Patients with Fontan palliation who underwent CMR with 4D-Flow acquisition between February 2018 and July 2021 in one tertiary center for clinical indication were included in the study. Exclusion criteria were the presence of atrio-pulmonary Fontan and non-diagnostic 4D-Flow CMR images. The Institutional Review Board of our hospital approved this study, Protocol No. 13756.

Surgical history and clinical data were abstracted from the hospital records: gender, age at CMR, diagnosis, age at Glenn anastomosis if appropriate, age and type of Fontan intervention, and length of follow-up after Fontan palliation. The last echocardiogram report was considered for the evaluation of the degree of atrio-ventricular valve/s regurgitation. Moreover, the following late complications were recorded: cyanosis, arrhythmias, exercise intolerance, protein-losing enteropathy, plastic bronchitis, hepatic, and renal complications.

Fontan patients were considered having overt heart failure if they showed at admission at least one of these manifestations: pleural effusions, ascites, edema, declining albumin, thrombocytopenia, and coagulopathy.

Lung spirometry and Cardiopulmonary test (CPET): CPET was performed on an electrically braked cycle ergometer (Ergostik, Geratherm, Germany) according to ATS/ACCP recommendations [13] in a subpopulation of 34 patients. The protocol included three stages: resting, unloaded pedaling, and exercise and was set to achieve peak exercise in ~ 10 min. The external work rate was continuously incremented using the ramp protocol; CPETs were interrupted when patients reached maximal effort. A CPET with a respiratory exchange ratio (RER) ≥ 1.05 was considered maximal for metabolic stress. A breath-by-breath analysis of expiratory gases and ventilation was performed. Pulse oximetry oxygen saturation was also monitored [14].

We analyzed the following variables: oxygen uptake (VO2), and its relationship with heart rate and external work (pulse oxygen or VO2/HR and VO2/work slope); peak oxygen uptake normalized for the body surface area (VO2peak ml/min/kg) CO2 production (VCO2) and gas ventilatory equivalents (VEVO2, VEVCO2); the physiological dead space to the tidal volume ratio (Vd/Vt); minute ventilation expressed as the highest value recorded either during exercise or at the first-time recovery phase (VEpeak); and Maximal Voluntary Ventilation (MVV), Maximal as estimated multiplying FEV1 value by a correction factor of 40 [15].

Baseline Lung function includes the spirometry for static and dynamic lung volume measurements: total lung capacity (TLC); slow vital capacity (SVC); forced vital capacity (FVC); and derived indices as forced expiratory volume in the first second and its percentage of vital capacity (FEV1/VC ratio). Spirometry was performed by experienced technologists. Three spirometric measurements were obtained, and the highest values were chosen in conformity with ATS/ERS standards [14, 16].

A 1.5 Tesla CMR scanner (Signa Artist, GE Healthcare) and 3 Tesla CMR scanner (Ingenia, Philips Healthcare) were used. A comprehensive CMR evaluation was performed following the examination protocol previously published [17].

Briefly, functionally single-ventricle short axis was visualized from the base to the apex, using a cardiac-cine-balanced steady-state free-precession (SSFP) pulse sequence with the following parameters: retrospective ECG gating, field of view 340–360 mm, flip angle 35–50°, TE 1.4–1.9 ms, TR 2.8–3.8 ms, slice thickness 6–8 mm, number of signal averages 1–3, and reconstructed cardiac phases 30. The CMR study was completed using a contrast-enhanced (gadopentetate dimeglumine 0.2–0.4 ml/kg). MR angiographic sequence or a time-resolved angiography for the anatomic evaluation of the Fontan pathway was performed. In patients aged < 8 years or with incapacity to collaborate the CMR exam was performed on deep sedation using titrated propofol.

