POLITECNICO DI MILANO
Dottorato di Ricerca in Bioingegneria
Ph.D. Degree in Bioengineering
Computational models of the
hemodynamics in stage 1 palliations for the
treatment of single ventricle heart diseases
Chiara Corsini
Relatore: Prof. Giancarlo Pennati
Tutor: Prof. Gabriele Dubini
Coordinatore: Prof. Maria Gabriella Signorini
XXV Ciclo
2010-2012
Acknowledgements
I would like to acknowledge my advisor, Professor Giancarlo Pennati, for unfailingly guiding my work in the last three years. His technical and scientific expertise was precious to me as I learned so much from his ever questioning approach to scientific problems.
My PhD work would not have been possible without the constant support and collaboration offered by Professor Francesco Migliavacca at the Laboratory of Biological Structure Mechanics, where I spent my PhD. I gratefully acknowledge also Professor Gabriele Dubini for his wise and meaningful suggestions during these years.
All of them have encouraged me to believe in myself and taught me, at the same time, how to make a research collaboration fruitful and effective.
I am grateful to Fondation Leducq, Paris, for granting this work within the Transatlantic Networks of Excellence in Cardiovascular Research Program. The collaborative research risen from this project was very challenging but exciting at the same time. In particular, I would like to thank Dr. Hsia and Daria Cosentino, for launching and cooperating in the first part of the work; Giovanni Biglino and Silvia Schievano for their prompt contribution to the second and third parts of the research work.
Finally, special and sincere acknowledgements are directed to my gorgeous family, Andrea and my friends for supporting and advising me all along these years, and to LaBS folks, who shared with me the crucial aspects of this experience, from both emotional and professional points of view. I really enjoyed working with all of you!
Contents
Nomenclature
Extended abstract ... i
Preface ... 1
Chapter 1
Stage 1 palliations for single ventricle heart diseases: surgical and
modeling remarks
1.1
Introduction ... 4
1.2
Surgical stage 1 procedures ... 6
1.3
Hybrid stage 1 procedure ... 8
1.4
Aortic coarctation ... 10
1.5
Mathematical modeling approaches ... 12
1.6
State-of-the-art on stage 1 palliation modeling ... 16
1.7
State-of-the-art on aortic coarctation modeling ... 20
1.8
Limitations and future advances ... 22
Chapter 2
Multidomain models of
different stage 1 configurations
2.1
Introduction ... 26
2.2
Multidomain models of the hybrid Norwood circulation:
preliminary results ... 27
2.3
Effects of pulmonary artery banding and retrograde aortic arch
obstruction on cerebral and coronary perfusion ... 35
2.4
Hybrid approach versus surgical procedure ... 43
Chapter 3
Mathematical models of stage 1 circulation with aortic coarctation23
3.1
Introduction ... 52
3.2
Multidomain model ... 53
3.2.1 Stand-alone 3D model ... 56
3.2.2 In silico reproduction of the in vitro model ... 58
3.2.3 Characterization of the in vitro manifold and implementation in the computational model ... 63
3.2.4 Simulation of more realistic conditions ... 71
3.3
Modeling for clinical use: lumped parameter approach ... 73
3.3.1 Characterization of the coarctation and the shunt ... 74
3.3.2 Validation with severe coarctation and without coarctation ... 79
3.4
Conclusions ... 83
Chapter 4
Patient-specific numerical models of the systemic-to-pulmonary shunt
4.1
Introduction ... 86
4.2
Methodological approach ... 87
4.2.1 Mesh sensitivity analysis ... 89
4.2.2 Sensitivity analysis of the flow regime model ... 95
4.2.3 Sensitivity analysis of the boundary conditions ... 96
4.3
Patient-specific modeling to evaluate clinical data consistency 99
4.4
Analysis of local hemodynamics in terms of hydraulic behavior
of the shunt ... 103
4.5
Conclusions ... 111
Conclusion ... 113
Nomenclature
Nomenclature
3D three-dimensional
∆Pm pressure drop across the manifold
AoA ascending aorta
BA balloon angioplasty
BSA body surface area
C compliance
CHb hemoglobin content
CI coarctation index
CO cardiac output
Co Courant number
CoA coarctation of the aorta CO2D cerebral oxygen delivery
CFD computational fluid dynamics
CS central shunt
DA ductus arteriosus
