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A pulsatile hemodynamic evaluation of the commercially available bifurcated Y-graft Fontan modification and comparison with the lateral tunnel and extracardiac conduits
Accepted Manuscript A Pulsatile Hemodynamic Evaluation of the Commercially Available Bifurcated YGraft Fontan Modification and Comparison to the Lateral Tunnel and Extracardiac Conduits Phillip M. Trusty, BS, Maria Restrepo, PhD, Kirk R. Kanter, MD, Ajit P. Yoganathan, PhD, Mark A. Fogel, MD, Timothy C. Slesnick, MD PII:
S0022-5223(16)00418-9
DOI:
10.1016/j.jtcvs.2016.03.019
Reference:
YMTC 10407
To appear in:
The Journal of Thoracic and Cardiovascular Surgery
Received Date: 16 December 2015 Revised Date:
1 March 2016
Accepted Date: 5 March 2016
Please cite this article as: Trusty PM, Restrepo M, Kanter KR, Yoganathan AP, Fogel MA, Slesnick TC, A Pulsatile Hemodynamic Evaluation of the Commercially Available Bifurcated Y-Graft Fontan Modification and Comparison to the Lateral Tunnel and Extracardiac Conduits, The Journal of Thoracic and Cardiovascular Surgery (2016), doi: 10.1016/j.jtcvs.2016.03.019. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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A Pulsatile Hemodynamic Evaluation of the Commercially Available Bifurcated Y-Graft Fontan Modification and Comparison to the Lateral Tunnel and Extracardiac Conduits
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Wallace H. Coulter Department of Biomedical Engineering, Georgia Institute of Technology and Emory University, Atlanta, GA 2 Division of Cardiothoracic Surgery, Children’s Healthcare of Atlanta, Emory University School of Medicine, Atlanta, GA, USA. 3 Division of Cardiology, Children’s Hospital of Philadelphia, Philadelphia, PA, USA 4 Department of Pediatrics, Division of Cardiology, Children’s Healthcare of Atlanta, Emory University School of Medicine, Atlanta, GA, USA
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Abbreviations and Acronyms: BSA (body surface area), CFD (computational fluid dynamics), CMR (cardiac magnetic resonance), ECC (extra cardiac conduit), HFD (hepatic flow distribution), iPL (indexed power loss), IVC (inferior vena cava), LPA (left pulmonary artery), LT (lateral tunnel), PA (pulmonary artery), PAVMs (pulmonary arteriovenous malformations), PFD (pulmonary flow distribution), PI (pulsatility index), PL (power loss), RPA (right pulmonary artery), SVC (superior vena cava), TCPC (total cavopulmonary connection)
Funding/Disclosures: P.M. Trusty: None. M. Restrepo: None. M.A. Fogel: Research Grant; Modest; NIH R01. Consultant/Advisory Board; Modest; Edwards Life Sciences - MRI Core Lab. Other; Modest; AMAG FACT trial site, Cooley's Anemia Foundation MRI Core Lab. K. Kanter: None. A.P. Yoganathan:None. T.C. Slesnick: None. Conflict of Interest: None of the authors have potential conflicts of interest in relation to the presented work.
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Phillip M. Trusty, BS1; Maria Restrepo, PhD1; Kirk R. Kanter, MD2; Ajit P. Yoganathan, PhD1; Mark A. Fogel, MD3; Timothy C. Slesnick, MD4
Corresponding Author: Ajit P. Yoganathan [email protected] 404-502-7869
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Abstract
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Objective: Fontan completion, resulting in a total cavopulmonary connection (TCPC), is
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accomplished using either a lateral tunnel (LT), extracardiac conduit (ECC), or bifurcated Y-graft.
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The use of Y-grafts is hypothesized to provide symmetric hepatic blood flow distribution to the
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lungs, a factor related to pulmonary arteriovenous malformations. The present study evaluates
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the hemodynamic performance of the largest commercially available Y-graft cohort to date,
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highlights 6 representative cases and compares commercially available Y-graft performance
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with LT/ECC connections.
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Methods: Thirty commercially available Y-graft and 30 LT/ECC patients were analyzed. TCPC
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anatomies and flow waveforms were reconstructed using cardiac magnetic resonance (CMR)
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images and phase-contrast CMR. Computational fluid dynamic simulations were performed to
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quantify TCPC power loss, resistance, and hepatic flow distribution (HFD). Comparisons
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between graft types were investigated.
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Results: TCPC resistance was significantly higher for Y-grafts. HFD was similar overall, but shows
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discrepancies at extreme values with more unbalanced flow in the Y-graft cohort. Power loss
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was more sensitive to LPA stenosis in the Y-graft cohort. Prediction of Y-graft HFD is multi-
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factorial.
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Conclusions: Commercially available Y-grafts do not inherently provide more balanced HFD than
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LT/ECC connections, which are more energetically favorable and less sensitive to PA stenosis.
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Graft type should be considered on an individual basis as hemodynamic performance is based
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on a combination of factors, including pulmonary flow distribution, PA stenosis and superior
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vena cava positioning.
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Central Message:
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Commercially available Y-grafts do not inherently provide more balanced HFD or favorable
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energetic performance than traditional Fontans.
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Perspective Statement:
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Despite intuition, commercially available Y-grafts do not inherently provide more balanced
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hepatic flow distribution than traditional Fontan connections. Several Y-grafts were seen to
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direct nearly all hepatic flow to a single lung. Graft type should be considered on an individual
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basis as hemodynamic performance is governed by several factors including pulmonary flow
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distribution and PA stenosis.
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Introduction The Fontan procedure is the final operation in a series of palliative surgical procedures
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for patients with single ventricle congenital heart defects, in which the inferior vena cava (IVC)
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is connected to the pulmonary arteries (PA) by way of an extra-cardiac conduit or intra-cardiac
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tunnel. This operation results in the total cavopulmonary connection (TCPC) which bypasses the
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ventricle with passive venous return into the PAs.1 With advances in medical imaging, clinicians
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can better determine the appropriate course of action for individual patients prior to surgery.
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Previous studies focused on surgical planning for the Fontan operation in an attempt to
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maximize energy efficiency, obtain equal hepatic flow distribution (HFD) and potentially
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improve the patient’s long term quality of life.2–4 Further efforts characterized the effect of
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baffle diameter, vessel offset, connection angle and surgical approach on the power loss and
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HFD of the TCPC.5–10
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Currently, surgeons complete the Fontan connection using either a lateral tunnel (LT) baffle, extracardiac conduit (ECC) or Y-graft. The ECC is typically positioned externally around
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the margin of the heart while the LT is positioned through the atrium. LT/ECC connections are
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usually anastomosed to the mid-PA, inferior to the superior vena cava (SVC) anastomosis from
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the prior bidirectional Glenn. Alternatively, the commercially available bifurcated Y-graft
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modification splits inferior systemic venous return through two branches toward the left and
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right pulmonary arteries. Y-Grafts are utilized in an effort to obtain balanced HFD to the left and
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right lungs, a factor in preventing pulmonary arteriovenous malformations (PAVMs), a potential
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Fontan patient morbidity.11,12 The TCPCs from two patients employing each option are shown in
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Figure 1.
