Background It is known that respiration modulates cavopulmonary flows, but there is little data comparing mean flows under breath holding and free breathing conditions to isolate the respiratory effects, as well as 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 based on beat-averaged flow was employed to quantify the respiratory effect. Results Flow during inspiration was significantly 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 significant 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.

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.

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.

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.