Christopher M. Haggerty

Simulating hemodynamics of the Fontan Y-graft based on patient-specific in vivo connections

BACKGROUNDnUsing a bifurcated Y-graft as the Fontan baffle is hypothesized to streamline and improve flow dynamics through the total cavopulmonary connection (TCPC). This study conducted numerical simulations to evaluate this hypothesis using postoperative data from 5 patients.nnnMETHODSnPatients were imaged with cardiac magnetic resonance or computed tomography after receiving a bifurcated aorto-iliac Y-graft as their Fontan conduit. Numerical simulations were performed using in vivo flow rates, as well as 2 levels of simulated exercise. Two TCPC models were virtually created for each patient to serve as the basis for hemodynamic comparison. Comparative metrics included connection flow resistance and inferior vena caval flow distribution.nnnRESULTSnResults demonstrate good hemodynamic outcomes for the Y-graft options. The consistency of inferior vena caval flow distribution was improved over TCPC controls, whereas the connection resistances were generally no different from the TCPC values, except for 1 case in which there was a marked improvement under both resting and exercise conditions. Examination of the connection hemodynamics as they relate to surgical Y-graft implementation identified critical strategies and modifications that are needed to potentially realize the theoretical efficiency of such bifurcated connection designs.nnnCONCLUSIONSnFive consecutive patients received a Y-graft connection to complete their Fontan procedure with positive hemodynamic results. Refining the surgical technique for implementation should result in further energetic improvements that may help improve long-term outcomes.

Experimental and numeric investigation of Impella pumps as cavopulmonary assistance for a failing Fontan.

OBJECTIVEnThis study sought to evaluate the performance of microaxial ventricular assist devices for the purposes of supporting failing Fontan physiology by decreasing central venous pressure.nnnMETHODSnThree Abiomed Impella pumps (Abiomed, Inc, Danvers, Mass) were evaluated in a mock circulatory system of the Fontan circuit. The local response of pressures and flows to pump function was assessed as a function of pump speed and pulmonary vascular resistance at a high baseline central venous pressure. For one device, subsequent modeling studies were conducted using a lumped parameter model of the single ventricle circuit.nnnRESULTSnThe left ventricular devices (Impella 2.5, 5.0) were shown to be suboptimal as single device solutions for cavopulmonary support. The small area of these devices relative to vessel diameter led to significant flow recirculation without an obstructive separator in place. Furthermore, downstream pressure augmentation adversely affected the pressure in the superior vena cava. The use of 2 devices would be mandatory for successful support. The right-sided device (Impella RP), whose outflow was positioned in the left pulmonary artery, demonstrated decreased flow recirculation and did not impede superior caval venous flow. Although static pressure is still required to drive flow through the opposite lung, numeric modeling demonstrated the potential for modest but significant improvements in lowering the central venous pressure (2-8 mm Hg).nnnCONCLUSIONSnLeft-sided microaxial pumps are not well suited for cavopulmonary support because of severe flow recirculation and the need for multiple devices. The right-ventricular Impella device provides improved performance by directing flow into the pulmonary artery, resulting in modest decreases in central venous pressure.

Comparing Pre- and Post-Operative Fontan Hemodynamic Simulations: Implications for the Reliability of Surgical Planning

Virtual modeling of cardiothoracic surgery is a new paradigm that allows for systematic exploration of various operative strategies and uses engineering principles to predict the optimal patient-specific plan. This study investigates the predictive accuracy of such methods for the surgical palliation of single ventricle heart defects. Computational fluid dynamics (CFD)-based surgical planning was used to model the Fontan procedure for four patients prior to surgery. The objective for each was to identify the operative strategy that best distributed hepatic blood flow to the pulmonary arteries. Post-operative magnetic resonance data were acquired to compare (via CFD) the post-operative hemodynamics with predictions. Despite variations in physiologic boundary conditions (e.g., cardiac output, venous flows) and the exact geometry of the surgical baffle, sufficient agreement was observed with respect to hepatic flow distribution (90% confidence interval—14xa0±xa04.3% difference). There was also good agreement of flow-normalized energetic efficiency predictions (19xa0±xa04.8% error). The hemodynamic outcomes of prospective patient-specific surgical planning of the Fontan procedure are described for the first time with good quantitative comparisons between preoperatively predicted and postoperative simulations. These results demonstrate that surgical planning can be a useful tool for single ventricle cardiothoracic surgery with the ability to deliver significant clinical impact.

