Modeling of end-tidal to transcutaneous oxygen pressure difference dynamics during cardio-pulmonary exercise testing: A retrospective study

BACKGROUND AND OBJECTIVE: During cardiopulmonary exercise testing (CPET), the alveolar-arterial oxygen gradient varies with exercise intensity, but direct assessment is limited by the invasiveness of arterial sampling. End-tidal oxygen pressure (PETO₂) and transcutaneous oxygen pressure (PtcO₂) may provide non-invasive estimates of alveolar and arterial oxygen pressures. This study aimed to identify the statistical model that best characterize the relationship between the end-tidal to transcutaneous oxygen pressure gradient (ET-tcDO₂) and oxygen uptake (VO₂) in apparently healthy individuals. METHODS: This retrospective study analyzed medical records (n = 68) from incremental CPETs performed with PtcO₂ monitoring. Breath‑by‑breath PETO₂ and VO₂ data from CPET tests were also extracted. The relationship between ET‑tcDO₂ and VO₂ was then characterized using functional data analysis (FDA) and polynomial regression models of increasing degree. RESULTS: FDA demonstrated high modeling accuracy (adjusted R² = 0.96). ET-tcDO2 decreased at onset of exercise, until VO2 = 1.26 L/min, and then increased until maximal exercise. Among polynomial regression models, the polynomial of the third degree best described this relationship (adjusted R² = 0.48). At the individual level, most responses followed a polynomial of the third degree, with 75% of participants exhibiting an adjusted R² above 0.7. CONCLUSION: Continuous PtcO₂ monitoring enables detailed characterization of exercise-induced alveolar-arterial gas exchange in apparently healthy individuals. ET-tcDO₂ follows a polynomial of the third-degree during CPET. The combined use of PETO₂ and PtcO₂ may offer a simple, non-invasive approach for assessing alveolar-arterial gas exchange in routine clinical practice that should be further investigated.