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For the inherently inefficient circulation in Fontan patients, all attempts to minimize the ventricle’s workload are clinically valuable. Surgeons can repair branch pulmonary artery
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stenosis, determine energetically favorable baffle placement, and choose which connection
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type to use. However, surgeons may not have complete freedom in these choices as they are
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limited by the specific patient’s anatomy and condition.
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The potential energetic and HFD differences associated with graft choice have not been fully analyzed in large cohorts under pulsatile simulation conditions. Previous studies showed
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exploratory and preliminary results discussing the effectiveness of Y-graft use, but included very
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small sample sizes.13–18 Some of these studies employed time-averaged boundary conditions
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which can cause non-negligible differences in TCPC hemodynamics.9,19 The present study
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compares hemodynamic performance between LT/ECC and commercially available Y-graft
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TCPCs with the largest patient cohort to date while employing patient-specific transient flow
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boundary conditions in order to provide insights into graft choice for Fontan completion.
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Methods
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Patient Cohort
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A total of 60 patients were used in this study. All LT/ECC patient data (n=30) were
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received from Children’s Healthcare of Philadelphia and all Y-Graft patient data (n=30) were
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received from Children’s Healthcare of Atlanta. Thirty consecutive Y-grafts performed by Dr.
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Kirk Kanter were used; LT/ECC patients were chosen to match age and BSA between groups. All
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Y-graft Fontan connections used commercially available Y-grafts. Clinical data including age,
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gender, and BSA were obtained for each patient. The Institutional Review Boards of the
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institutions involved approved this study.
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Anatomy and Velocity Reconstruction
Patient-specific anatomies were reconstructed from axial CMR images using methods
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previously developed.20,21 Geomagic Studio® (Geomagic Inc., Research Triangle Park, NC) was
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then used to fit a surface around the reconstructed point-cloud and export the surface for mesh
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generation. Patient-specific blood flow waveforms were calculated from phase contrast MRI
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images for all vessels of interest using previously validated methods.22,23
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Computational Fluid Dynamics
All geometries were imported into GAMBIT (ANSYS Inc., Lebanon, NH) for mesh
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generation. To provide computational stability, flow extensions of 10mm and 50mm were
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added to all inlets and outlets, respectively. All computational fluid dynamics (CFD) simulations
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were completed using a previously developed and validated in-house immersed boundary
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solver.4,5,14,24 The unsteady flow waveforms extracted from PC-MRI were imposed as boundary
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conditions for each inlet and outlet. A total of 6 cardiac cycles were simulated for each patient
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to overcome transition effects and achieve periodic stability; the final cycle was used for
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hemodynamic analysis.
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Data Analysis Calculations were performed to compute power loss (PL), indexed power loss (iPL), TCPC resistance, pulsatility index (PI) and percent stenosis. Power loss was calculated as follows:
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where p is the static pressure, ρ is the blood density, A is the area of the inlet/outlet and v the
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velocity. Power loss was calculated for 200 points throughout the cardiac cycle and then
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averaged to obtain an overall PL value. Power loss was indexed (iPL) as (
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resistance was calculated
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surface area (m2), ∆ is the pressure drop and ρ is the blood density (kg·m-3).
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Pulsatility index was defined as follows:
/
) and a TCPC
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where Qs is the systemic venous flow (L·s-1), BSA is the body
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where Qmax, Qmin, and Qavg are the maximum, minimum and average flow rates during the
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cardiac cycle, respectively, as determined from the phase contrast CMR data. Percent stenosis
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was calculated for the pulmonary arteries using the following equation:
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where . , and . ,1 are the minimum cross sectional areas of the LPA and RPA, and
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.!", and .!",1 are the average cross sectional areas of the LPA and RPA. A combined
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outlet stenosis value was used in place of unilateral PA stenosis measurements to give a more
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accurate representation of the total outlet obstruction to flow and allow a better comparison
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between patients. Additionally, IVCs and SVCs showed negligible percent stenosis and therefore 8
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were not included in this calculation. All vessel areas for this metric were calculated using
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Vascular Modeling Toolkit version 1.0.1 (Orobix, Bergamo, Italy). Hepatic flow distribution (HFD) was evaluated using particle tracking. Particles were
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seeded at the base of the LT/ECC or Y-graft at 200 time points throughout the cardiac cycle. An
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additional four cardiac cycles (with no particle seeding) were simulated to ensure that the
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residence time of the particles would not exceed the simulation time. Particles exiting at each
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outlet were counted and HFD was defined as the ratio of particles leaving the LPA: 67$)89%(
67$)89%( 0 67$)89%(1
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HFD will be discussed in terms of “HFD deviation from 50%”, and an even (50/50) split of
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hepatic flow to the LPA and RPA is assumed as ideal. This convention gives a “0” HFD deviation
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for balanced HFD cases, and an HFD deviation of 50 for extreme cases (all hepatic flow to either
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the LPA or RPA). Particle residence time was calculated by recording the amount of time
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required for each particle to exit the TCPC domain. An overall pulmonary flow distribution (PFD)
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was calculated as the ratio of LPA flow to total pulmonary flow:
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Statistics
The Anderson-Darling test was used to determine normality for each parameter within
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both patient groups. Depending on normality, either a Wilcoxon rank sum test (non-normal
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distribution) or a two-sample t-test (normal distribution) was used to test for equal medians
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between patient groups using SPSS (IBM Corp., Version 23, Armonk, NY). An asterisk (*) is used
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to signify statistical significance using α=0.05. Values are shown as mean± standard deviation.
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Results
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The commercially available Y-graft evaluation includes an investigation of several
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hemodynamic metrics within the Y-graft cohort, then an examination of both ideally and poorly
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performing Y-grafts (in terms of HFD), and finally, a comparison between Y-graft and LT/ECC
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performance.
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Y-Graft Hemodynamic Performance
Figure 2 shows several clinically important hemodynamic metrics for the Y-graft cohort.
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The average PFD and HFD deviations from an ideal 50/50 split were 13.4±10.6 and 25.9±15.1,
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respectively (Fig. 2a). Fig. 2b illustrates the effect of LPA stenosis on indexed power loss. A
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moderate positive correlation (r=0.67, p<0.01) was found between LPA stenosis and indexed
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power loss. The distributions of TCPC resistance and HFD deviation are shown in Fig. 2c and 2d.
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The distribution of TCPC resistance is skewed to the right with approximately 53% and 70% of Y-
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graft patients having resistances less than 1 and 2 Wood Units respectively. HFD deviation was
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relatively evenly distributed (Fig. 2d). Only 13% of Y-graft patients have HFD deviations less
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than 10 (ideal) and 24% of Y-graft patients have deviations greater than 40 (poor).