A 4D-Flow CMR sequence was also prescribed in axial or coronal orientation covering the entire thorax with the following parameters: for both CMR machine: field of view 250–400 mm, TR 3.8–5.3 ms, TE 2.0–3.2 ms, reconstructed cardiac phases 20–32, acquisition time 5–12 min slice thickness 2.2–3.0 mm, VENC according to the velocity in the aorta in the first stage of the study and around 70–100 cm/s subsequently whereas in 1.5 T GE scanner view per segment and flip angle were respectively 2–3, and 13–15 and in 3T Philips scanner turbo field echo (TFE) factor) was 2–3 and flip angle 8–9°.

The SSFP images were evaluated by means of a commercially available software (Mass plus; version 4.0, MR Analytical Software Systems, Leiden, The Netherlands). Ventricular volumes, mass (indexed to body surface area), and the ejection fraction were calculated. 4D-Flow CMR data were processed using Arterys Cardio AIMR (Arterys Inc., San Francisco, CA).

Blood flow quantification was performed by reformatting 4D-Flow CMR data in all Fontan circuit flow; sectional mean wall shear stress, flow eccentricity, and angle jet in the IVC conduit/tunnel-PA conduit were automatically calculated.

Pulmonary arteries and Fontan conduit diameters were measured in axial and latero-lateral planes using multiplanar reformatting of volumetric 3D SSFP in the diastolic phase. The angle between pulmonary arteries and IVC conduit/tunnel-PA conduit was also calculated. Systemic-pulmonary collateral flows (QSPCs) were calculated as: left pulmonary veins flow + right pulmonary veins flow (and) minus right pulmonary artery flow + left pulmonary artery flow [18]; these values were normalized to body surface area. Effective cardiac index (CI) was calculated as (QAo flow − QSPCs)/BSA [19].

Blood flow for pulmonary branches was considered asymmetric if the RPA/LPA flow ratio was > 1.56 (predominant flow for the RPA) or < 0.75 (predominant flow for the RPA) [20].

The segmentation of the Fontan circuit was manually performed on the phase with the highest contrast on magnitude-weighted velocity images and/or in the phase contrast MR angiography of the 4D-Flow data using open-source TK-SNAP software [21]. Streamlines quantification and visualization were done using ParaView software [22] and were generated using Paraview software, taking as seed points the planes in the SVC and IVC, with a forward integrator type (Runge-Kutta 4-5). 4D-Flow CMR-derived energetics parameters were evaluated in the Fontan confluence including the IVC-Conduit/tunnel-PA, proximal pulmonary branches, and superior vena cava (Fig. 1). The cutting planes were manually placed around 1.5 cm from the bifurcation.

Post-processing 4D-flow MRI data in an extracardiac conduit Fontan patient

Kinetic energy and energy loss were calculated in a custom-developed software, as previously described [23]. Both parameters were calculated at node level in the 3D domain and then were integrated in a volume of interest, and then normalized by volume (uW/ml) (Fig. 1). KE represents the amount of energy that the blood flow possesses due to its motion; EL represents the amount of KE within the blood flow lost per second due to viscosity-induced frictional forces [3]. In addition, EL index (EL/KE) was calculated as the ratio of normalized EL/normalized KE. EL index (EL/KE) has been described, which can be used as a marker of flow efficiency [7].

Continuous variables were expressed as mean ± standard deviation (SD) or median (interquartile range IQR: 25th; 75th percentiles) if skewed. Categorical variables were expressed as absolute frequency and percentage.

The correlation between continuous variables was tested with age-adjusted Pearson’s partial correlation coefficient. Leverage points were evaluated in the linear regression analyses: these points were not considered if greater than three times the value of their mean. Age and gender were used as cofactors when appropriate. The effect of gender was evaluated by analyzing the two groups separately. Student’s independent t-test was used to compare means between groups of quantitative variables. Statistical analyses performed were considered significant with a p value < 0.05. All statistical analyses were performed using SPSS (IBM Corp. Released 2017. IBM SPSS Statistics for Windows, Version 25.0. Armonk, NY: IBM Corp.).