Passive performance evaluation and validation of a viscous impeller pump for subpulmonary fontan circulatory support.

Patients with single ventricle defects undergoing the Fontan procedure eventually face Fontan failure. Long-term cavopulmonary assist devices using rotary pump technologies are currently being developed as a subpulmonary power source to prevent and treat Fontan failure. Low hydraulic resistance is a critical safety requirement in the event of pump failure (0 RPM) as a modest 2 mmHg cavopulmonary pressure drop can compromise patient hemodynamics. The goal of this study is therefore to assess the passive performance of a viscous impeller pump (VIP) we are developing for Fontan patients, and validate flow simulations against in-vitro data. Two different blade heights (1.09 mm vs 1.62 mm) and a blank housing model were tested using a mock circulatory loop (MCL) with cardiac output ranging from 3 to 11 L/min. Three-dimensional flow simulations were performed and compared against MCL data. In-silico and MCL results demonstrated a pressure drop of < 2 mmHg at a cardiac output of 7 L/min for both blade heights. There was good agreement between simulation and MCL results for pressure loss (mean difference - 0.23 mmHg 95% CI [0.24-0.71]). Compared to the blank housing model, low wall shear stress area and oscillatory shear index on the pump surface were low, and mean washout times were within 2 s. This study demonstrated the low resistance characteristic of current VIP designs in the failed condition that results in clinically acceptable minimal pressure loss without increased washout time as compared to a blank housing model under normal cardiac output in Fontan patients.

Passive Performance Evaluation and Validation of a Viscous Impeller Pump for Subpulmonary Fontan Circulatory Support.

Patients with single ventricle defects undergoing the Fontan procedure eventually face Fontan failure. Long-term cavopulmonary assist devices using rotary pump technologies are currently being developed as a subpulmonary power source to prevent and treat Fontan failure. Low hydraulic resistance is a critical safety requirement in the event of pump failure (0 RPM) as a modest 2 mmHg cavopulmonary pressure drop can compromise patient hemodynamics. The goal of this study is therefore to assess the passive performance for a viscous impeller pump (VIP) we are developing for Fontan patients, and validate flow simulations against in-vitro data. Two different blade heights (1.09 mm vs 1.62 mm) and a blank housing model were tested using a mock circulatory loop (MCL) with cardiac output ranging from 3 to 11 L/min. Three-dimensional flow simulations were performed and compared against MCL data. In-silico and MCL results demonstrated a clinically insignificant pressure drop of $<$ 2 mmHg at a cardiac output of 7 L/min for both blade heights. There was good agreement between simulation and MCL results for pressure loss (mean difference -0.23 mmHg 95% CI [0.24 -0.71]). Compared to the blank housing model, low wall shear stress area and oscillatory shear index on the pump surface were low, and mean washout times were within 2 seconds. This study demonstrated the low resistance characteristic of current VIP designs in the failed condition that results in clinically acceptable minimal pressure loss with low risk of thrombosis.

Computational Modeling to Support Surgical Decision Making in Single Ventricle Physiology.

Many of the advances in congenital heart surgery were built upon lessons and insights gained from model simulations. While animal and mock-circuit models have historically been the main arena to test new operative techniques and concepts, the recognition that complex cardiovascular anatomy and circulation can be modeled mathematically ushered a new era of collaboration between surgeons and engineers. In 1996, the computational age in congenital heart surgery began when investigators in London and Milan tapped the power of the computer to simulate the Fontan procedure and introduced operative improvements. Since then, computational modeling has led to numerous contributions in congenial heart surgery as continuing sophistication and advances in numerical and imaging methods furthered the ability to refine anatomic and physiologic details. Idealized generic models have given way to precise patient-specific simulations of the 3-dimensional anatomy, reconstructed circulation, affected hemodynamics, and altered physiology. Tools to perform virtual surgery, and predict flow dynamic and circulatory results, have been developed for some of the most complex defects, such as those requiring single ventricle palliation. In today's quest for personalized medicine and precision care, computational modeling's role to assist surgical planning in complex congenital heart surgery will continue to grow and evolve. With ever closer collaboration between surgeons and engineers, and clear understanding of modeling limitations, computational simulations can be a valuable adjunct to support preoperative surgical decision making.

An interactive simulation tool for patient-specific clinical decision support in single-ventricle physiology.

OBJECTIVE: Modeling of single-ventricle circulations has yielded important insights into their unique flow dynamics and physiology. Here we translated a state-of-the-art mathematical model into a patient-specific clinical decision support interactive Web-based simulation tool and show validation for all 3 stages of single-ventricular palliation. METHODS: Via the adoption a validated lumped parameter method, complete cardiovascular-pulmonary circulatory models of all 3 stages of single-ventricle physiology were created within a simulation tool. The closed-loop univentricular heart model includes scaling for growth and respiratory effects, and typical patient-specific parameters are entered through an intuitive user interface. The effects of medical or surgical interventions can be simulated and compared. To validate the simulator, patient parameters were collected from catheterization reports. Four simulator outputs were compared against catheterization findings: pulmonary to systemic flow ratio (Qp:Qs), systemic arterial saturation (SaO2), mean pulmonary arterial pressure (mPAp), and systemic-venous oxygen difference (SaO2-SvO2). RESULTS: Data from 60 reports were used. Compared with the clinical values, the simulator results were not significantly different in mean Qp:Qs, SaO2, or mPAp (P > .09). There was a statistical but clinically insignificant difference in average SaO-SvO2 (average difference 1%, P < .01). Linear regression analyses revealed a good prediction for each variable (Qp:Qs, R2 = 0.79; SaO2, R2 = 0.64; mPAp, R2 = 0.69; SaO2-SvO2, R2 = 0.93). CONCLUSIONS: This simulator responds quickly and predicts patient-specific hemodynamics with good clinical accuracy. By predicting postoperative and postintervention hemodynamics in all 3 stages of single-ventricle physiology, the simulator could assist in clinical decision-making, training, and consultation. Continuing model refinement and validation will further its application to the bedside.