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Representative Y-Graft Cases
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Figure 3 shows streamlines for six representative cases from the 30 patient Y-graft cohort. Patients Y1, Y2, andY3 show balanced HFD (low HFD deviation) while the final three (Y4,
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Y5, andY6) are unbalanced. Y1 represents an “ideal” case with balanced PFD, normal SVC
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positioning and no mid-PA stenosis. Y2 represents uneven PFD with mid-PA stenosis on the low
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PFD side. Y3 has bilateral SVCs, balanced PFD and mid-PA stenosis. Y4 has balanced PFD with
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the SVC positioned superior to a y-branch anastomosis. Y5 has unbalanced PFD with mid-PA
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stenosis on the high PFD side and Y6 represents balanced PFD with y-branch stenosis. HFD to
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the LPA is noted for each model in Figure 3.
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Y-graft vs LT/ECC
To further evaluate the commercially available Y-graft, a direct hemodynamic comparison was performed between the Y-graft and LT/ECC cohorts. Clinical data are shown in
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Table 1 for each graft type. Patients were selected to best match age and BSA between groups.
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Y-graft resistance was significantly higher (p=0.03) than LT/ECC resistance, with averages of
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1.51±1.52 and 0.56±0.54 WU, respectively (Fig. 4a). Overall, HFD deviation was not significantly
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different between the groups with an average of 21±13 for the LT/ECC and 26±15 for Y-grafts
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(Fig. 4b). Figure 4c shows no significant differences in particle residence time.
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Figure 4d and 4e compare the distribution of TCPC resistance and HFD deviation
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between graft types. Almost 77% of LT/ECC patients have a resistance less than 1 WU,
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compared to 53% of the Y-grafts. Additionally, 30% of commercially available Y-graft patients
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exhibit a resistance greater than 2 WU, while only 3% of the LT/ECC cohort have values this high
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(Fig. 4d). Focusing on the two extremes of HFD deviation, 30% of LT/ECC patients had ideal HFD
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deviations (<10) compared to only 10% of Y-graft patients. At the other extreme, more than
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twice as many Y-graft patients (23%) have HFD deviations greater than 40 (poor) than LT/ECCs
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(10%).
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Figure 4f compares the sensitivity of iPL to LPA stenosis for both graft types. Commercially
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available Y-grafts were found to be more sensitive to LPA stenosis in terms of iPL with a
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significant difference in the two correlations (Fisher’s exact test, one-tailed, z=-1.76, p=0.04).
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Discussion
Capitalizing on the relatively large sample size in this study, the following section evaluates commercially available Y-graft performance and generalizes differences between
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graft types. Both clinical and physiological implications are discussed. Six Y-grafts that illustrate
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common anatomical parameter combinations will be emphasized and study limitations will be
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considered.
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Y-Graft Hemodynamic Performance
As expected, PFD deviation is relatively low and on average there is about a 25%
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difference in overall flow rates to the LPA and RPA. This flow split is determined partially by
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branch pulmonary artery stenosis.25 HFD deviation was higher and more variable than
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expected and will be discussed in the following section. The effect of LPA stenosis on iPL is also
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an expected result. Blood accelerates through the stenosis region and then decelerates as the
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vessel returns to its original size according to the conservation of momentum principle. These
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changes in velocity result in an increased pressure drop across the TCPC (lower efficiency)
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which leads to an increased iPL. The LPA is a common site for stenosis and Fig. 2b shows that
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the most severe LPA stenosis anatomies have the highest iPL.10
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The distribution of TCPC resistance raises concern for a number of Y-graft patients. In this study, Y-graft TCPCs are seen to have resistances as high as 4.7 Wood units, and this
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increased level of vascular resistance could lead to venous hypertension/congestion. Liver
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failure and protein-losing enteropathy are thought to be direct results of increased Fontan
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venous pressures.26–28 The relatively even distribution of HFD deviation suggests that the Y-
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graft is not a “one size fits all” solution to accomplish evenly split hepatic flow to the lungs.
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While 13% of Y-graft patients had very balanced HFD, nearly a quarter of patients had >90%
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hepatic flow directed to either the left or right lung. The effectiveness of Y-graft use seems to
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be multi-factorial and extremely patient specific, suggesting the need for pre-operative,
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individual surgical planning via computational modeling.
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Representative Y-Graft Cases
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The ability of a Y-graft to evenly split hepatic flow is highly dependent on PFD, the position of branch PA stenosis and SVC flow collision, among other factors. A difference in PFD
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can cause two Y-grafts with very similar anatomies to have drastically different HFDs. This can
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be seen in Fig. 3, where Y2 and Y5 have PFDs of 24% and 65%, respectively. Despite similar
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anatomies and Y-grafts, the HFD for these cases are considerably different (48% and 81% to the
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LPA, respectively). A comparison between Y1 and Y4 illustrates the effect of SVC flow collision
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on HFD. Both cases have balanced PFD and similar sized PAs, but Y1 has a medially positioned
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SVC, while Y4’s SVC is positioned more superior to the left Y-branch. The SVC flow collision in Y4
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forces some SVC flow into the left Y-branch which causes hepatic flow to preferentially course
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through the right Y-branch. The result is a very unbalanced HFD for Y4 (12% to the LPA) and
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relatively balanced HFD for Y1 (65% to the LPA).
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Stenosis is commonly seen in Fontan anatomies;10 Y2, Y3, Y5 and Y6 all have severe stenosis in the TCPC. For single SVC anatomies (Y2, Y5), mid-PA stenosis influences SVC flow
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which will preferentially follow the path of least resistance (in these cases towards the RPA).
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For an unbalanced PFD case, if the mid-PA stenosis is located on the same side as the PA
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receiving the majority of pulmonary flow, then the stenosis blocks most SVC flow and this PA
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receives the majority of its flow from the hepatics and will therefore have an unbalanced HFD
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(Y5). However, the opposite is true if the stenosis is located on the low PFD side; in this case
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hepatic flow is able to move towards both PAs resulting in more balanced HFD (Y2). For
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bilateral SVC anatomies (Y3), mid-PA stenosis can “separate” the two SVC flows and HFD will be
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highly dependent on the individual SVC and overall PA flows. If SVC flows are similar and PFD is
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balanced, a balanced HFD is expected. Y6 shows a case with severe stenosis located in the left
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Y-branch, and as a result hepatic flow is very limited through the stenosed branch. With the full
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range of flow rates, flow ratios, pulsatilities, unique geometries and stenosis locations, the
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ability of a specific Y-graft to achieve balanced HFD may not be intuitive. Again, Y-graft
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performance needs to be evaluated on an individual basis and all of these factors must be
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considered.
