• ventilatory efficiency M0 [ Time Frame: Month 0 ]
  ventilatory efficiency, e.g. the VE/VCO2 slope, measured by CPET
• ventilatory efficiency M6 [ Time Frame: Month 6 ]
  ventilatory efficiency, e.g. the VE/VCO2 slope, measured by

• VO2 max M0 [ Time Frame: Month 0 ]
  maximum oxygen uptake mesured by Cardio-pulmonary exercise test (CPET)
• VO2 max M6 [ Time Frame: Month 6 ]
  maximum oxygen uptake mesured by Cardio-pulmonary exercise test (CPET)
• ventilatory anaerobic threshold M0 [ Time Frame: Month 0 ]
  VAT using Beaver's method
• ventilatory anaerobic threshold M6 [ Time Frame: Month 6 ]
  VAT using Beaver's method
• oxygen pulse M0 [ Time Frame: Month 0 ]
  ratio VO2/heart rate M0
• oxygen pulse M6 [ Time Frame: Month 6 ]
  ratio VO2/heart rate M6
• OUES M0 [ Time Frame: Month 0 ]
  oxygen uptake efficiency slope mesured by CPET
• OUES M6 [ Time Frame: Month 6 ]
  oxygen uptake efficiency slope mesured by CPET
• NYHA functional class M0 [ Time Frame: Month 0 ]
  Functional class from I to IV (New York Heart Association Functional classification).
• NYHA functional class M6 [ Time Frame: Month 6 ]
  Functional class from I to IV (New York Heart Association Functional classification).
• blood pressure M0 [ Time Frame: Month 0 ]
  SV function evaluation with non-invasive imaging
• blood pressure M6 [ Time Frame: Month 6 ]
  function evaluation with non-invasive imaging
• oxygen saturation SaO2 [ Time Frame: Month 0 ]
  oxygen saturation measured using a transcutaneous sensor
• oxygen saturation SaO2 [ Time Frame: Month 6 ]
  oxygen saturation measured using a transcutaneous sensor
• 6-minute walk test (6MWT) [ Time Frame: Month 0 ]
  6-minute walk test
• 6-minute walk test (6MWT) [ Time Frame: Month 6 ]
  6-minute walk test
• Health-related quality of life [ Time Frame: Month 0 ]
  The SF-36 questionnaire
• Health-related quality of life [ Time Frame: Month 6 ]
  The SF-36 questionnaire
• Systemic blood flow [ Time Frame: Month 0 ]
  SV function with echocardiography
• Systemic blood flow [ Time Frame: Month 6 ]
  SV function with echocardiography
• SV systolic ejection fraction [ Time Frame: Month 0 ]
  SV function with echocardiography
• SV systolic ejection fraction [ Time Frame: Month 6 ]
  SV function with echocardiography
• 2D strain SV function [ Time Frame: Month 0 ]
  SV function with echocardiography
• 2D strain SV function [ Time Frame: Month 6 ]
  SV function with echocardiography
• Systemic blood flows in phase contrast [ Time Frame: Month 0 ]
  SV function evaluation with MRI
• Systemic blood flows in phase contrast [ Time Frame: Month 6 ]
  SV function evaluation with MRI
• Pulmonary blood flows in phase contrast [ Time Frame: Month 0 ]
  SV function evaluation with MRI
• Pulmonary blood flows in phase contrast [ Time Frame: Month 6 ]
  SV function evaluation with MRI
• SV systolic ejection fraction [ Time Frame: Month 0 ]
  SV function evaluation with MRI
• SV systolic ejection fraction [ Time Frame: Month 6 ]
  SV function evaluation with MRI
• SV systolic ejection volume [ Time Frame: Month 0 ]
  SV function evaluation with MRI
• SV systolic ejection volume [ Time Frame: Month 6 ]
  SV function evaluation with MRI
• NT Pro BNP [ Time Frame: Month 0 ]
  blood test checked
• NT Pro BNP [ Time Frame: Month 6 ]
  blood test checked
• forced expiratory volume in 1 s (FEV1 ) [ Time Frame: month 0 ]
  FEV1 spirometry
• forced expiratory volume in 1 s (FEV1 ) [ Time Frame: month 6 ]
  FEV1 spirometry
• Forced vital capacity FVC [ Time Frame: month 0 ]
  FVC spirometry
• Forced vital capacity FVC [ Time Frame: month 6 ]
  FVC spirometry
• FEV1% [ Time Frame: month 0 ]
  FEV1/FEVC ratio
• FEV1% [ Time Frame: month 6 ]
  FEV1/FEVC ratio
• DEMM25/75 [ Time Frame: month 0 ]
  DEMM25/75 measured by spirometry
• DEMM25/75 [ Time Frame: month 6 ]
  DEMM25/75 measured by spirometry
• Capillary lung volume [ Time Frame: month 0 ]
  pulmonary CO/NO transfer (patient seated and lying down)
• Capillary lung volume [ Time Frame: month 6 ]
  pulmonary CO/NO transfer (patient seated and lying down)
• Cardiac catheterization [ Time Frame: month 0 ]
  pulmonary arterial pressure mmHg
• Cardiac catheterization [ Time Frame: month 6 ]
  pulmonary arterial pressure mmHg
• percentage of patients compliant at 6 months of study treatment [ Time Frame: month 6 ]
  percentage of patients compliant
• AE [ Time Frame: month 6 ]
  type of Averse events
• SAE [ Time Frame: month 6 ]
  type of serious Averse events