DAo descending aorta
dAoA distal ascending aorta dIA distal innominate artery FSI fluid-structure interaction HLHS hypoplastic left heart syndrome
Nomenclature
HR heart rate
IA innominate artery
IVC inferior vena cava
LCA left carotid artery
LES large eddy simulation
LPA left pulmonary artery
LPM lumped parameter model
lRPA lower right pulmonary artery LSA left subclavian artery
mBTS modified Blalock-Taussig shunt MRA magnetic resonance angiography
MRI magnetic resonance imaging
NAO neoaortic arch obstruction ODE ordinary differential equations
O2C oxygen consumption
PA pulmonary artery
PAB pulmonary artery banding
pAoA proximal ascending aorta pIA proximal innominate artery
P-V pressure-volume
PVR pulmonary vascular resistance PVWP pulmonary venous wedge pressure
Nomenclature
Qp pulmonary blood flow
Qs systemic blood flow
Qp/Qs pulmonary-systemic flow ratio
R resistance
RAAO retrograde aortic arch obstruction RCA right carotid artery
Rep peak Reynolds number
RPA right pulmonary artery
RSA right subclavian artery
RVPAS right ventricle-pulmonary artery shunt SatART arterial oxygen saturation
SatPV pulmonary vein saturation SatVEN mixed venous oxygen saturation shunt systemic-to-pulmonary shunt SO2D systemic arterial oxygen delivery
SVC superior vena cava
SVR systemic vascular resistance uRPA upper right pulmonary artery
Wo Womersley number
Extended abstract
Single ventricle heart diseases encompass a wide range of abnormalities in cardiac anatomy leading to only one fully developed, functioning ventricle, which supports both systemic and pulmonary circulations. They represent about 1.4% of congenital cardiovascular defects, and, if left untreated, are certainly fatal.
The most common form of single ventricle malformation is the hypoplastic left heart syndrome (HLHS), mainly characterized by underdeveloped left ventricle and ascending aorta (AoA). The fetus is generally unaffected by the anatomical abnormality, but the natural decrease in pulmonary vascular resistance (PVR) shortly after birth results in a volume shift from the systemic to the pulmonary circulation, and, as the ductus arteriosus (DA) closes, systemic perfusion is further impaired.
In the last decades, various surgical procedures have been developed to treat infants with such complex cardiovascular defects. The ultimate goal of surgeries is to bypass the dysfunctional, or missing, ventricle by connecting the systemic venous return to the pulmonary arterial system, thus creating a circulation in series powered by a single ventricle. However, at birth the lungs are immature and the PVR is naturally high, precluding a series circulation in the neonatal period. Thus, multi-staged management is necessary. Stage 1 surgery, also called ‘Norwood procedure’, occurs as a neonate in order to provide an unrestrictive systemic outflow tract with a balanced pulmonary flow (Qp). This is usually allowed by reconstructing the AoA and placing a systemic-to-pulmonary shunt (shunt). In the first few months, the PVR decreases, thus accomplishing physiological conditions for stage 2 surgery. At about 6 months, the shunt is removed and the superior vena cava is attached to the pulmonary arterial vasculature to provide Qp. During the first 3/4 years of life the proportion of systemic venous return from the lower body progressively increases; therefore, at stage 3, also the inferior vena cava flow is rerouted to the pulmonary vasculature.
There is wide variation in anatomy and physiology between individuals requiring single ventricle surgery, with several alternative procedures at each stage. The resulting different local hemodynamics may differently affect global parameters such as flow distribution and systemic oxygen delivery (SO2D). As these effects are difficult to predict clinically, mathematical and computational models can help exploring various anatomical configurations.
The present Ph.D. dissertation will focus on stage 1 palliation, and will be divided in four chapters whose contents are resumed below.
Chapter 1 – Stage 1 palliations: surgical and modeling remarks
This Chapter presents different stage 1 procedures from the surgical point of view, and analyzes the state of the art of the modeling approaches.