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Y-graft vs LT/ECC
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Average TCPC resistance was approximately 3x higher for Y-grafts than LT/ECCs. The most apparent explanation stems from the geometry of the Y-graft itself. With commercially
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available Y-grafts, the diameter of the base of the Y-graft is evenly split between the two
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branches. For example, a 20mm (diameter) Y-graft will have two 10mm (diameter) branches. A
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basic calculation reveals that preserving diameter does not preserve area; this design results in
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a 50% reduction in cross-sectional area as blood moves into the Y-graft branches. In essence, Y-
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grafts have a built-in 50% long segment “stenosis”. Additionally, resistance is inversely
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proportional to the fourth power of the radius (Poiseuille’s law). Therefore, blood traveling
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through 2 Y-branches with diameters half the size of the LT/ECC would analytically yield an 8x
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increase in resistance. According to the results presented here, the reduction in diameter and
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cross sectional area inherent to commercially available Y-grafts has a greater negative impact
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on TCPC resistance than the venous flow collisions inherent to LT/ECCs. Therefore, “custom” Y-
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grafts which preserve cross sectional area as shown by Yang et al. may be the more promising
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option for successful Y-graft use, and may potentially provide an advantage over the LT/ECC
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Fontan pathway.15,18
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Overall, HFD deviation was not significantly different between graft types. Both graft types were highly variable. The most important observation is the discrepancy in the number of
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patients at both the lowest (ideal) and highest (poor) HFD deviation (Fig. 4e). In this study, Y-
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grafts have almost twice as many poor differential cases as ideal cases, while LT/ECCs show the
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opposite trend. These results highlight the need for pre-operative planning with Y-graft use as
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well as the design of individualized Y-grafts which are tailored for a specific patient. Potential
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modifications such as the angle of origin, branch size and overall shape could lead to better HFD
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performance on an individual basis. Particle residence time (PRT) was investigated for its potential impact on flow-induced
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thrombus formation, as well as to provide quantitative information about flow stagnation.29,30
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No differences were seen for minimum, mean or maximum residence times. As seen in Fig. 4f,
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Y-graft iPL was more sensitive to LPA stenosis than LT/ECC iPL. PA stenosis may have a greater
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influence on the reduced flow through the closest Y-branch than it does on the total hepatic
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flow traveling through a LT/ECC connection. This difference would lead to a greater effect of
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stenosis on iPL for Y-grafts than LT/ECCs.
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Several additional comparisons between graft types are important to note. First, PFD
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was identical between graft types at 46% to the left lung, indicating that graft type does not
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affect PFD. Additionally, there was significantly more PA stenosis (p=0.02) in the LT/ECC cohort
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as seen in Table 1. Even with significantly more PA stenosis, which is known to increase
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resistance, the LT/ECC cohort still had lower TCPC resistance than the Y-grafts. This strengthens
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the finding that commercially available Y-grafts inherently create higher TCPC resistance.
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Finally, the LT/ECC group includes both intra-atrial and extracardiac connections, while Y-grafts
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are always extracardiac. In this study, 10 of the LT/ECCs were intra-atrial. To ensure that these
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results were a consequence of graft type and not the intra-atrial vs. extracardiac variation, a
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sub-comparison was performed between these two groups. No statistically significant
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differences were found in this sub-comparison for iPL, PFD, or HFD, and therefore the
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comparative results presented here are due to graft choice and not its location.
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Limitations
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The period of time between the Fontan operation and the MRI scan was not identical for all
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patients included in this study. Y-graft patients were scanned within one month of the surgery,
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while LT/ECC patients were scanned around 6 months after the operation. This difference in
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recovery time may lead to variations in the amount of physiological adaptation which could
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affect flow rates (as seen in the difference between systemic venous flows) and pulmonary
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vascular resistance and therefore TCPC resistance and HFD. Additionally, the spatial velocity
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profiles used at the inlets of the CFD domain are assumed to be parabolic, which are not
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identical to the spatial velocity profiles obtained from the MRI velocity segmentations.
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However, this assumption is expected to have only minimal effects on the bulk hemodynamic
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metrics. From a modeling perspective, this study focused only on the TCPC geometry and did
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not include interactions with the rest of the cardiovascular or pulmonary circulation. This
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localized TCPC modeling may omit significant global influences on the system. Finally, due to
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limited data, the hepatic veins were not included in this modeling. The inclusion of the hepatic
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veins and their respective flow rates could potentially influence the mixing of hepatic factor
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with the IVC flow and therefore alter the presented HFD results. This study also assumes rigid
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walls in the CFD simulations.
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Conclusion
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In this study, commercially available Y-grafts were found to have highly variable HFD due to a combination of factors including pulmonary flow distribution to the left and right
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lungs, presence and location of stenosis, and SVC positioning. HFD distributions suggest that
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LT/ECCs are more likely to avoid extremely unbalanced hepatic flow to the lungs. Y-graft HFD
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performance is multi-factorial, which requires patient specific computational modeling. Y-grafts
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were found to be more sensitive in terms of indexed power loss to LPA stenosis. Overall, Y-
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grafts show significantly higher resistance than LT/ECCs due in part to the intrinsic graft
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geometry, which calls for a better Y-graft design.
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hepatocellular carcinoma. J Thorac Cardiovasc Surg. 2005;129(6):1348-1352.
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deposition in intracranial aneurysms. Ann Biomed Eng. 2010;38(10):3058-3069.
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doi:10.1007/s10439-010-0065-8.
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Figure Legends
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Figure 1: (a) TCPC with an extracardiac baffle. (b) TCPC with a Y-Graft. (SVC: superior vena cava.
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LPA: left pulmonary artery. RPA: right pulmonary artery.)
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Figure 2: Y-Graft hemodynamic performance. (a) Pulmonary and hepatic flow distribution (PFD
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and HFD) deviations. (b) Effect of LPA stenosis on index power loss (iPL). (c) Distribution of TCPC
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resistance. (d) Distribution of HFD deviation.
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Figure 3: Representative Y-Graft cases with HFD noted. Streamlines colored by origin: IVC(blue),
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SVC(green) and LSVC(red). Percentages shown are percent of hepatic flow to the left pulmonary
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artery.
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Figure 4: Hemodynamic Comparison of the Y-graft and LT/ECC. (a) Indexed power loss. (b) HFD
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deviation from 50%. (c) Particle residence time. (d) Distribution of iPL. (e) Distribution of HFD
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deviation. (f) Effect of LPA stenosis on iPL.
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Central Picture: Fontan Y-graft hepatic flow distribution; streamlines colored by inlet vessel.
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0.67 ± 0.12
0.66 ± 0.20
0.19
Age (years)
3.65 ± 1.14
3.68 ± 2.49
0.17
70%
PA Stenosis (%)*
33.99 ± 12.60
Systemic Venous Flow (L/min)* Pulmonary Flow Distribution (% LPA)
Pulsatility Index (SVC)
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0.19
26.82 ± 10.69
0.02
1.57 ± 0.59
<0.01
46.01 ± 15.00
46.00 ± 16.78
0.99
13.13 ± 7.91
13.40 ± 10.60
0.78
0.66 ± 0.65
0.93 ± 0.72
0.14
0.42 ± 0.12
0.76
0.51 ± 0.50
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BSA: body surface area; PA: pulmonary artery; PFD: pulmonary flow distribution. Systemic venous return is defined as the sum of all TCPC inlet flow rates. Pulmonary flow distribution is the percent of total inlet flow that exits through the left pulmonary artery. An “*” represents statistically significant differences.
