Recovery during mid-term mechanical support of fontan circulation in sheep

Role and Applications of Experimental Animal Models of Fontan Circulation

Over the last four decades, the Fontan operation has been the treatment of choice for children born with complex congenital heart diseases and a single-ventricle physiology. However, therapeutic options remain limited and despite ongoing improvements in initial surgical repair, patients still experience a multiplicity of cardiovascular complications. The causes for cardiovascular failure are multifactorial and include systemic ventricular dysfunction, pulmonary vascular resistance, atrioventricular valve regurgitation, arrhythmia, development of collaterals, protein-losing enteropathy, hepatic dysfunction, and plastic bronchitis, among others. The mechanisms leading to these late complications remain to be fully elucidated. Experimental animal models have been developed as preclinical steps that enable a better understanding of the underlying pathophysiology. They furthermore play a key role in the evaluation of the efficacy and safety of new medical devices prior to their use in human clinical studies. However, these experimental models have several limitations. In this review, we aim to provide an overview of the evolution and progress of the various types of experimental animal models used in the Fontan procedure published to date in the literature. A special focus is placed on experimental studies performed on animal models of the Fontan procedure with or without mechanical circulatory support as well as a description of their impact in the evolution of the Fontan design. We also highlight the contribution of animal models to our understanding of the pathophysiology and assess forthcoming developments that may improve the contribution of animal models for the testing of new therapeutic solutions.

Approaches to Establish Extracardiac Total Cavopulmonary Connections in Animal Models—A Review

Background:

Long-term survival of patients with a single ventricle palliated with a Fontan procedure is still limited. No curative treatment options are available. To investigate the pathophysiology and potential treatment options, such as mechanical circulatory support (MCS), appropriate large animal models are required. The aim of this review was to analyze all full-text manuscripts presenting approaches for an extracardiac total cavopulmonary connection (TCPC) animal model to identify the feasibility and limitations in the acute and chronic setting.

Methods:

A literature search was performed for full-text publications presenting large animal models with extracardiac TCPCs on Pubmed and Embase. Out of 454 reviewed papers, 23 manuscripts fulfilled the inclusion criteria. Surgical procedures were categorized and hemodynamic changes at the transition from the biventricular to the univentricular condition analyzed.

Results:

Surgical procedures varied especially regarding coronary venous flow handling and anatomic shape of the TCPC. In most studies (n = 14), the main pulmonary artery was clamped and the coronary venous flow redirected by additional surgical interventions. Only in five reports, the caval veins were connected to the right pulmonary artery to create a true TCPC shape, whereas in all others (n = 18), the veins were connected to the main pulmonary artery. An elevated pulmonary vascular resistance was identified as a limiting hemodynamic factor for TCPC completion in healthy animals.

Conclusions:

A variety of acute TCPC animal models were successfully established with and without MCS, reflecting the most important hemodynamic features of a Fontan circulation; however, chronic animal models were not reported.

Concurrent use of continuous and pulsatile flow Ventricular Assist Device on a fontan patient: A simulation study

The aim of this work is to develop and test a lumped parameter model of the cardiovascular system to simulate the concurrent use of pulsatile (PVAD) and continuous flow (CVAD) ventricular assist device (VAD) on Fontan patients. Echocardiographic and hemodynamic data of five Fontan patients were retrospectively collected and used to simulate the patients' baseline hemodynamics. Then, for each patient, the following assistance modality was simulated for the cavopulmonary and the single ventricle (SV): (a) CVAD for cavopulmonary assistance (RCF) and PVAD assisting the SV (LCF) (RPF + LCF), (b) CVAD assisting SV and PVAD for cavopulmonary assistance (LPF + RCF). The numerical model can well reproduce patients' baseline. The cardiac output increases more importantly in the LCF + RPF configuration (35 vs. 8%). Ventricular volume decreases more evidently in the configuration LCF + RPF (28 vs. 6%), atrial pressure decreases in the LCF + RPF modality (10%), while it slightly increases in the RCF + LPF modality. The pulmonary arterial pressure slightly decreases (increases) in the configuration RCF + LPF (LCF + RPF). Ventricular external work increases in both configurations because of the total increment of the cardiac output. However, artero-ventricular coupling improves in both configurations: RCF + LPF-14%, LCF + RPF-41%. The pulsatility index decreases (increases) by 8% (13.8%) in the configuration LCF + RPF (RCF + LPF). A model could permit us to simulate extreme physiological conditions of the implantation of both CF and PF VAD on the Fontan patient and could permit to choose the proper VAD on the base of patients' condition. The configuration LCF + RPF seems to maximize the hemodynamic benefits.

The use of a numerical model to simulate the cavo-pulmonary assistance in Fontan circulation: a preliminary verification

The lack of an established experience on the use of VAD for the cavo-pulmonary assistance leads to the need of dedicated VADs development and animal experiments. A dedicated numerical model could support clinical and experimental strategies design and new VADs testing. The aim of this work is to perform a preliminary verification of a lumped parameter model of the cardiovascular system to simulate Fontan physiology and the effect of cavo-pulmonary assistance. Literature data of 4 pigs were used to simulate animals' baseline, and then the model was tested in simulating Fontan circulation and cavo-pulmonary-assisted condition comparing the simulation outcome (Sim) with measured literature data (Me). The results show that the numerical model can well reproduce experimental data in all three conditions (baseline, Fontan and assisted Fontan) [cardiac output (l/min): Me = 2.8 ± 1.7, Sim = 2.8 ± 1.8; ejection fraction (%): Me = 57 ± 17, Sim = 54 ± 17; arterial systemic pressure (mmHg): Me = 41.8 ± 18.6, Sim = 43.8 ± 18.1; pulmonary arterial pressure (mmHg): Me = 15.4 ± 8.9, Sim = 17.7 ± 9.9; caval pressure (mmHg): Me = 6.8 ± 4.1, Sim = 7 ± 4.6]. Systolic elastance, arterial systemic and arterial pulmonary resistances increase (10, 69, and 100 %) passing from the biventricular circulation to the Fontan physiology and then decrease (21, 39, and 50 %) once the VAD was implanted. The ventricular external work decreases (71 %) passing from the biventricular circulation to the Fontan physiology and it increases three times after the VAD implantation in parallel with the VAD power consumption. A numerical model could support clinicians in an innovative and challenging field as the use of VAD to assist the Fontan physiology and it could be helpful to personalize the VAD insertion on the base of ventricular systo-diastolic function, circulatory parameters and energetic variables.

