Abstract Background In Fontan patients, the slope of invasively measured pulmonary artery pressures plotted against cardiac output (CO) during exercise has prognostic significance. We propose a methodology using exercise echocardiography combined with cardiopulmonary exercise testing and simultaneous peripheral venous pressure measurements (CPET-echo-PVP) to calculate PVP/CO slopes as a non-invasive alternative. Purposes This study aimed at assessing (1) PVP/CO slopes and (2) their relationship with peak oxygen consumption in Fontan patients. Material and Methods Consecutive adult Fontan patients seen in the outpatient clinic at UZ Leuven, Belgium were included in the study. All patients underwent CPET-echo-PVP (peripheral intravenous line connected to the pressure transducer zeroed at the mid-axillary level). Bloodwork was performed, including N-terminal prohormone of brain natriuretic peptide (NTproBNP). The PVP/CO slope was calculated as a linear approximation using linear regression analysis from pressure-flow plots for each patient. Univariable regression analysis was performed to identify potential determinants of PVP/CO slope. Results In total 20 patients (mean age 30.5±10.4 (range 20-60), 11[55%] females, dominant left ventricle in 15 [75%]) were included. NT-proBNP was 297±235 pg/mL. Patients performed staged exercise test achieving workload of 102±30 Watts with a peak oxygen consumption (peakVO2) of 19.2±3.8 mL/kg/min. During exercise cardiac output (CO), heart rate (HR) and PVP all increased (mean difference: 4.1±2.2L/min, 57±26bpm, 10.4±5.9mmHg, respectively). Stroke volume index (SVind) increased from baseline to peak exercise in 9 (45%) (mean difference -2.6±12.0mL/m2). The average PVP/CO slope was 2.21±1.53mmHg/L*min, with the value exceeding 3mmHg/L*min in 5 (25%) patients. PeakVO2, peak Workload during exercise, NTproBNP, New York Heart Association (NYHA) class, and rest-to-peak change in tissue-Doppler e’ (delta e’) of the lateral wall of dominant ventricle were significantly related to the PVP/CO slope (all p<0.05). Conclusions CPET-echo with PVP is a reliable method to identify determinants of exercise limitation in Fontan patients. The PVP/CO slope, which reflects total pulmonary resistance, is indeed related to peak oxygen consumption, heart rate reserve and diastolic function. Since it reflects hydraulic load to the systemic venous system it may be an important target in Fontan patients.