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Background: Patients who have undergone a Fontan operation (FO) may suffer from both systolic and diastolic single ventricle (SV) dysfunction., Aim: The aim of the study was to quantify non-invasively the systolic and diastolic single ventricle function in adult FO patients and to assess its effect on exercise tolerance., Methods: We investigated 21 patients (12F; 9M) after FO with morphological left ventricle with a mean age of 26 +/- 6 years, and 17 age-matched, healthy people (control group). Pulse-wave Doppler signals from the mitral inflow, aortic flow and tissue Doppler imaging of the mitral annulus were obtained. A cardiopulmonary exercise test (CPET) was performed., Results: The Fontan patients, as compared to the controls, had a lower value of SV ejection fraction (SVEF), E velocity, E/A ratio, aortic valve velocity time integral, exercise time, SatO2, VO(2peak) and significantly higher E/E' ratio, VE/VCO(2peak) and VE/VO(2peak'). We found negative correlations between E/E' and exercise time and VO(2peak). Positive correlations were found between E/E' and VE/VCO(2peak) and age at surgery. S' velocity correlated positively with SVEF, VO(2peak) and negatively with E/E' ratio. Patients after FO with E/E' ratio > or = 12 had a significantly lower oxygen uptake when compared to those with E/E' ratio < 12., Conclusion: Adult patients after Fontan operations are characterized by both systolic and diastolic dysfunction of the single ventricle, associated with severe exercise intolerance reflected by shorter duration of exercise with decreased oxygen uptake and increased ventilatory response. E/E' ratio assessed by Tissue Doppler echocardiography can be a powerful predictor of oxygen uptake and ventilatory response.