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53%
2.07 ± 0.61
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PFD Deviation from 50% (percentage points) Pulsatility Index (IVC)
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p-value
BSA (m2)
Gender (male)
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Table 1: Averaged clinical data for all patients separated by graft type. LT/ECC Y-Graft (n=30) (n=30)
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Table 1:
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S0022-5223(16)00418-9
DOI:
10.1016/j.jtcvs.2016.03.019
Reference:
YMTC 10407
To appear in:
The Journal of Thoracic and Cardiovascular Surgery
Received Date: 16 December 2015 Revised Date:
1 March 2016
Accepted Date: 5 March 2016
Please cite this article as: Trusty PM, Restrepo M, Kanter KR, Yoganathan AP, Fogel MA, Slesnick TC, A Pulsatile Hemodynamic Evaluation of the Commercially Available Bifurcated Y-Graft Fontan Modification and Comparison to the Lateral Tunnel and Extracardiac Conduits, The Journal of Thoracic and Cardiovascular Surgery (2016), doi: 10.1016/j.jtcvs.2016.03.019. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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A Pulsatile Hemodynamic Evaluation of the Commercially Available Bifurcated Y-Graft Fontan Modification and Comparison to the Lateral Tunnel and Extracardiac Conduits
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Wallace H. Coulter Department of Biomedical Engineering, Georgia Institute of Technology and Emory University, Atlanta, GA 2 Division of Cardiothoracic Surgery, Children’s Healthcare of Atlanta, Emory University School of Medicine, Atlanta, GA, USA. 3 Division of Cardiology, Children’s Hospital of Philadelphia, Philadelphia, PA, USA 4 Department of Pediatrics, Division of Cardiology, Children’s Healthcare of Atlanta, Emory University School of Medicine, Atlanta, GA, USA
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Abbreviations and Acronyms: BSA (body surface area), CFD (computational fluid dynamics), CMR (cardiac magnetic resonance), ECC (extra cardiac conduit), HFD (hepatic flow distribution), iPL (indexed power loss), IVC (inferior vena cava), LPA (left pulmonary artery), LT (lateral tunnel), PA (pulmonary artery), PAVMs (pulmonary arteriovenous malformations), PFD (pulmonary flow distribution), PI (pulsatility index), PL (power loss), RPA (right pulmonary artery), SVC (superior vena cava), TCPC (total cavopulmonary connection)
Funding/Disclosures: P.M. Trusty: None. M. Restrepo: None. M.A. Fogel: Research Grant; Modest; NIH R01. Consultant/Advisory Board; Modest; Edwards Life Sciences - MRI Core Lab. Other; Modest; AMAG FACT trial site, Cooley's Anemia Foundation MRI Core Lab. K. Kanter: None. A.P. Yoganathan:None. T.C. Slesnick: None. Conflict of Interest: None of the authors have potential conflicts of interest in relation to the presented work.
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Phillip M. Trusty, BS1; Maria Restrepo, PhD1; Kirk R. Kanter, MD2; Ajit P. Yoganathan, PhD1; Mark A. Fogel, MD3; Timothy C. Slesnick, MD4
Corresponding Author: Ajit P. Yoganathan [email protected] 404-502-7869
Article Word Count: 3343 1
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Abstract
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Word count: 225
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Objective: Fontan completion, resulting in a total cavopulmonary connection (TCPC), is
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accomplished using either a lateral tunnel (LT), extracardiac conduit (ECC), or bifurcated Y-graft.
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The use of Y-grafts is hypothesized to provide symmetric hepatic blood flow distribution to the
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lungs, a factor related to pulmonary arteriovenous malformations. The present study evaluates
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the hemodynamic performance of the largest commercially available Y-graft cohort to date,
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highlights 6 representative cases and compares commercially available Y-graft performance
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with LT/ECC connections.
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Methods: Thirty commercially available Y-graft and 30 LT/ECC patients were analyzed. TCPC
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anatomies and flow waveforms were reconstructed using cardiac magnetic resonance (CMR)
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images and phase-contrast CMR. Computational fluid dynamic simulations were performed to
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quantify TCPC power loss, resistance, and hepatic flow distribution (HFD). Comparisons
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between graft types were investigated.
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Results: TCPC resistance was significantly higher for Y-grafts. HFD was similar overall, but shows
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discrepancies at extreme values with more unbalanced flow in the Y-graft cohort. Power loss
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was more sensitive to LPA stenosis in the Y-graft cohort. Prediction of Y-graft HFD is multi-
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factorial.
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Conclusions: Commercially available Y-grafts do not inherently provide more balanced HFD than
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LT/ECC connections, which are more energetically favorable and less sensitive to PA stenosis.
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Graft type should be considered on an individual basis as hemodynamic performance is based
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on a combination of factors, including pulmonary flow distribution, PA stenosis and superior
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vena cava positioning.
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Central Message:
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Commercially available Y-grafts do not inherently provide more balanced HFD or favorable
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energetic performance than traditional Fontans.
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Perspective Statement:
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Despite intuition, commercially available Y-grafts do not inherently provide more balanced
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hepatic flow distribution than traditional Fontan connections. Several Y-grafts were seen to
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direct nearly all hepatic flow to a single lung. Graft type should be considered on an individual
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basis as hemodynamic performance is governed by several factors including pulmonary flow
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distribution and PA stenosis.
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Introduction The Fontan procedure is the final operation in a series of palliative surgical procedures
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for patients with single ventricle congenital heart defects, in which the inferior vena cava (IVC)
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is connected to the pulmonary arteries (PA) by way of an extra-cardiac conduit or intra-cardiac
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tunnel. This operation results in the total cavopulmonary connection (TCPC) which bypasses the
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ventricle with passive venous return into the PAs.1 With advances in medical imaging, clinicians
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can better determine the appropriate course of action for individual patients prior to surgery.
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Previous studies focused on surgical planning for the Fontan operation in an attempt to
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maximize energy efficiency, obtain equal hepatic flow distribution (HFD) and potentially
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improve the patient’s long term quality of life.2–4 Further efforts characterized the effect of
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baffle diameter, vessel offset, connection angle and surgical approach on the power loss and
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HFD of the TCPC.5–10
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Currently, surgeons complete the Fontan connection using either a lateral tunnel (LT) baffle, extracardiac conduit (ECC) or Y-graft. The ECC is typically positioned externally around
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the margin of the heart while the LT is positioned through the atrium. LT/ECC connections are
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usually anastomosed to the mid-PA, inferior to the superior vena cava (SVC) anastomosis from
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the prior bidirectional Glenn. Alternatively, the commercially available bifurcated Y-graft
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modification splits inferior systemic venous return through two branches toward the left and
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right pulmonary arteries. Y-Grafts are utilized in an effort to obtain balanced HFD to the left and
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right lungs, a factor in preventing pulmonary arteriovenous malformations (PAVMs), a potential
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Fontan patient morbidity.11,12 The TCPCs from two patients employing each option are shown in
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Figure 1.