Fontan Liver Disease: Review of an Emerging Epidemic and Management Options

OPINION STATEMENT

Adults with complex congenital heart disease that resulted in a Fontan procedure frequently experience late cardiac failure. Increasingly, liver disease is recognized as an important complication of single-ventricle anatomy and Fontan physiology; however, there is no consensus regarding liver evaluation in this population. Here, we review what is known about liver disease in this unique group and propose screening and prevention measures. We also review controversial treatment areas including assist devices and transplantation, with a review of outcomes in isolated heart and combined heart-liver transplant.

Experimental measurements of energy augmentation for mechanical circulatory assistance in a patient-specific Fontan model

A mechanical blood pump specifically designed to increase pressure in the great veins would improve hemodynamic stability in adolescent and adult Fontan patients having dysfunctional cavopulmonary circulation. This study investigates the impact of axial-flow blood pumps on pressure, flow rate, and energy augmentation in the total cavopulmonary circulation (TCPC) using a patient-specific Fontan model. The experiments were conducted for three mechanical support configurations, which included an axial-flow impeller alone in the inferior vena cava (IVC) and an impeller with one of two different protective stent designs. All of the pump configurations led to an increase in pressure generation and flow in the Fontan circuit. The increase in IVC flow was found to augment pulmonary arterial flow, having only a small impact on the pressure and flow in the superior vena cava (SVC). Retrograde flow was neither observed nor measured from the TCPC junction into the SVC. All of the pump configurations enhanced the rate of power gain of the cavopulmonary circulation by adding energy and rotational force to the fluid flow. We measured an enhancement of forward flow into the TCPC junction, reduction in IVC pressure, and only minimally increased pulmonary arterial pressure under conditions of pump support.

Maintenance dose of warfarin in sheep and effect of diet: a preliminary report

Sheep models are widely used to evaluate the feasibility of various cardiac assist devices. Anticoagulation therapy postoperatively, however, is seldomly reported on. Continuous heparin infusion is often used, but is cumbersome due to long-term line management with the risk of infection and dislodgement. We contemplated using warfarin instead and started a pilot dose-finding study. Three sheep were given oral warfarin between 0.1 and 0.3 mg/kg/day. Prothrombin time was monitored and INR was calculated daily. If the INR did not reach a target of 2.5-3.5, warfarin dose was doubled. We found that sheep required a dose of warfarin between 1.6 and 2.4 mg/kg/day to raise the INR to the target zone. In a subsequent study to evaluate the effect of diet on INR in sheep, three sheep were fed alfalfa hay or alfalfa pellets in a crossover design. All the animals were given warfarin at the dose of 1.6 mg/kg. The diet was switched when the INR reached the target zone of 2.5-3.5. Hay-fed animals reached the target INR on days 6 and 7. On the other hand, pellet-fed animals did not reach the target value by day 7 with the initial dose and required 2.4 mg/kg of warfarin to achieve the goal. Hay raised the INR faster and higher than pellets with the same warfarin dose. Hay may be advantageous when using oral warfarin therapy in sheep.

Mechanisms of systemic adaptation to univentricular Fontan conversion

OBJECTIVE

After univentricular Fontan conversion, systemic venous pressure serves as the sole driving force for transpulmonary blood flow. Consequently, systemic venous return is markedly altered and ventricular filling is subnormal. The mechanisms and time course of systemic adaptation to Fontan conversion are incompletely understood. We hypothesized that acute elevation in systemic venous pressure induces an adaptive response similar to conversion to a univentricular Fontan circulation.

METHODS

Adjustable vessel occluders were placed around the superior and inferior vena cavae in juvenile sheep. After 1-week recovery, occluders were tightened to acutely increase and maintain systemic venous pressure at 15 mm Hg (n = 6), simulating 1-stage Fontan conversion. Control animals (n = 4) received identical surgery, but venous pressure was not manipulated.

RESULTS

Cardiac index decreased significantly (3.9 ± 1.0 mL/min/m(2) to 2.7 ± 0.7 mL/min/m(2), P < .001) and then normalized to control at 2 weeks. Circulating blood volume increased (100 ± 9.4 mL/kg vs 85.5 ± 8.4 mL/kg, P = .034) as a persistent response. Cardiac reserve improved and was not different from control by week 3. Resting heart rate decreased in both groups. Oxygen extraction (arteriovenous oxygen difference) and neurohormonal mediators increased transiently and then normalized by week 2.

CONCLUSIONS

Adaptation to global elevation in systemic venous pressure to Fontan levels is complete within 2 weeks. Increased blood volume and reduced heart rate are persistent responses. Increased oxygen extraction and neurohormonal up-regulation are temporary responses that normalize with recovery of cardiac output. With improved physiologic understanding of systemic adaptation to Fontan conversion, approaches to single-ventricle palliation can be more objectively assessed and optimized.