• VO2 max M0 [ Time Frame: Month 0 ]
  maximum oxygen uptake mesured by Cardio-pulmonary exercise test (CPET)
• VO2 max M6 [ Time Frame: Month 6 ]
  maximum oxygen uptake mesured by Cardio-pulmonary exercise test (CPET)
• ventilatory anaerobic threshold M0 [ Time Frame: Month 0 ]
  VAT using Beaver's method
• ventilatory anaerobic threshold M6 [ Time Frame: Month 6 ]
  VAT using Beaver's method
• oxygen pulse M0 [ Time Frame: Month 0 ]
  ratio VO2/heart rate M0
• oxygen pulse M6 [ Time Frame: Month 6 ]
  ratio VO2/heart rate M6
• OUES M0 [ Time Frame: Month 0 ]
  oxygen uptake efficiency slope mesured by CPET
• OUES M6 [ Time Frame: Month 6 ]
  oxygen uptake efficiency slope mesured by CPET
• NYHA functional class M0 [ Time Frame: Month 0 ]
  o Functional class from I to IV (New York Heart Association Functional classification).
• NYHA functional class M6 [ Time Frame: Month 6 ]
  o Functional class from I to IV (New York Heart Association Functional classification).
• blood pressure M0 [ Time Frame: Month 0 ]
  SV function evaluation with non-invasive imaging
• blood pressure M6 [ Time Frame: Month 6 ]
  function evaluation with non-invasive imaging
• oxygen saturation [ Time Frame: Month 0 ]
  oxygen saturation measured using a transcutaneous sensor
• oxygen saturation [ Time Frame: Month 6 ]
  oxygen saturation measured using a transcutaneous sensor
• SV function M0 [ Time Frame: Month 0 ]
  echocardiography
• SV function M6 [ Time Frame: Month 6 ]
  echocardiography

50 Patients with a single ventricle (e.g. univentricular heart), as defined by the ACC-CHD classification, with a mean pulmonary arterial pressure (mPAP) > 15 mmHg and a trans-pulmonary gradient (TPG) > 5 mmHg, and aged 15 years old and above, will be prospectively recruited in the participating centres during their regular follow-up.

Patients wil be randomised into 2 groups:

• Patients randomised in the group 1 will receive sildenafil in 3 oral doses of 20 mg per day (t.i.d.), as defined in the marketing authorization indicated for PAH in adolescent and adult patients, and for a period of 6 months.
• Patients in the group 2 will receive a placebo (t.i.d.), for the same period of 6 months. To guarantee the double blind, capsules will be similar in size and colour and will be differentiated only by a vial number regarding to the randomization list. The clinical trials unit of the sponsor's pharmacy will centralize treatment allocation and supply to the participating centres. Drug management (reception, storage, delivery and traceability) will be ensured by the pharmacies of the participating centres.

After the 6 month-treatment period, patients will be followed for 3 months, and undergo at least 2 safety visits (1 and 3 months after intervention, and if necessary, any supplementary unscheduled visits). In accordance with the recommendations of the drug notice, the treatment will be suspended progressively over 1 week (20 mg b.i.d for 3 days, then 20 mg q.d. for 4 days, and then stopped) with a reinforcement of the surveillance. Patients will be able to contact an emergency number during this period and the investigator may decide to continue open treatment with sildenafil if clinically justified.