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For the inherently inefficient circulation in Fontan patients, all attempts to minimize the ventricle’s workload are clinically valuable. Surgeons can repair branch pulmonary artery
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stenosis, determine energetically favorable baffle placement, and choose which connection
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type to use. However, surgeons may not have complete freedom in these choices as they are
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limited by the specific patient’s anatomy and condition.
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The potential energetic and HFD differences associated with graft choice have not been fully analyzed in large cohorts under pulsatile simulation conditions. Previous studies showed
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exploratory and preliminary results discussing the effectiveness of Y-graft use, but included very
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small sample sizes.13–18 Some of these studies employed time-averaged boundary conditions
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which can cause non-negligible differences in TCPC hemodynamics.9,19 The present study
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compares hemodynamic performance between LT/ECC and commercially available Y-graft
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TCPCs with the largest patient cohort to date while employing patient-specific transient flow
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boundary conditions in order to provide insights into graft choice for Fontan completion.
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Methods
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Patient Cohort
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A total of 60 patients were used in this study. All LT/ECC patient data (n=30) were
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received from Children’s Healthcare of Philadelphia and all Y-Graft patient data (n=30) were
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received from Children’s Healthcare of Atlanta. Thirty consecutive Y-grafts performed by Dr.
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Kirk Kanter were used; LT/ECC patients were chosen to match age and BSA between groups. All
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Y-graft Fontan connections used commercially available Y-grafts. Clinical data including age,
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gender, and BSA were obtained for each patient. The Institutional Review Boards of the
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institutions involved approved this study.
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Anatomy and Velocity Reconstruction
Patient-specific anatomies were reconstructed from axial CMR images using methods
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previously developed.20,21 Geomagic Studio® (Geomagic Inc., Research Triangle Park, NC) was
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then used to fit a surface around the reconstructed point-cloud and export the surface for mesh
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generation. Patient-specific blood flow waveforms were calculated from phase contrast MRI
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images for all vessels of interest using previously validated methods.22,23
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Computational Fluid Dynamics
All geometries were imported into GAMBIT (ANSYS Inc., Lebanon, NH) for mesh
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generation. To provide computational stability, flow extensions of 10mm and 50mm were
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added to all inlets and outlets, respectively. All computational fluid dynamics (CFD) simulations
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were completed using a previously developed and validated in-house immersed boundary
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solver.4,5,14,24 The unsteady flow waveforms extracted from PC-MRI were imposed as boundary
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conditions for each inlet and outlet. A total of 6 cardiac cycles were simulated for each patient
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to overcome transition effects and achieve periodic stability; the final cycle was used for
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hemodynamic analysis.
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Data Analysis Calculations were performed to compute power loss (PL), indexed power loss (iPL), TCPC resistance, pulsatility index (PI) and percent stenosis. Power loss was calculated as follows:
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where p is the static pressure, ρ is the blood density, A is the area of the inlet/outlet and v the
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velocity. Power loss was calculated for 200 points throughout the cardiac cycle and then
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averaged to obtain an overall PL value. Power loss was indexed (iPL) as (
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resistance was calculated
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surface area (m2), ∆ is the pressure drop and ρ is the blood density (kg·m-3).
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Pulsatility index was defined as follows:
/
) and a TCPC
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where Qs is the systemic venous flow (L·s-1), BSA is the body
2 ∗ !"
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where Qmax, Qmin, and Qavg are the maximum, minimum and average flow rates during the
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cardiac cycle, respectively, as determined from the phase contrast CMR data. Percent stenosis
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was calculated for the pulmonary arteries using the following equation:
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. , 0 . ,1
2 .!", 0 .!",1
where . , and . ,1 are the minimum cross sectional areas of the LPA and RPA, and
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.!", and .!",1 are the average cross sectional areas of the LPA and RPA. A combined
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outlet stenosis value was used in place of unilateral PA stenosis measurements to give a more
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accurate representation of the total outlet obstruction to flow and allow a better comparison
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between patients. Additionally, IVCs and SVCs showed negligible percent stenosis and therefore 8
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were not included in this calculation. All vessel areas for this metric were calculated using
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Vascular Modeling Toolkit version 1.0.1 (Orobix, Bergamo, Italy). Hepatic flow distribution (HFD) was evaluated using particle tracking. Particles were
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seeded at the base of the LT/ECC or Y-graft at 200 time points throughout the cardiac cycle. An
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additional four cardiac cycles (with no particle seeding) were simulated to ensure that the
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residence time of the particles would not exceed the simulation time. Particles exiting at each
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outlet were counted and HFD was defined as the ratio of particles leaving the LPA: 67$)89%(
67$)89%( 0 67$)89%(1
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HFD will be discussed in terms of “HFD deviation from 50%”, and an even (50/50) split of
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hepatic flow to the LPA and RPA is assumed as ideal. This convention gives a “0” HFD deviation
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for balanced HFD cases, and an HFD deviation of 50 for extreme cases (all hepatic flow to either
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the LPA or RPA). Particle residence time was calculated by recording the amount of time
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required for each particle to exit the TCPC domain. An overall pulmonary flow distribution (PFD)
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was calculated as the ratio of LPA flow to total pulmonary flow:
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:;< =<: >?
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Statistics
The Anderson-Darling test was used to determine normality for each parameter within
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both patient groups. Depending on normality, either a Wilcoxon rank sum test (non-normal
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distribution) or a two-sample t-test (normal distribution) was used to test for equal medians
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between patient groups using SPSS (IBM Corp., Version 23, Armonk, NY). An asterisk (*) is used
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to signify statistical significance using α=0.05. Values are shown as mean± standard deviation.
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Results
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The commercially available Y-graft evaluation includes an investigation of several
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hemodynamic metrics within the Y-graft cohort, then an examination of both ideally and poorly
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performing Y-grafts (in terms of HFD), and finally, a comparison between Y-graft and LT/ECC
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performance.
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Y-Graft Hemodynamic Performance
Figure 2 shows several clinically important hemodynamic metrics for the Y-graft cohort.
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The average PFD and HFD deviations from an ideal 50/50 split were 13.4±10.6 and 25.9±15.1,
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respectively (Fig. 2a). Fig. 2b illustrates the effect of LPA stenosis on indexed power loss. A
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moderate positive correlation (r=0.67, p<0.01) was found between LPA stenosis and indexed
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power loss. The distributions of TCPC resistance and HFD deviation are shown in Fig. 2c and 2d.
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The distribution of TCPC resistance is skewed to the right with approximately 53% and 70% of Y-
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graft patients having resistances less than 1 and 2 Wood Units respectively. HFD deviation was
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relatively evenly distributed (Fig. 2d). Only 13% of Y-graft patients have HFD deviations less
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than 10 (ideal) and 24% of Y-graft patients have deviations greater than 40 (poor).