In newborns with a univentricular circulation, a shunt is implanted to connect the systemic arterial circulation with the pulmonary arteries (PAs), creating a
Extended abstract
complex, non-physiologic parallel circuit, with the single ventricle pumping blood to both the systemic and pulmonary circulations. Furthermore, aortic arch reconstruction is performed to guarantee unobstructed flow from the functioning ventricle.
The features of the optimal shunt are listed below:
• it should be technically simple to implant and easily removable from the circulation when cavopulmonary connection is performed;
• it should provide even distribution of both cardiac output (CO) to the pulmonary and systemic circulations, and Qp to the right and left lungs; • in particular, if properly sized, it should allow to regulate Qp for maturation of
the pulmonary vasculature, in order to avoid PA distortion or stenosis, thus facilitating stage 2 procedure;
• it should allow proper SO2D and coronary perfusion to minimize ventricular volume overload;
• it should avoid pulmonary hypertension and maintain intermediate-term patency.
After the introduction of the first shunt in 1945, various modifications of the original procedure have been developed, connecting different vessels in different ways. Conventional surgical variants include the modified Blalock-Taussig shunt (mBTS), the central shunt (CS) and the Sano modification. The mBTS is a Gore-Tex conduit joining the innominate artery (IA) to the RPA, while the CS connects the AoA to the main PA. The last one is a more recent modification interposing a conduit between the right ventricle and a PA, known as ‘right ventricle-pulmonary artery shunt’ (RVPAS). This was introduced to avoid the diastolic run-off related to the previous techniques. In such circulations, indeed, Qp is forward throughout the cardiac cycle, potentially leading to pulmonary overcirculation and decreased coronary blood flow. With a RVPAS, instead, Qp is backward during diastole, thus facilitating coronary perfusion. Nevertheless, controversy remains about the potential salutary effects of the RVPAS, as the drawbacks of performing an incision in the systemic right ventricle might not be apparent for many years.
A more recent approach, also known as ‘hybrid Norwood’ (HN), to the management of HLHS has been developed as an alternative strategy to the surgical stage 1 palliations. It combines a surgical technique, i.e. branch pulmonary artery banding (PAB), and interventional cardiology techniques, i.e. DA stenting and balloon atrial septostomy. The circulatory arrangement achieved with the hybrid approach is different from that of the surgical palliation: besides the distinct way to regulate Qp (PAB vs. shunt), in the HN circulation the ventricular outflow tract is not reconstructed. Therefore, in case of concomitant aortic atresia or severe aortic stenosis, systemic and cerebral perfusion would rely on retrograde flow through the aortic arch.
Encouraged by improved techniques and increasing experience to overcome the early ‘learning curve’, some institutions have adopted the hybrid approach as the definitive strategy for stage 1 palliation of HLHS. Nevertheless, conflicting opinions about whether the HN provides major benefits are still present.
Extended abstract
HLHS patients are often affected by a concomitant disease, called coarctation of the aorta (CoA), which is a discrete narrowing of the proximal descending aorta (DAo) at the juxtaductal (i.e. adjacent to the DA) region. CoA can occur both in newborns and in the intervening years between the staged surgeries. Although various approaches (e.g. percutaneous balloon angioplasty, coarctectomy) have been introduced to repair this defect, an optimal treatment for CoA, able to free from re-obstruction of the aortic arch, does not exist.
Several factors, including geometrical and patient related characteristics, influence flow distribution and pressures in stage 1 circulation. Therefore, a thorough understanding of the hemodynamics of the shunt, of the HN and of the CoA, by means of mathematical modeling approaches, can assist the surgeons in pre-operative and post-operative management.
