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Introduction:Patients born with anatomical or functional single ventricle are treated with establishment of Fontan circulation, characterized by passive filling of pulmonary circulation. Patients with Fontan circulation have limited exercise capacity; however, their resting and exercise-augmented hemodynamic parameters have not been well described. Non-Invasive Cardiac System (NICaS, NI Medical) provides various hemodynamic measurements, using the whole body impedance cardiography. We have previously validated NICaS derived stroke volume (SV) with that from cardiac magnetic resonance.Methods:Twenty patients with Fontan circulation were assessed by cardiopulmonary exercise test (CPET). Their resting and post-exercise hemodynamic parameters were measured using NICaS.Results:Their mean age was 28?10 years; 13/20 (65%) were male. Their body mass index was 24?4.6 kg/m2. During CPET (Balke protocol) they achieved 6.8?1.7 METs (4.2-10). The average resting heart rate was 79?14 bpm that increased to 155?26. Their peak oxygen consumption (VO2max) measured at 66?13% of maximum predicted for their age and sex. Resting SV was 50?17 ml that remained unchanged immediately after exercise at 50?15ml. However, due to increase in the heart rate with exercise, cardiac index (CI) increased from 2.2?0.76 L/min/m2at rest to 2.8?0.82 L/min/m2post-exercise. Ten (50%) patients? resting CI was in the range of cardiogenic shock that is defined as CI <2.2 l/min/m2; these patients had markedly high total peripheral resistance index (TPRI) in comparison to the ones with normal CI (4501?1353 vs 2522?340 dn*s/cm5*m2, p<0.0001. Cardiac power index (CPI) at rest was 0.41?0.16 w/m2that increased to 0.58?0.18 w/m2after exercise. Significant reduction in TPRI was observed post-exercise (at rest: 3512?1398 vs post-exercise: 2851?985 dn*s/cm5*m2, p=0.04).Conclusion:This is the first study to report non-invasively obtained hemodynamics in patients with Fontan circulation. Patients with low cardiac index are characterized by markedly elevated TPRI. Exercise induced augmentation in CI was mainly due to increase in heart rate and reduction in TPRI, but not an increase in SV. Further studies are required to evaluate role of such measurements in routine clinical care.