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Representative Y-Graft Cases
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Figure 3 shows streamlines for six representative cases from the 30 patient Y-graft cohort. Patients Y1, Y2, andY3 show balanced HFD (low HFD deviation) while the final three (Y4,
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Y5, andY6) are unbalanced. Y1 represents an “ideal” case with balanced PFD, normal SVC
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positioning and no mid-PA stenosis. Y2 represents uneven PFD with mid-PA stenosis on the low
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PFD side. Y3 has bilateral SVCs, balanced PFD and mid-PA stenosis. Y4 has balanced PFD with
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the SVC positioned superior to a y-branch anastomosis. Y5 has unbalanced PFD with mid-PA
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stenosis on the high PFD side and Y6 represents balanced PFD with y-branch stenosis. HFD to
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the LPA is noted for each model in Figure 3.
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Y-graft vs LT/ECC
To further evaluate the commercially available Y-graft, a direct hemodynamic comparison was performed between the Y-graft and LT/ECC cohorts. Clinical data are shown in
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Table 1 for each graft type. Patients were selected to best match age and BSA between groups.
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Y-graft resistance was significantly higher (p=0.03) than LT/ECC resistance, with averages of
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1.51±1.52 and 0.56±0.54 WU, respectively (Fig. 4a). Overall, HFD deviation was not significantly
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different between the groups with an average of 21±13 for the LT/ECC and 26±15 for Y-grafts
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(Fig. 4b). Figure 4c shows no significant differences in particle residence time.
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Figure 4d and 4e compare the distribution of TCPC resistance and HFD deviation
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between graft types. Almost 77% of LT/ECC patients have a resistance less than 1 WU,
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compared to 53% of the Y-grafts. Additionally, 30% of commercially available Y-graft patients
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exhibit a resistance greater than 2 WU, while only 3% of the LT/ECC cohort have values this high
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(Fig. 4d). Focusing on the two extremes of HFD deviation, 30% of LT/ECC patients had ideal HFD
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deviations (<10) compared to only 10% of Y-graft patients. At the other extreme, more than
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twice as many Y-graft patients (23%) have HFD deviations greater than 40 (poor) than LT/ECCs
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(10%).
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Figure 4f compares the sensitivity of iPL to LPA stenosis for both graft types. Commercially
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available Y-grafts were found to be more sensitive to LPA stenosis in terms of iPL with a
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significant difference in the two correlations (Fisher’s exact test, one-tailed, z=-1.76, p=0.04).
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Discussion
Capitalizing on the relatively large sample size in this study, the following section evaluates commercially available Y-graft performance and generalizes differences between
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graft types. Both clinical and physiological implications are discussed. Six Y-grafts that illustrate
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common anatomical parameter combinations will be emphasized and study limitations will be
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considered.
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Y-Graft Hemodynamic Performance
As expected, PFD deviation is relatively low and on average there is about a 25%
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difference in overall flow rates to the LPA and RPA. This flow split is determined partially by
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branch pulmonary artery stenosis.25 HFD deviation was higher and more variable than
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expected and will be discussed in the following section. The effect of LPA stenosis on iPL is also
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an expected result. Blood accelerates through the stenosis region and then decelerates as the
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vessel returns to its original size according to the conservation of momentum principle. These
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changes in velocity result in an increased pressure drop across the TCPC (lower efficiency)
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which leads to an increased iPL. The LPA is a common site for stenosis and Fig. 2b shows that
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the most severe LPA stenosis anatomies have the highest iPL.10
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The distribution of TCPC resistance raises concern for a number of Y-graft patients. In this study, Y-graft TCPCs are seen to have resistances as high as 4.7 Wood units, and this
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increased level of vascular resistance could lead to venous hypertension/congestion. Liver
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failure and protein-losing enteropathy are thought to be direct results of increased Fontan
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venous pressures.26–28 The relatively even distribution of HFD deviation suggests that the Y-
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graft is not a “one size fits all” solution to accomplish evenly split hepatic flow to the lungs.
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While 13% of Y-graft patients had very balanced HFD, nearly a quarter of patients had >90%
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hepatic flow directed to either the left or right lung. The effectiveness of Y-graft use seems to
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be multi-factorial and extremely patient specific, suggesting the need for pre-operative,
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individual surgical planning via computational modeling.
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The ability of a Y-graft to evenly split hepatic flow is highly dependent on PFD, the position of branch PA stenosis and SVC flow collision, among other factors. A difference in PFD
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can cause two Y-grafts with very similar anatomies to have drastically different HFDs. This can
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be seen in Fig. 3, where Y2 and Y5 have PFDs of 24% and 65%, respectively. Despite similar
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anatomies and Y-grafts, the HFD for these cases are considerably different (48% and 81% to the
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LPA, respectively). A comparison between Y1 and Y4 illustrates the effect of SVC flow collision
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on HFD. Both cases have balanced PFD and similar sized PAs, but Y1 has a medially positioned
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SVC, while Y4’s SVC is positioned more superior to the left Y-branch. The SVC flow collision in Y4
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forces some SVC flow into the left Y-branch which causes hepatic flow to preferentially course
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through the right Y-branch. The result is a very unbalanced HFD for Y4 (12% to the LPA) and
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relatively balanced HFD for Y1 (65% to the LPA).
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Stenosis is commonly seen in Fontan anatomies;10 Y2, Y3, Y5 and Y6 all have severe stenosis in the TCPC. For single SVC anatomies (Y2, Y5), mid-PA stenosis influences SVC flow
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which will preferentially follow the path of least resistance (in these cases towards the RPA).
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For an unbalanced PFD case, if the mid-PA stenosis is located on the same side as the PA
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receiving the majority of pulmonary flow, then the stenosis blocks most SVC flow and this PA
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receives the majority of its flow from the hepatics and will therefore have an unbalanced HFD
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(Y5). However, the opposite is true if the stenosis is located on the low PFD side; in this case
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hepatic flow is able to move towards both PAs resulting in more balanced HFD (Y2). For
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bilateral SVC anatomies (Y3), mid-PA stenosis can “separate” the two SVC flows and HFD will be
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highly dependent on the individual SVC and overall PA flows. If SVC flows are similar and PFD is
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balanced, a balanced HFD is expected. Y6 shows a case with severe stenosis located in the left
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Y-branch, and as a result hepatic flow is very limited through the stenosed branch. With the full
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range of flow rates, flow ratios, pulsatilities, unique geometries and stenosis locations, the
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ability of a specific Y-graft to achieve balanced HFD may not be intuitive. Again, Y-graft
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performance needs to be evaluated on an individual basis and all of these factors must be
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considered.