The study will be conducted in compliance with the Good Clinical Practices protocol and Declaration of Helsinki principles. It was approved by a drawn National Ethics Committee (CPP) and by the French National Agency of Medicine and Health Products Safety (ANSM). Informed consent will be obtained from all patients and their parents or legal guardians for minors.

• Single-ventricle
• Pulmonary Hypertension
• Univentricular Heart

• Drug: Sildenafil
  Patients randomised in the group 1 will receive sildenafil in 3 oral doses of 20 mg per day
  Other Name: sildenafil group
• Drug: Placebos
  Patients randomised in the group placebo in 3 oral doses of per day
  Other Name: placebo group

• Experimental: sildenafil
  • Patients randomised in the group 1 will receive sildenafil in 3 oral doses of 20 mg per day (t.i.d.), as defined in the marketing authorization indicated for PAH in adolescent and adult patients, and for a period of 6 months.
  Intervention: Drug: Sildenafil
• Placebo Comparator: placebo
  • Patients in the group 2 will receive a placebo (t.i.d.), for the same period of 6 months. To guarantee the double blind, capsules will be similar in size and colour and will be differentiated only by a vial number regarding to the randomization list
  Intervention: Drug: Placebos

Inclusion Criteria:

1. 15 years of age and over.
2. Patient's weight over 20 kg
3. Patients with CHD with a single ventricular type defined by the classification of congenital heart diseases in Orphanet (53).
4. PAH defined by diagnostic catheterization with mean PAP > 15 mmHg and a trans-pulmonary gradient > 5 mmHg, performed as part of the usual follow-up. No definition of PAH in SV is available as a result of a particular physiology. Therefore, we chose the 15mmHg cut-off, which is used in clinical routine to allow or contra-indicate the Fontan procedure [50,51].
5. Appropriate written informed consent (adult patients, legal parents for teenagers), and formal assent (teenagers), should to be provided.
6. Beneficiary of a health insurance.

Exclusion Criteria:

1. Patient who is unable to perform a cardio-pulmonary exercise test.
2. Cardiac surgery planned during the trial.
3. Patient treated by any pulmonary arterial vasodilator drug, as defined in the 2015 PH guidelines (52), within 6 months before inclusion, regardless the duration and the type(s) (oral, intravenous, subcutaneous, inhaled) of administration.
4. Patient treated by Sildenafil or any other type of phosphodiesterase-type 5 inhibitor (such as tadalafil) within 6 months before inclusion, regardless the duration of administration.
5. Interventional cardiac catheterization planned during the trial (collateral occlusion, fenestration occlusion, stenting, angioplasty, ablation of rhythm disorder), other than during the screening.
6. Participation in another clinical trial or administration of an off-label drug in the 4 weeks preceding the screening.
7. Pregnancy, desire for pregnancy, absence of contraception during the study period.
8. Severe hepatic insufficiency (Child-Pugh C class).

9. Hypersensitivity to the active substance or to any of the excipients of the tablet:

   microcrystalline cellulose, calcium hydrogen phosphate anhydrous, croscarmellose sodium, stearate of magnesium, hypromellose, titanium dioxide (E171), monohydrate lactose, glycerol triacetate.

10. Combination with products called "nitric oxide donors" (such as amyl nitrite) or with nitrates in any form, due to the hypotensive effects of nitrates.
11. Concomitant administration of PDE5 inhibitors, such as Sildenafil, with guanylate cyclase stimulators, such as Riociguat.
12. Combination with the most potent inhibitors of CYP3A4 (eg ketoconazole, itraconazole, ritonavir).
13. Disposition to priapism, sclerosis of corpora cavernosa, disease of La Peyronie, sickle cell anemia, multiple myeloma, leukemia.
14. Uncontrolled hypotension or risk of hypotension: water depletion, obstruction to ejection of the left ventricle, dysfunction of the autonomic nervous system, patient under alpha-blocker.
15. Severe cardiovascular events, recent (<3 months) or not stabilized: myocardial infarction, unstable angina, sudden cardiac death, ventricular arrhythmia, cerebrovascular hemorrhage.
16. Active hemorrhagic disorders.
17. Active gastro-duodenal ulcer.
18. Patients with loss of vision of an eye due to non-arteritic anterior ischemic optic neuropathy (NAION), whether or not this event has been associated with previous exposure to a PDE5 inhibitor.