Comparison between PtCO(2) and PaCO(2) and Derived Parameters in Heart Failure Patients during Exercise: A Preliminary Study

Evaluation of arterial carbon dioxide pressure (PaCO(2)) 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 PaCO(2) by means of a non-invasive technique, transcutaneous PCO(2) (PtCO(2)), and to verify the correlation between PtCO(2) and PaCO(2) 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. PaCO(2) and PtCO(2) values were collected at rest and every 2 min during active pedaling. The uncertainty of PCO(2) and V(D)/V(T) measurements were determined by analyzing the error between the two methods. The accuracy of PtCO(2) measurements vs. PaCO(2) decreases towards the end of exercise. Therefore, a correction to PtCO(2) that keeps into account the time of the measurement was implemented with a multiple regression model. PtCO(2) and V(D)/V(T) changes at 6, 8 and 10 min vs. 2 min data were evaluated before and after PtCO(2) correction. PtCO(2) overestimates PaCO(2) 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 PCO(2) and V(D)/V(T) changes. In HF patients PtCO(2) is a reliable PaCO(2) 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 PtCO(2) of PaCO(2) and V(D)/V(T).