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Y-graft vs LT/ECC
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Average TCPC resistance was approximately 3x higher for Y-grafts than LT/ECCs. The most apparent explanation stems from the geometry of the Y-graft itself. With commercially
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available Y-grafts, the diameter of the base of the Y-graft is evenly split between the two
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branches. For example, a 20mm (diameter) Y-graft will have two 10mm (diameter) branches. A
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basic calculation reveals that preserving diameter does not preserve area; this design results in
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a 50% reduction in cross-sectional area as blood moves into the Y-graft branches. In essence, Y-
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grafts have a built-in 50% long segment “stenosis”. Additionally, resistance is inversely
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proportional to the fourth power of the radius (Poiseuille’s law). Therefore, blood traveling
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through 2 Y-branches with diameters half the size of the LT/ECC would analytically yield an 8x
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increase in resistance. According to the results presented here, the reduction in diameter and
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cross sectional area inherent to commercially available Y-grafts has a greater negative impact
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on TCPC resistance than the venous flow collisions inherent to LT/ECCs. Therefore, “custom” Y-
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grafts which preserve cross sectional area as shown by Yang et al. may be the more promising
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option for successful Y-graft use, and may potentially provide an advantage over the LT/ECC
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Fontan pathway.15,18
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Overall, HFD deviation was not significantly different between graft types. Both graft types were highly variable. The most important observation is the discrepancy in the number of
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patients at both the lowest (ideal) and highest (poor) HFD deviation (Fig. 4e). In this study, Y-
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grafts have almost twice as many poor differential cases as ideal cases, while LT/ECCs show the
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opposite trend. These results highlight the need for pre-operative planning with Y-graft use as
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well as the design of individualized Y-grafts which are tailored for a specific patient. Potential
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modifications such as the angle of origin, branch size and overall shape could lead to better HFD
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performance on an individual basis. Particle residence time (PRT) was investigated for its potential impact on flow-induced
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thrombus formation, as well as to provide quantitative information about flow stagnation.29,30
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No differences were seen for minimum, mean or maximum residence times. As seen in Fig. 4f,
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Y-graft iPL was more sensitive to LPA stenosis than LT/ECC iPL. PA stenosis may have a greater
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influence on the reduced flow through the closest Y-branch than it does on the total hepatic
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flow traveling through a LT/ECC connection. This difference would lead to a greater effect of
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stenosis on iPL for Y-grafts than LT/ECCs.
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Several additional comparisons between graft types are important to note. First, PFD
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was identical between graft types at 46% to the left lung, indicating that graft type does not
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affect PFD. Additionally, there was significantly more PA stenosis (p=0.02) in the LT/ECC cohort
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as seen in Table 1. Even with significantly more PA stenosis, which is known to increase
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resistance, the LT/ECC cohort still had lower TCPC resistance than the Y-grafts. This strengthens
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the finding that commercially available Y-grafts inherently create higher TCPC resistance.
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Finally, the LT/ECC group includes both intra-atrial and extracardiac connections, while Y-grafts
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are always extracardiac. In this study, 10 of the LT/ECCs were intra-atrial. To ensure that these
328
results were a consequence of graft type and not the intra-atrial vs. extracardiac variation, a
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sub-comparison was performed between these two groups. No statistically significant
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differences were found in this sub-comparison for iPL, PFD, or HFD, and therefore the
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comparative results presented here are due to graft choice and not its location.
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Limitations
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The period of time between the Fontan operation and the MRI scan was not identical for all
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patients included in this study. Y-graft patients were scanned within one month of the surgery,
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while LT/ECC patients were scanned around 6 months after the operation. This difference in
337
recovery time may lead to variations in the amount of physiological adaptation which could
338
affect flow rates (as seen in the difference between systemic venous flows) and pulmonary
339
vascular resistance and therefore TCPC resistance and HFD. Additionally, the spatial velocity
340
profiles used at the inlets of the CFD domain are assumed to be parabolic, which are not
341
identical to the spatial velocity profiles obtained from the MRI velocity segmentations.
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However, this assumption is expected to have only minimal effects on the bulk hemodynamic
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metrics. From a modeling perspective, this study focused only on the TCPC geometry and did
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not include interactions with the rest of the cardiovascular or pulmonary circulation. This
345
localized TCPC modeling may omit significant global influences on the system. Finally, due to
346
limited data, the hepatic veins were not included in this modeling. The inclusion of the hepatic
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veins and their respective flow rates could potentially influence the mixing of hepatic factor
348
with the IVC flow and therefore alter the presented HFD results. This study also assumes rigid
349
walls in the CFD simulations.
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Conclusion
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In this study, commercially available Y-grafts were found to have highly variable HFD due to a combination of factors including pulmonary flow distribution to the left and right
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lungs, presence and location of stenosis, and SVC positioning. HFD distributions suggest that
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LT/ECCs are more likely to avoid extremely unbalanced hepatic flow to the lungs. Y-graft HFD
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performance is multi-factorial, which requires patient specific computational modeling. Y-grafts
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were found to be more sensitive in terms of indexed power loss to LPA stenosis. Overall, Y-
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grafts show significantly higher resistance than LT/ECCs due in part to the intrinsic graft
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geometry, which calls for a better Y-graft design.
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Figure Legends
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Figure 1: (a) TCPC with an extracardiac baffle. (b) TCPC with a Y-Graft. (SVC: superior vena cava.
464
LPA: left pulmonary artery. RPA: right pulmonary artery.)
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Figure 2: Y-Graft hemodynamic performance. (a) Pulmonary and hepatic flow distribution (PFD
467
and HFD) deviations. (b) Effect of LPA stenosis on index power loss (iPL). (c) Distribution of TCPC
468
resistance. (d) Distribution of HFD deviation.
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Figure 3: Representative Y-Graft cases with HFD noted. Streamlines colored by origin: IVC(blue),
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SVC(green) and LSVC(red). Percentages shown are percent of hepatic flow to the left pulmonary
472
artery.
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Figure 4: Hemodynamic Comparison of the Y-graft and LT/ECC. (a) Indexed power loss. (b) HFD
475
deviation from 50%. (c) Particle residence time. (d) Distribution of iPL. (e) Distribution of HFD
476
deviation. (f) Effect of LPA stenosis on iPL.
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Central Picture: Fontan Y-graft hepatic flow distribution; streamlines colored by inlet vessel.
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0.67 ± 0.12
0.66 ± 0.20
0.19
Age (years)
3.65 ± 1.14
3.68 ± 2.49
0.17
70%
PA Stenosis (%)*
33.99 ± 12.60
Systemic Venous Flow (L/min)* Pulmonary Flow Distribution (% LPA)
Pulsatility Index (SVC)
492
0.19
26.82 ± 10.69
0.02
1.57 ± 0.59
<0.01
46.01 ± 15.00
46.00 ± 16.78
0.99
13.13 ± 7.91
13.40 ± 10.60
0.78
0.66 ± 0.65
0.93 ± 0.72
0.14
0.42 ± 0.12
0.76
0.51 ± 0.50
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BSA: body surface area; PA: pulmonary artery; PFD: pulmonary flow distribution. Systemic venous return is defined as the sum of all TCPC inlet flow rates. Pulmonary flow distribution is the percent of total inlet flow that exits through the left pulmonary artery. An “*” represents statistically significant differences.
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53%
2.07 ± 0.61
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p-value
BSA (m2)
Gender (male)
485 486 487 488 489
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Table 1: Averaged clinical data for all patients separated by graft type. LT/ECC Y-Graft (n=30) (n=30)
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Table 1:
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