There are various mathematical methods available in the engineering field to model cardiovascular hemodynamics. They include Computational Fluid Dynamics (CFD) techniques applied to both three-dimensional (3D) and zero-dimensional models, also known as lumped parameter models (LPMs). 3D modeling enable a local representation of the hemodynamic field in specific portions of the cardiovascular system. LPMs, instead, can describe the relative effects on the global circulation. They are electric analogues composed of i) resistors, ii) inductors and iii) capacitors, representing the i) viscous and ii) inertial properties of blood, and iii) the elastic behavior of the vessel walls. With LPMs it is also possible to reproduce heart contraction and valves unidirectional flow. These models are particularly useful in simulating both the global PVR and systemic vascular resistance (SVR), and the local arterial, capillary and venous portions of the circulation, as well as localized pressure drops, e.g. due to a CoA or a shunt.
Another approach to evaluate the local and global hemodynamics, with their mutual interactions, is the so-called multiscale or multidomain method, coupling a 3D domain with a LPM. In this case, defining proper interface conditions is fundamental, as the different dimensions of the coupled models lead to distinct mathematical descriptions.
The literature on shunt modeling is very limited, if compared to that on modeling of the other two stages, due to the fact that geometry of shunt and surroundings is very individual, and that fluid dynamics in the shunt are complex, with significant pressure gradients and velocities. Moreover, accurate clinical data in the post-operative setting are hardly available to build and validate the model, owing to the very young age of the patients. Last but not least, for an effective analysis of shunt performance modeling should include biological phenomena, e.g. progressive shunt narrowing, growth and distortion of the adjacent vessels. However, a number of in vitro and in silico studies concerning shunts for univentricular circulation can be found in the literature. Some of them investigated geometrical features, such as shunt diameter and the angle of the anastomoses, using either 3D models or LPMs. Later on, multidomain models were developed both experimentally and computationally. The latter were used to compare different surgical stage 1 palliations in terms of ventricular performance,
Extended abstract
pulmonary and coronary perfusions, or to optimize shunt geometry maximizing physiological parameters.
Concerning CoA modeling, in vitro and in silico studies are dated more recently if compared with shunt modeling. On the other hand, several patient-specific models were developed, even adopting a multidomain approach, or incorporating vessel wall deformation.
Chapter 2 – Multidomain models of different stage 1 configurations
In this Chapter, the multiscale modeling approach is adopted to investigate the effects of bilateral PAB, ductal stenting and retrograde aortic arch obstruction (RAAO) on the local and global hemodynamics of the hybrid stage 1 procedure. The ultimate goal is to provide surgeons with pre-operative guidance for clinical decision support, by shedding light on the circulation following a hybrid palliation, compared with conventional surgical procedures.
Due to the low PVR/SVR ratio during the first few months of life, bilateral PA bands need to be sufficiently tight to equalize the two resistances, thus achieving a balanced pulmonary-to-systemic flow ratio (Qp/Qs), without resulting in suboptimal oxygen availability.
The multiscale models built in this study combined a detailed, though idealized, 3D domain of the cardiovascular portion undergoing HN palliation, with a detailed LPM of the remaining circulatory system. This configuration was described by a closed loop to avoid arbitrary forcing of boundary conditions, and allow the two domains to interact with each other. Coupling was accomplished by uniformly applying time-dependent pressures, yielded by the LPM, to the boundary sections of the 3D domain, while passing flow rates across the inlet and outlets, computed in the 3D domain at each time step, as the forcing terms for the LPM. A wall boundary condition was applied to the aortic inlet section to replicate aortic valve atresia, which is often associated with aortic hypoplasia.
The 3D domains were based on previous models of mBTS and RVPAS. In addition, the AoA was reduced in diameter to represent congenital hypoplasia, the main PA was included in the models as the outflow tract of the single ventricle, the LPA and RPA were narrowed at their mid-sections, and the DA was included as a patent vessel. To investigate the influence of different PAB, five models were built with PAB diameter varying from 1.5 mm to 3.5 mm (step 0.5 mm). Another model with 3.5 mm PAB had a DA diameter increased from 7 to 8 mm, to analyze the effect of stent expansion.
The LPM represented a generic univentricular newborn and consisted of five blocks (i.e. heart, upper and lower systemic circulation, pulmonary circulation and coronary circulation) similar to previously developed models. However, connections with the 3D domain were different to account for the specific HN arrangement.
