Leveraging Preclinical Modeling for Clinical Advancements in Single Ventricle Physiology: Spotlight on the Fontan Circulation

Preclinical modeling of human circulation has been instrumental in advancing cardiovascular medicine. Alongside clinical research, the armamentarium of computational (e.g., lumped parameter or computational fluid dynamics) and experimental (e.g., benchtop or animal) models have substantially enhanced our understanding of risk factors and root causes for circulatory diseases. Recent innovations are further disrupting the boundaries of these preclinical models toward patient-specific simulations, surgical planning, and postoperative outcome prediction. This fast-paced progress empowers preclinical modeling to increasingly delve into the intricacies of single ventricle physiology, a rare and heterogeneous congenital heart disease that remains inadequately understood. Here, we review the current landscape of preclinical modeling (computational and experimental) proposed to advance clinical management of a prominent yet complex subset of single ventricle physiology: patients who have undergone Fontan-type surgical corrections. Further, we explore recent innovations and emerging technologies that are poised to bridge the gap between preclinical Fontan modeling and clinical implementation.

Investigation of a chronic single-stage sheep Fontan model

Objectives

Our goal was to conduct a hemodynamic analysis of a novel animal model of Fontan physiology. Poor late-term outcomes in Fontan patients are believed to arise from Fontan-induced hemodynamics, but the mechanisms remain poorly understood. Recent advances in surgical experimentation have resulted in the development of a chronic sheep model of Fontan physiology; however, detailed analysis of this model is lacking.

Methods

We created a single-stage Fontan model in juvenile sheep with normal biventricular circulation. The superior vena cava was anastomosed to the main pulmonary artery, and the inferior vena cava was connected to the main pulmonary artery using an expanded polytetrafluoroethylene conduit. Longitudinal hemodynamics, including catheterization and magnetic resonance imaging were evaluated.

Results

Four out of 12 animals survived, with the longest surviving animal living 3 years after single-stage Fontan. We showed a significant era effect regarding survival (1 out of 8 and subsequently 3 out of 4 animals surviving beyond 2 months) attributed in large part to the procedural learning curve. Key characteristics of Fontan hemodynamics, namely systemic venous hypertension and low normal cardiac output, were observed. However, recapitulation of passive human Fontan hemodynamics is affected by volume loading of the right ventricle given an anatomic difference in sheep azygous venous anatomy draining to the coronary sinus.

Conclusions

A significant learning curve exists to ensure long-term survival and future surgical modifications, including banding of the main pulmonary artery and ligation of the azygous to coronary sinus connection are promising strategies to improve the fidelity of model hemodynamics.

Feasibility of an Animal Model for Cavopulmonary Support With a Double-Outflow Pump

Both single- and double-outflow cavopulmonary assist devices (CPADs) were recently proposed for the Fontan population, whereas single-outflow configurations were evaluated in large animal trials and double-outflow concepts are lacking an equivalent in vivo assessment. The aim of this study was to test the hemodynamic properties of a double-outflow CPAD device in an acute sheep model. The two inflow cannulae of a CPAD were anastomosed to the caval veins. Outflow graft connection was performed via end-to-side anastomosis to the right (RPA) and main pulmonary artery (MPA). Speed ramp protocols were conducted, and hemodynamic effects were monitored in terms of caval flows, cardiac output (CO), central venous pressure (CVP), pulmonary artery pressure (PAP), and left atrial pressure (LAP). Six experiments were conducted (53.35 ± 5.1 kg). In three experiments, the animal model was established, the CPAD was examined, and restoration of biventricular equivalency in terms of venous return was achieved. Venous pressures (CVP) declined linearly with increasing pump speed (r > 0.879), whereas caval flow (r > 0.973), CO (r > 0.993), PAP (r > 0.973), and LAP (r > 0.408) increased. Despite the considerable complexity of the sheep model caused by the sheep pulmonary arterial anatomy that requires substantial graft bending, the CPAD was evaluated in three acute experiments and showed the potential to completely substitute a subpulmonary ventricle.

Computational analysis of serum-derived extracellular vesicle miRNAs in juvenile sheep model of single stage Fontan procedure

Patients with single ventricle heart defects requires a series of staged open-heart procedures, termed Fontan palliation. However, while lifesaving, these operations are associated with significant morbidity and early mortality. The attendant complications are thought to arise in response to the abnormal hemodynamics induced by Fontan palliation, although the pathophysiology underlying these physicochemical changes in cardiovascular and other organs remain unknown. Here, we investigated the microRNA (miRNA) content in serum and serum-derived extracellular vesicles (EVs) by sequencing small RNAs from a physiologically relevant sheep model of the Fontan operation. The differential expression analysis identified the enriched miRNA clusters in (1) serum vs. serum-derived EVs and (2) pre-Fontan EVs vs. post-Fontan EVs. Metascape analysis showed that the overexpressed subset of EV miRNAs by Fontan procedure target liver-specific cells, underscoring a potentially important pathway involved in the liver dysfunction that occurs as a consequence of Fontan palliation. We also found that post-Fontan EV miRNAs were associated with senescence and cell death, whereas pre-Fontan EV miRNAs were associated with stem cell maintenance and epithelial-to-mesenchymal transition. This study shows great potential to identify novel circulating EV biomarkers from Fontan sheep serum that may be used for the diagnosis, prognosis, and therapeutics for patients that have undergone Fontan palliation.

Physiological Fontan Procedure

Objective: The conventional Fontan circulation deviates the superior vena cava (SVC = 1/3 of the systemic venous return) toward the right lung (3/5 of total lung volume) and the inferior vena cava (IVC = 2/3 of the systemic venous return) toward the left lung (2/5 of total lung volume). A "physiological" Fontan deviating the SVC toward the left lung and the IVC toward the right lung was compared with the conventional setting by computational fluid dynamics, studying whether this setting achieves a more favorable hemodynamics than the conventional Fontan circulation. Materials and Methods: An in-silico 3D parametric model of the Fontan procedure was developed using idealized vascular geometries with invariant sizes of SVC, IVC, right pulmonary artery (RPA), and left pulmonary artery (LPA), steady inflow velocities at IVC and SVC, and constant equal outflow pressures at RPA and LPA. These parameters were set to perform finite-volume incompressible steady flow simulations, assuming a single-phase, Newtonian, isothermal, laminar blood flow. Numerically converged finite-volume mass and momentum flow balances determined the inlet pressures and the outflow rates. Numerical closed-path integration of energy fluxes across domain boundaries determined the flow energy loss rate through the Fontan circulation. The comparison evaluated: (1) mean IVC pressure; (2) energy loss rate; (3) kinetic energy maximum value throughout the domain volume. Results: The comparison of the physiological vs. conventional Fontan provided these results: (1) mean IVC pressure 13.9 vs. 14.1 mmHg (= 0.2 mmHg reduction); (2) energy loss rate 5.55 vs. 6.61 mW (= 16% reduction); (3) maximum kinetic energy 283 vs. 396 J/m3 (= 29% reduction). Conclusions: A more physiological flow distribution is accompanied by a reduction of mean IVC pressure and by substantial reductions of energy loss rate and of peak kinetic energy. The potential clinical impact of these hemodynamic changes in reducing the incidence and severity of the adverse long-term effects of the Fontan circulation, in particular liver failure and protein-losing enteropathy, still remains to be assessed and will be the subject of future work.