• Strain correlation with ventricular volume [ Time Frame: 1 year ]
  Strain correlation with ventricular volume measured by magnetic resonance imaging
• Strain correlation with clinical parameters [ Time Frame: 1 year ]
  Strain correlation with clinical parameters such as presence of arrhythmias on Holter-EKG, maximal oxygen consumption on cardio-pulmonary exercise testing (ml/(kg*min))

• T1 values compared to previously established normal cohort [ Time Frame: 1 year ]
  T1 values compared to previously established normal cohort
• Blood and exhaled biomarkers of heart failure correlation with T1 mapping [ Time Frame: 1 year ]
  Blood and exhaled biomarkers of heart failure correlation with T1 mapping
• Intraventricular blood flow correlation with cardiac function [ Time Frame: 1 year ]
  Intraventricular blood flow correlation with cardiac ejection fraction measured by magnetic resonance imaging

Single ventricle defects make up the severe end of the congenital heart disease spectrum. The Fontan operation leads to a complete redirection of systemic venous blood outside of the heart and directly into the lungs. Patients with single ventricles suffer from multiple complications. Their survival has improved over the past decades, but is still severely compromised compared to the general population.

Their evaluation includes echocardiography and functional status by history and/or exercise testing. In longer intervals or if echocardiography does not allow visualization of all cardiovascular structures, cardiac magnetic resonance (CMR) is employed. Many patients also undergo more invasive cardiac catheterization.

In single ventricle patients, cardiac imaging has to address the questions of the patency of the Fontan pathways, i.e. all systemic veins, the Fontan conduit, and the pulmonary arteries, and of the function of the single ventricle (including myocardial function and valve function).

By using conventional imaging methods in Fontan patients, Ghelani et al. identified a CMR-based ventricular end-diastolic volume of > 125 ml/m2 and an echocardiographic global circumferential strain (GCS) value of higher than -17% to be strong predictors for a combined adverse outcome of death or heart transplantation. While interobserver reproducibility of single ventricle ejection fraction is similarly high by echocardiography, CMR is better in reliably measuring ventricular mass and diastolic volume and can provide additional information by MR feature tracking (strain), T1 mapping, and 4D flow measurements. Several substances that can be measured in the peripheral blood are being increasingly investigated as biomarkers of heart failure.

In conclusion, several advanced CMR sequences and new biomarkers have a potential role in the assessment and risk stratification of single ventricle patients. Every single published study has elucidated a particular use and aspect of these parameters, but broader correlations and prognostic values are still unclear.

The investigators hypothesize that myocardial strain (by feature tracking), myocardial fibrosis (by T1 mapping), and intracardiac flow disturbances (by 4D flow) along with biomarkers are diagnostic for single ventricle dysfunction and correlate with known prognostic factors.

This is a single center, prospective, observational cohort study. There will be no randomisation or blinding. Study setting: outpatients, cardiology clinic and radiology department, academic hospital. Every patient will be examined twice with a one-year interval (MR will only be repeated if clinically indicated).

• Diagnostic Test: Cardiac Magnetic Resonance Imaging
  Cardiac Magnetic Resonance Imaging (non-invasive, with i.v. application of contrast), awake or (if clinically indicated) in general anesthesia
• Diagnostic Test: Blood draw for hematocrit and heart failure biomarkers
  Approximately 10 ml of blood will be drawn before administration of contrast medium.
• Diagnostic Test: Cardiopulmonary exercise test
  In patients 8 years of age or older: on a cycle ergometer with breath-by-breath analysis, continuous ECG and SpO2 monitoring during exercise, after a baseline spirometry and bodyplethysmography
• Diagnostic Test: Exhalomics
  Measurement of exhaled molecules by mass spectrometry; patients breathe into a mouthpiece for 15 seconds 6 times (total time requirement: about 5 minutes)
• Diagnostic Test: Quality of life questionnaire
  Questionnaire to be filled out by the Patient regarding quality of life perception

Inclusion Criteria:

• Patients of any age with functionally single ventricle (patients under age 8 who need anesthesia for CMR will not be actively recruited. They may be approached to participate only if the anesthesia and CMR examination have been planned independently for clinical purposes)
• Written informed consent

Exclusion Criteria:

• Inability to follow the procedures of the study, e.g. due to language problems, psychological disorders, inability to give assent or consent, etc. of the participant and/or his/her parents or legal caregivers
• MR-incompatible implanted or accidentally incorporated metal device or claustrophobia that prohibits use of magnetic resonance imaging (patient and guardians fill out a questionnaire).
• Pregnancy
• Participation in another study is not an exclusion criterion, e.g. in a therapeutic trial.