Pulsatile simulations were run using the CFD finite volume code ANSYS Fluent (ANSYS Inc., Canonsburg, PA, USA), and time-averaged values of Qp/Qs, CO and SO2D were calculated for each HN model.
Extended abstract
Simulation results highlighted the importance of the degree of PAB in determining the global hemodynamic performance of the hybrid procedure. The model with 2 mm PAB achieved maximal SO2D and provided the best Qp/Qs, among those obtained with the PAB sizes modeled in the study. On the contrary, changes in DA diameter did not result in substantial differences (lower than 0.5%) in the investigated quantities. This observation can be explained by the balance between PVR and SVR, existing in the parallel configuration following the HN. While varying PAB leads to significant changes to the relatively low PVR, ductal stent diameter has little impact on the SVR.
Remarkable hemodynamic differences between the models with 1.5 and 3.5 mm PAB were observed in diastole. With tighter PAB, diastolic runoff was primarily directed to the DAo from the upper aortic branches, bypassing the PAs; whereas looser banding led to increased diastolic flow into the pulmonary rather than the systemic circulation, thus augmenting Qp.
These findings have potential clinical ramifications. First, the current practice to oversize the ductal stent to achieve maximal systemic flow (Qs) into the aorta is unnecessary. Secondly, the choice of PAB tightness, presently based on anecdotal experience, may be guided by the modeling results in order to achieve optimal clinical outcome.
Based on the above preliminary findings, the best performing HN model (2 mm PAB) was further used to study the influence of RAAO, due to aortic arch hypoplasia or isthmus coarctation, on the necessarily retrograde cerebral and coronary perfusions. Multiscale coupling described earlier was adopted, using the same LPM. The 3D domain was modified by varying, in a first set, the AoA and aortic arch diameters from 5 to 2 mm (step 1 mm), and, in a second set, the isthmus coarctation diameter from 5.0 mm (no coarctation) to 2.5 mm.
Physiological parameters, such as oxygen deliveries, systemic arterial (SatART) and mixed venous (SatVEN) oxygen saturations, as well as ventricular performance parameters, were calculated from the resulting flow rates and pressures.
Simulation results showed that progressively increasing the aortic arch hypoplasia or isthmus coarctation caused a continuous decrease in coronary and cerebral perfusion. However, the detrimental effect was significant when hypoplasia or stenosis was smaller than 3 mm. This suggests that caution should be taken in proceeding with hybrid palliation when pre-operative imaging data reveal severe isthmus narrowing or aortic arch hypoplasia.
The present results do not suffice to predict clinical outcomes or dictate management decisions, but may recommend pre-interventional imaging of the aortic arch and isthmus to exclude potential suboptimal cerebral and coronary perfusion after a HN procedure.
Once the hemodynamic effects of different HN configurations have been explored, the hybrid palliation was compared with the standard surgical procedures. Again, adopting the same multidomain approach and same LPM, results from the HN model with 2 mm PAB were analyzed in comparison with those obtained from previous analogous studies using a model with 3.5 mm mBTS
Extended abstract
and another one with 5 mm RVPAS. These specific 3D domains were selected as they had shown the best hemodynamic and physiological performance with respect to the other, previously simulated, geometries with different PAB or shunt sizes. Dimensions of common vessels were the same for all 3D models.
With 2 mm PAB, PA flow and pressure were higher compared to the values predicted by the other models, resulting in a substantially higher Qp/Qs. However, the CO was lower than that reported by the mBTS and RVPAS models, indicating a poorer systemic perfusion. As a consequence, also coronary and cerebral blood flows through the retrograde aortic arch were lower than those in the surgical models. Furthermore, notwithstanding a higher SatART, the HN model exhibited lower SatVEN. Due to the lower Qs, oxygen deliveries were considerably poorer in the HN model. Similarly, single ventricle performance of both surgical models resulted better than the HN one, in terms of ejection fraction and mechanical efficiency. The poorer ventricular performance in the HN model could be explained by the combination of the obligatory volume load on the single ventricle and the additional afterload brought on by the absence of aortic reconstruction and by PAB.