Individualized computer-based surgical planning to address pulmonary arteriovenous malformations in patients with a single ventricle with an interrupted inferior vena cava and azygous continuation

OBJECTIVEnPulmonary arteriovenous malformations caused by abnormal hepatic flow distribution can develop in patients with a single ventricle with an interrupted inferior vena cava. However, preoperatively determining the hepatic baffle design that optimizes hepatic flow distribution is far from trivial. The current study combines virtual surgery and numeric simulations to identify potential surgical strategies for patients with an interrupted inferior vena cava.nnnMETHODSnFive patients with an interrupted inferior vena cava and severe pulmonary arteriovenous malformations were enrolled. Their in vivo anatomies were reconstructed from magnetic resonance imaging (n = 4) and computed tomography (n = 1), and alternate virtual surgery options (intracardiac/extracardiac, Y-grafts, hepato-to-azygous shunts, and azygous-to-hepatic shunts) were generated for each. Hepatic flow distribution was assessed for all options using a fully validated computational flow solver.nnnRESULTSnFor patients with a single superior vena cava (n = 3), intracardiac/extracardiac connections proved dangerous, because even a small left or right offset led to a highly preferential hepatic flow distribution to the associated lung. The best results were obtained with either a Y-graft spanning the Kawashima to split the flow or hepato-to-azygous shunts to promote mixing. For patients with bilateral superior vena cavae (n = 2), results depended on the balance between the left and right superior inflows. When those were equal, connecting the hepatic baffle between the superior vena cavae performed well, but other options should be pursued otherwise.nnnCONCLUSIONSnThis study demonstrates how virtual surgery environments can benefit the clinical community, especially for patients with a single ventricle with an interrupted inferior vena cava. Furthermore, the sensitivity of the optimal baffle design to the superior inflows underscores the need to characterize both preoperative anatomy and flows to identify the best option.

Preliminary Clinical Experience with a Bifurcated Y-Graft Fontan Procedure—A Feasibility Study

OBJECTIVEnOptimizing flow and diminishing power loss in the Fontan circuit can improve hemodynamic efficiency, potentially improving the long-term outcomes. Computerized modeling has predicted improved energetics with a Y-graft Fontan.nnnMETHODSnFrom August to December 2010, 6 consecutive children underwent completion Fontan (n=3) or Fontan revision (n=3) using a bifurcated polytetrafluoroethylene Y-graft (18×9×9 mm in 2, 20×10×10 mm in 4) connecting the inferior vena cava to the right and left pulmonary arteries with separate graft limbs. The patents underwent magnetic resonance imaging (n=5) or computed tomography (n=1). Computational fluid dynamics assessed Fontan hemodynamics, power loss, and inferior vena cava flow splits to the branch pulmonary arteries. The clinical parameters were compared with those from 12 patients immediately preceding the present series who had undergone a lateral Fontan procedure.nnnRESULTSnDespite longer crossclamp and bypass times (not statistically significant), the Y-graft Fontan patients had postoperative courses similar to those of the conventional Fontan patients. Other than 2 early readmissions for pleural effusions managed with diuretics, at 6 to 12 months of follow-up (mean, 8 months), all 6 patients had done well. Postoperative flow modeling demonstrated a balanced distribution of inferior vena cava flow to both pulmonary arteries with minimal flow disturbance. Improvements in hemodynamics and efficiency were noted when the Y-graft branches were anastomosed distally and aligned tangentially with the branch pulmonary arteries.nnnCONCLUSIONSnThe present preliminary surgical experience has demonstrated the clinical feasibility of the bifurcated Y-graft Fontan. Computational fluid dynamics showed acceptable hemodynamics with low calculated power losses and a balanced distribution of inferior vena cava flow to the pulmonary arteries as long as the branch grafts were anastomosed distally.

