Hemodynamics of the VenusP Valve System™-an in vitro study.

This study aims to evaluate the fluid dynamic characteristics of the VenusP Valve System™ under varying cardiac outputs in vitro. A thorough hemodynamic study of the valve under physiological cardiac conditions was conducted and served as an independent assessment of the performance of the valve. Flow fields downstream of the valve near the pulmonary bifurcation were quantitatively studied by two-dimensional Particle Image Velocimetry (PIV). The obtained flow field was analyzed for potential regions of flow stasis and recirculation, and elevated shear stress and turbulence. High-speed en face imaging capturing the leaflet motion provided data for leaflet kinematic modeling. The experimental conditions for PIV studies were in accordance with ISO 5840-1:2021 standard, and two valves with different lengths and different orientations were studied. Results show good hemodynamics performance for the tested valves according to ISO 5840 standard without significant regions of flow stasis. Observed shear stress values are all well below established hemolysis limits.

Fontan Geometry and Hemodynamics Are Associated With Quality of Life in Adolescents and Young Adults.

BACKGROUND: Despite favorable short-term outcomes, Fontan palliation is associated with comorbidities and diminished quality of life (QOL) in the years after completion. We hypothesized that poor Fontan hemodynamics and ventricular function are associated with worse QOL. METHODS: This was a single-center study of Fontan survivors aged more than 12 years. Subjects completed a cardiac magnetic resonance scan and QOL questionnaire. Cardiac magnetic resonance-derived variables included Fontan geometry, and hemodynamics. Computational fluid dynamics simulations quantified power loss, pressure drop, and total cavopulmonary connection resistance across the Fontan. Quality of life was assessed by completion of the Pediatric Quality of Life Inventory. Longitudinal and cross-sectional comparisons were made between cardiac magnetic resonance and computational fluid dynamics parameters with patient-reported QOL. RESULTS: We studied 77 Fontan patients, median age 19.7 years (interquartile range, 17.1 to 23.6), median time from Fontan completion 16 years (interquartile range, 13 to 20). Longitudinal data were available for 48 patients; median time between cardiac magnetic resonance and QOL was 8.1 years (interquartile range, 7 to 9.4). Median patient-reported Pediatric Quality of Life Inventory total score was 80 (interquartile range, 67.4 to 88). Greater power loss and smaller left pulmonary artery diameter at baseline were associated with worse QOL at follow-up. Greater pressure drop was associated with worse QOL at the same time point. CONCLUSIONS: For Fontan survivors, measures of computational fluid dynamics hemodynamics and geometry are associated with worse QOL. Interventional strategies targeted at optimizing the Fontan may improve QOL.

The Advantages of Viscous Dissipation Rate over Simplified Power Loss as a Fontan Hemodynamic Metric.

Flow efficiency through the Fontan connection is an important factor related to patient outcomes. It can be quantified using either a simplified power loss or a viscous dissipation rate metric. Though practically equivalent in simplified Fontan circulation models, these metrics are not identical. Investigation is needed to evaluate the advantages and disadvantages of these metrics for their use in in vivo or more physiologically-accurate Fontan modeling. Thus, simplified power loss and viscous dissipation rate are compared theoretically, computationally, and statistically in this study. Theoretical analysis was employed to assess the assumptions made for each metric and its clinical calculability. Computational simulations were then performed to obtain these two metrics. The results showed that apparent simplified power loss was always greater than the viscous dissipation rate for each patient. This discrepancy can be attributed to the assumptions derived in theoretical analysis. Their effects were also deliberately quantified in this study. Furthermore, statistical analysis was conducted to assess the correlation between the two metrics. Viscous dissipation rate and its indexed quantity show significant, strong, linear correlation to simplified power loss and its indexed quantity (p < 0.001, r > 0.99) under certain assumptions. In conclusion, viscous dissipation rate was found to be more advantageous than simplified power loss as a hemodynamic metric because of its lack of limiting assumptions and calculability in the clinic. Moreover, in addition to providing a time-averaged bulk measurement like simplified power loss, viscous dissipation rate has spatial distribution contours and time-resolved values that may provide additional clinical insight. Finally, viscous dissipation rate could maintain the relationship between Fontan connection flow efficiency and patient outcomes found in previous studies. Consequently, future Fontan hemodynamic studies should calculate both simplified power loss and viscous dissipation rate to maintain ties to previous studies, but also provide the most accurate measure of flow efficiency. Additional attention should be paid to the assumptions required for each metric.

Using a Novel In Vitro Fontan Model and Condition-Specific Real-Time MRI Data to Examine Hemodynamic Effects of Respiration and Exercise.

Several studies exist modeling the Fontan connection to understand its hemodynamic ties to patient outcomes (Chopski in: Experimental and Computational Assessment of Mechanical Circulatory Assistance of a Patient-Specific Fontan Vessel Configuration. Dissertation, 2013; Khiabani et al. in J Biomech 45:2376-2381, 2012; Taylor and Figueroa in Annu Rev Biomed 11:109-134, 2009; Vukicevic et al. in ASAIO J 59:253-260, 2013). The most patient-accurate of these studies include flexible, patient-specific total cavopulmonary connections. This study improves Fontan hemodynamic modeling by validating Fontan model flexibility against a patient-specific bulk compliance value, and employing real-time phase contrast magnetic resonance flow data. The improved model was employed to acquire velocity field information under breath-held, free-breathing, and exercise conditions to investigate the effect of these conditions on clinically important Fontan hemodynamic metrics including power loss and viscous dissipation rate. The velocity data, obtained by stereoscopic particle image velocimetry, was visualized for qualitative three-dimensional flow field comparisons between the conditions. Key hemodynamic metrics were calculated from the velocity data and used to quantitatively compare the flow conditions. The data shows a multi-factorial and extremely patient-specific nature to Fontan hemodynamics.