In Vitro Validation of a Multiscale Patient-Specific Norwood Palliation Model.

In Norwood physiology, shunt size and the occurrence of coarctation can affect hemodynamics significantly. The aim of the study was to validate an in vitro model of the Norwood circulation against clinical measurements for patients presenting differing aortic morphologies. The mock circulatory system used coupled a lumped parameter network of the neonatal Norwood circulation with modified Blalock-Taussig (mBT) shunt with a three-dimensional aorta model. Five postoperative aortic arch anatomies of differing morphologies were reconstructed from imaging data, and the system was tuned to patient-specific clinical values. Experimentally measured flow rates and pressures were compared with clinical measurements. Time-based experimental and clinical pressure and flow signals within the aorta and pulmonary circulation branches agreed closely (0.72 < R < 0.95) for the five patients, whereas mean values within the systemic and pulmonary branches showed no significant differences (95% confidence interval). We validated an experimental multiscale model of the Norwood circulation with mBT shunt by showing it capable of reproducing clinical pressure and flow rates at various positions of the circulation with very good fidelity across a range of patient physiologies and morphologies. The multiscale aspect of the model provides a means to study variables in isolation with their effects both locally and at the system level. The model serves as a tool to further the understanding of the complex physiology of single-ventricle circulation.

In Vitro Assessment of the Assisted Bidirectional Glenn Procedure for Stage One Single Ventricle Repair.

This in vitro study compares the hemodynamic performance of the Norwood and the Glenn circulations to assess the performance of a novel assisted bidirectional Glenn (ABG) procedure for stage one single ventricle surgery. In the ABG, the flow in a bidirectional Glenn procedure is assisted by injection of a high-energy flow stream from the systemic circulation using an aorta-caval shunt with nozzle. The aim is to explore experimentally the potential of the ABG as a surgical alternative to current surgical practice. The experiments are directly compared against previously published numerical simulations. A multiscale mock circulatory system was used to measure the hemodynamic performance of the three circulations. For each circulation, the system was tested using both low and high values of pulmonary vascular resistance. Resulting parameters measured were: pressure and flow rate at left/right pulmonary artery and superior vena cava (SVC). Systemic oxygen delivery (OD) was calculated. A parametric study of the ratio of ABG nozzle to shunt diameter was done. We report time-based comparisons with numerical simulations for the three surgical variants tested. The ABG circulation demonstrated an increase of 30-38% in pulmonary flow with a 2-3.7 mmHg increase in SVC pressure compared to the Glenn and a 4-14% higher systemic OD than either the Norwood or the Glenn. The nozzle/shunt diameter ratio affected the local hemodynamics. These experimental results agreed with those of the numerical model: mean flow values were not significantly different (p > 0.05) while mean pressures were comparable within 1.2 mmHg. The results verify the approaches providing two tools to study this complicated circulation. Using a realistic experimental model we demonstrate the performance of a novel surgical procedure with potential to improve patient hemodynamics in early palliation of the univentricular circulation.

Control of respiration-driven retrograde flow in the subdiaphragmatic venous return of the Fontan circulation.

Respiration influences the subdiaphragmatic venous return in the total cavopulmonary connection (TCPC) of the Fontan circulation whereby both the inferior vena cava (IVC) and hepatic vein flows can experience retrograde motion. Controlling retrograde flows could improve patient outcomes. Using a patient-specific model within a Fontan mock circulatory system with respiration, we inserted a valve into the IVC to examine its effects on local hemodynamics while varying retrograde volumes by changing vascular impedances. A bovine valved conduit reduced IVC retrograde flow to within 3% of antegrade flow in all cases. The valve closed only under conditions supporting retrograde flow and its effects on local hemodynamics increased with larger retrograde volume. Liver and TCPC pressures improved only when the valve leaflets were closed whereas cycle-averaged pressures improved only slightly (<1 mm Hg). Increased pulmonary vascular resistance raised mean circulation pressures, but the valve functioned and cardiac output improved and stabilized. Power loss across the TCPC improved by 12%-15% (p < 0.05) with a valve. The effectiveness of valve therapy is dependent on patient vascular impedance.

Mock circulatory system of the Fontan circulation to study respiration effects on venous flow behavior.

We describe an in vitro model of the Fontan circulation with respiration to study subdiaphragmatic venous flow behavior. The venous and arterial connections of a total cavopulmonary connection (TCPC) test section were coupled with a physical lumped parameter (LP) model of the circulation. Intrathoracic and subdiaphragmatic pressure changes associated with normal breathing were applied. This system was tuned for two patients (5 years, 0.67 m2; 10 years, 1.2 m2) to physiological values. System function was verified by comparison to the analytical model on which it was based and by consistency with published clinical measurements. Overall, subdiaphragmatic venous flow was influenced by respiration. Flow within the arteries and veins increased during inspiration but decreased during expiration, with retrograde flow in the inferior venous territories. System pressures and flows showed close agreement with the analytical LP model (p < 0.05). The ratio of the flow rates occurring during inspiration to expiration were within the clinical range of values reported elsewhere. The approach used to set up and control the model was effective and provided reasonable comparisons with clinical data.