Fontan hemodynamics from 100 patient-specific cardiac magnetic resonance studies: a computational fluid dynamics analysis.

OBJECTIVESnThis study sought to quantify average hemodynamic metrics of the Fontan connection as reference for future investigations, compare connection types (intra-atrial vs extracardiac), and identify functional correlates using computational fluid dynamics in a large patient-specific cohort. Fontan hemodynamics, particularly power losses, are hypothesized to vary considerably among patients with a single ventricle and adversely affect systemic hemodynamics and ventricular function if suboptimal.nnnMETHODSnFontan connection models were created from cardiac magnetic resonance scans for 100 patients. Phase velocity cardiac magnetic resonance in the aorta, vena cavae, and pulmonary arteries was used to prescribe patient-specific time-averaged flow boundary conditions for computational fluid dynamics with a customized, validated solver. Comparison with 4-dimensional cardiac magnetic resonance velocity data from selected patients was used to provide additional verification of simulations. Indexed Fontan power loss, connection resistance, and hepatic flow distribution were quantified and correlated with systemic patient characteristics.nnnRESULTSnIndexed power loss varied by 2 orders of magnitude, whereas, on average, Fontan resistance was 15% to 20% of published values of pulmonary vascular resistance in single ventricles. A significant inverse relationship was observed between indexed power loss and both systemic venous flow and cardiac index. Comparison by connection type showed no differences between intra-atrial and extracardiac connections. Instead, the least efficient connections revealed adverse consequences from localized Fontan pathway stenosis.nnnCONCLUSIONSnFontan power loss varies from patient to patient, and elevated levels are correlated with lower systemic flow and cardiac index. Fontan connection type does not influence hemodynamic efficiency, but an undersized or stenosed Fontan pathway or pulmonary arteries can be highly dissipative.

Visualization of flow structures in Fontan patients using 3-dimensional phase contrast magnetic resonance imaging

OBJECTIVEnOur objective was to analyze 3-dimensional (3D) blood flow patterns within the total cavopulmonary connection (TCPC) using in vivo phase contrast magnetic resonance imaging (PC MRI).nnnMETHODSnSixteen single-ventricle patients were prospectively recruited at 2 leading pediatric institutions for PC MRI evaluation of their Fontan pathway. Patients were divided into 2 groups. Group 1 comprised 8 patients with an extracardiac (EC) TCPC, and group 2 comprised 8 patients with a lateral tunnel (LT) TCPC. A coronal stack of 5 to 10 contiguous PC MRI slices with 3D velocity encoding (5-9 ms resolution) was acquired and a volumetric flow field was reconstructed.nnnRESULTSnAnalysis revealed large vortices in LT TCPCs and helical flow structures in EC TCPCs. On average, there was no difference between LT and EC TCPCs in the proportion of inferior vena cava flow going to the left pulmonary artery (43% ± 7% vs 46% ± 5%; P = .34). However, for EC TCPCs, the presence of a caval offset was a primary determinant of inferior vena caval flow distribution to the pulmonary arteries with a significant bias to the offset side.nnnCONCLUSIONSn3D flow structures within LT and EC TCPCs were reconstructed and analyzed for the first time using PC MRI. TCPC flow patterns were shown to be different, not only on the basis of LT or EC considerations, but with significant influence from the superior vena cava connection as well. This work adds to the ongoing body of research demonstrating the impact of TCPC geometry on the overall hemodynamic profile.