Local Hemodynamic Differences Between Commercially Available Y-Grafts and Traditional Fontan Baffles Under Simulated Exercise Conditions: Implications for Exercise Tolerance.

Fontan completion, resulting in a total cavopulmonary connection (TCPC), is accomplished using a lateral tunnel (LT), extracardiac conduit (ECC), or recently a bifurcated Y-graft. The local energetic differences between these graft types have not been substantially analyzed under exercise conditions. The present study evaluates the energetic performance of Y-grafts under simulated exercise conditions, compares their performance to the previous LT/ECC Fontan options, and discusses implications for exercise tolerance and hemodynamic predictability. Twenty Y-graft and 20 LT/ECC patients were analyzed. TCPC anatomies and flow waveforms were reconstructed using patient-specific cardiac magnetic resonance (CMR) images and phase-contrast CMR. Computational fluid dynamics simulations were performed to quantify indexed power loss (iPL) under both resting and simulated exercise conditions. Comparisons between graft types were investigated. iPL was significantly higher (p < 0.01) for Y-grafts at all activity levels. No significant interaction effects were observed between graft type and activity level. iPL at rest was strongly correlated (r 2 = 0.97, p < 0.001) with iPL at moderate exercise for Y-grafts, but less so for the LT/ECC cohort (r 2 = 0.66, p < 0.001). Similar results were seen for intense exercise, with a strong correlation for Y-grafts (r 2 = 0.94, p < 0.001) and a moderate correlation for LT/ECC (r 2 = 0.52, p < 0.001). Commercially available Y-grafts were found to have significantly higher iPL at all activity levels, suggesting worse exercise tolerance than the LT/ECC alternatives. Y-grafts offered impressive hemodynamic predictability which was not seen in the LT/ECC cohort. Our results encourage the further evaluation of an area-preserving Y-graft design to offer both improved energetic performance and hemodynamic predictability. Commercial Y-grafts show worse energetics, but more predictable responses than traditional Fontan connections under simulated exercise conditions. During simulated exercise conditions, commercially available Y-grafts show predictable but inferior energetic performance compared to lateral tunnel and extracardiac conduit Fontan connections, suggesting poorer exercise capacity. If Y-graft use is continued, these results encourage further evaluation of a cross sectional area-preserving Y-graft design as a additional alternative for Fontan completion.

In Vitro Examination of the HeartWare CircuLite Ventricular Assist Device in the Fontan Connection.

The failing Fontan physiology may benefit from ventricular assist device (VAD) mechanical circulatory support, although a subpulmonary VAD placed at the Fontan connection has never successfully supported the Fontan circulation long term. The HeartWare CircuLite continuous flow VAD was examined for Fontan circulatory support in an in vitro mock circulation. The VAD was tested in three different scenarios: VAD in parallel, baffle restricted VAD in parallel, and VAD in series. Successful support was defined as simultaneous decrease in inferior vena cava (IVC) pressure of 5 mm Hg or more and an increase in cardiac output (CO) to 4.25 L/min or greater. The VAD in parallel scenario resulted in a CO decrease to 3.46 L/min and 2.22 mm Hg decrease in IVC pressure. The baffle restricted VAD in parallel scenario resulted in a CO increase to 3.9 L/min increase in CO and 20.5 mm Hg decrease in IVC pressure (at 90% restriction). The VAD in series scenario resulted in a CO of 1.75 L/min and 5.9 mm Hg decrease in IVC pressure. We successfully modeled chronic failing Fontan physiology using patient-specific hemodynamic and anatomic data. Although unsuccessful in supporting Fontan patients as defined here, the HeartWare CircuLite VAD demonstrates the possibility to reduce Fontan pressure and increase CO with a VAD in the Fontan connection. This study provides insight into pump performance and design issues when attempting to support Fontan circulation. Refinements in VAD design with specific parameters to help support this patient population is the subject of our future work.

Respiratory Effects on Fontan Circulation During Rest and Exercise Using Real-Time Cardiac Magnetic Resonance Imaging.

BACKGROUND: It is known that respiration modulates cavopulmonary flows, but little data compare mean flows under breath-holding and free-breathing conditions to isolate the respiratory effects and effects of exercise on the respiratory modulation. METHODS: Real-time phase-contrast magnetic resonance combined with a novel method to track respiration on the same image acquisition was used to investigate respiratory effects on Fontan caval and aortic flows under breath-holding, free-breathing, and exercise conditions. Respiratory phasicity indices that were based on beat-averaged flow were used to quantify the respiratory effect. RESULTS: Flow during inspiration was substantially higher than expiration under the free-breathing and exercise conditions for both inferior vena cava (inspiration/expiration: 1.6 ± 0.5 and 1.8 ± 0.5, respectively) and superior vena cava (inspiration/expiration: 1.9 ± 0.6 and 2.6 ± 2.0, respectively). Changes from rest to exercise in the respiratory phasicity index for these vessels further showed the impact of respiration. Total systemic venous flow showed no significant statistical difference between the breath-holding and free-breathing conditions. In addition, no substantial difference was found between the descending aorta and inferior vena cava mean flows under either resting or exercise conditions. CONCLUSIONS: This study demonstrated that inferior vena cava and superior vena cava flow time variance is dominated by respiratory effects, which can be detected by the respiratory phasicity index. However, the minimal respiration influence on net flow validates the routine use of breath-holding techniques to measure mean flows in Fontan patients. Moreover, the mean flows in the inferior vena cava and descending aorta are interchangeable.