Comparison between PtCO₂ and PaCO₂ and Derived Parameters in Heart Failure Patients during Exercise: A Preliminary...

Evaluation of arterial carbon dioxide pressure (PaCO₂) and dead space to tidal volume ratio (V_D/V_T) during exercise is important for the identification of exercise limitation causes in heart failure (HF). However, repeated sampling of arterial or arterialized ear lobe capillary blood may be clumsy. The aim of our study was to estimate PaCO₂ by means of a non-invasive technique, transcutaneous PCO₂ (PtCO₂), and to verify the correlation between PtCO₂ and PaCO₂ and between their derived parameters, such as V_D/V_T, during exercise in HF patients. 29 cardiopulmonary exercise tests (CPET) performed on a bike with a ramp protocol aimed at achieving maximal effort in ≈10 min were analyzed. PaCO₂ and PtCO₂ values were collected at rest and every 2 min during active pedaling. The uncertainty of PCO₂ and V_D/V_T measurements were determined by analyzing the error between the two methods. The accuracy of PtCO₂ measurements vs. PaCO₂ decreases towards the end of exercise. Therefore, a correction to PtCO₂ that keeps into account the time of the measurement was implemented with a multiple regression model. PtCO₂ and V_D/V_T changes at 6, 8 and 10 min vs. 2 min data were evaluated before and after PtCO₂ correction. PtCO₂ overestimates PaCO₂ for high timestamps (median error 2.45, IQR -0.635-5.405, at 10 min vs. 2 min, p-value = 0.011), while the error is negligible after correction (median error 0.50, IQR = -2.21-3.19, p-value > 0.05). The correction allows removing differences also in PCO₂ and V_D/V_T changes. In HF patients PtCO₂ is a reliable PaCO₂ estimation at rest and at low exercise intensity. At high exercise intensity the overall response appears delayed but reproducible and the error can be overcome by mathematical modeling allowing an accurate estimation by PtCO₂ of PaCO₂ and V_D/V_T.