Numerical, Hydraulic, and Hemolytic Evaluation of an Intravascular Axial Flow Blood Pump to Mechanically Support Fontan Patients

Currently available mechanical circulatory support systems are limited for adolescent and adult patients with a Fontan physiology. To address this growing need, we are developing a collapsible, percutaneously-inserted, axial flow blood pump to support the cavopulmonary circulation in Fontan patients. During the first phase of development, the design and experimental evaluation of an axial flow blood pump was performed. We completed numerical modeling of the pump using computational fluid dynamics analysis, hydraulic testing of a plastic pump prototype, and blood bag experiments (nxa0=xa07) to measure the levels of hemolysis produced by the pump. Statistical analyses using regression were performed. The prototype with a 4-bladed impeller generated a pressure rise of 2–30xa0mmHg with a flow rate of 0.5–4xa0L/min for 3000–6000xa0RPM. A comparison of the experimental performance data to the numerical predictions demonstrated an excellent agreement with a maximum deviation being less than 6%. A linear increase in the plasma-free hemoglobin (pfHb) levels during the 6-h experiments was found, as desired. The maximum pfHb level was measured to be 21xa0mg/dL, and the average normalized index of hemolysis was determined to be 0.0097xa0g/100xa0L for all experiments. The hydraulic performance of the prototype and level of hemolysis are indicative of significant progress in the design of this blood pump. These results support the continued development of this intravascular pump as a bridge‐to‐transplant, bridge‐to‐recovery, bridge-to-hemodynamic stability, or bridge-to-surgical reconstruction for Fontan patients.

Geometric characterization of patient-specific total cavopulmonary connections and its relationship to hemodynamics.

Total cavopulmonary connection (TCPC) geometries have great variability. Geometric features, such as diameter, connection angle, and distance between vessels, are hypothesized to affect the energetics and flow dynamics within the connection. This study aimed to identify important geometric characteristics that can influence TCPC hemodynamics. Anatomies from 108 consecutive patients were reconstructed from cardiac magnetic resonance (CMR) images and analyzed for their geometric features. Vessel flow rates were computed from phase contrast CMR. Computational fluid dynamics simulations were carried out to quantify the indexed power loss and hepatic flow distribution. TCPC indexed power loss correlated inversely with minimum Fontan pathway (FP), left pulmonary artery, and right pulmonary artery diameters. Cardiac index correlated with minimum FP diameter and superior vena cava (SVC) minimum/maximum diameter ratio. Hepatic flow distribution correlated with caval offset, pulmonary flow distribution, and the angle between FP and SVC. These correlations can have important implications for future connection design and patient follow-up.

Uniquely shaped cardiovascular stents enhance the pressure generation of intravascular blood pumps

OBJECTIVEnAdvances in the geometric design of blood-contacting components are critically important as the use of minimally invasive, intravascular blood pumps becomes more pervasive in the treatment of adult and pediatric patients with congestive heart failure. The present study reports on the evaluation of uniquely shaped filaments and diffuser blades in the development of a protective stent for an intravascular cavopulmonary assist device for patients with a single ventricle.nnnMETHODSnWe performed numeric modeling, hydraulic testing of 11 stents with an axial flow blood pump, and blood bag experiments (n = 6) of the top-performing stent geometries to measure the levels of hemolysis. A direct comparison using statistical analyses, including regression analysis and analysis of variance, was completed.nnnRESULTSnThe stent geometry with straight filaments and diffuser blades that extended to the vessel wall outperformed all other stent configurations. The pump with this particular stent was able to generate pressures of 2 to 32 mm Hg for flow rates of 0.5 to 4 L/min at 5000 to 7000 RPM. A comparison of the experimental performance data to the numeric predictions demonstrated an excellent agreement within 16%. The addition of diffuser blades to the stent reduced the flow vorticity at the pump outlet. The average and maximum normalized index of hemolysis level was 0.0056 g/100 L and 0.0064 g/100 L, respectively.nnnCONCLUSIONSnThe specialized design of the stents, which protect the vessel wall from the rotating components of the pump, proved to be advantageous by further augmenting the pressure generation of the pump, reducing the flow vorticity at the pump outlet, and enhancing flow control.