, and at peak exercise, across three identical treadmill tests, using the exponential rise technique for measuring mixed venous PCO₂ and CCO₂ and estimating venous-arterio carbon dioxide content difference (C_v-aCO₂). Measurements were divided into measured or estimated variables [heart rate (HR), oxygen consumption VO₂), volume of expired carbon dioxide (VCO_A end-tidal carbon dioxide (P_ETCO₂), arterial carbon dioxide partial pressure (P_aCO₂), venous carbon dioxide partial pressure (P_vCO₂), and C_v-aCO₂] and cardiorespiratory variables derived from the measured variables [Q_c, stroke volume (V_s , and arteriovenous oxygen difference (C_a-vO₂)]. In general, the derived cardiorespiratory variables demonstrated acceptable (R=0.61) to high (R >0.80) reproducibility, especially at higher intensities and peak exercise. Measured variables, excluding P_aCO₂ and C_v-aCO₂, also demonstrated acceptable (R=0.6 to 0.79) to high reliability. The current study demonstrated acceptable to high reproducibility of the exponential rise indirect Fick method in measurement of mixed venous PCO₂ and CCO₂ for estimation of Q_c during incremental treadmill exercise testing, especially at high-intensity and peak exercise.