The present results also predicted diastolic runoff through the DA into the branch PAs of the HN model. Although requiring further clinical investigation, this may be important in determining the clinical outcome of the hybrid strategy in the presence of aortic atresia, since it may cause a reduction of the obligatory retrograde coronary and cerebral perfusion.
Chapter 3 – Mathematical models of stage 1 circulation with aortic
coarctation
This Chapter presents a multiscale model, as well as a pure LPM, of a patient-specific aortic arch with CoA, included in a stage 1 circulation. The multidomain model was based on an analogous in vitro model, previously developed, to enable validation against experimental data. The pure LPM was built using the in silico results of the present study and mathematical models proposed in the literature. The ultimate goal is to show the potential use of the lumped parameter modeling approach within a clinical environment for clinical care of single ventricle palliations.
The patient-specific 3D model of the aortic arch, characterized by a mild coarctation index (CI=0.47), had one inlet, i.e. AoA, and five outlets, i.e. the three brachiocephalic vessels, DAo and mBTS. The model outlets were enlarged to cylindrical shapes to accommodate the circuit pipes, whereas three small ports were placed on the wall (one upstream the CoA and two downstream the CoA) to allow for pressure measurements. During pulsatile flow, pressures were measured by means of a catheter, while flow rates were acquired with ultrasonic probes right downstream the 3D model outlets.
The in vitro 3D phantom was inserted into a mock circulatory system through attachment at a ventricular assist device simulating the single ventricle. Three lumped impedances, made of resistances (R) and compliances (C) were used as
Extended abstract
downstream boundary conditions for the 3D model, namely the upper body (UB – merging the brachiocephalic outlets), lower body (LB – connected to the DAo outlet) and pulmonary (P – connected to the mBTS outlet) circulations. Compliances were Windkessel chambers of adjustable air volume. Resistances were needle-pinch valves with a non-linear behaviour. Circuital pipes were finally connected to an open-air reservoir providing a constant pressure and feeding back to the ventricle.
As a first step, in order to accurately replicate the 3D fluid dynamics simulated
in vitro, the virtual 3D geometry was subjected to sensitivity analyses of the mesh
density and of the numerical flow regime model, since the shunt and the CoA were characterized by likely transition to turbulence. Four flow regime models (i.e. laminar model, and k-ε, k-ω and Large Eddy Simulation –LES– turbulence
models) were tested using the CFD commercial code ANSYS Fluent, and the respective pressure drops were compared with the experimental data. Three of them produced very similar findings, whereas k-ε model was more inaccurate. The LES approach seemed slightly better than traditional turbulence models, but had a much greater computational cost.
Therefore, k-ω and laminar models were also used to simulate a multiscale
model which essentially reproduced the experimental setup. The inflow boundary condition was assigned, with a flat profile, as the Fourier series of the velocity waveform derived from the in vitro inflow. The same multiscale coupling approach, as that described in Chapter 2, was adopted. Overall, the two flow regime models performed the same, if compared with the experimental data. This suggests that, although transition to turbulence could affect hemodynamics of the shunt and CoA, such a condition did not influence flow regime in the other regions of the 3D model. However, flows and pressures predicted by both models presented a mismatch with the experimental signals, although capturing their main features.
Given the similar performance exhibited by multidomain models under different flow regime assumptions, the cause of disagreement was sought in other factors associated with numerical 3D modelling, e.g. fluid viscosity. A multiscale simulation with a lower viscosity assumption was carried out to account for possible uncertainties related to temperature variations during the experiments. Results demonstrated that even large (25%) variations of viscosity did not significantly affect the hemodynamics of the multiscale model. This suggested that working fluid viscosity would not play a major role in the accuracy of the outcomes, as the main factors responsible for energy dissipations in the investigated anatomy were the localized resistances represented by the CoA and the shunt. Furthermore, non-linear components, which are barely sensitive to viscous changes, were used as the physical elements representing peripheral vascular resistances.
Factors associated with the LPM were considered as the possible cause of incongruities with the experimental data. First, experimental pressures acting on compliances were derived through pure lumped parameter modelling, by imposing
Extended abstract
the in vitro flow rates across the UB, LB and P impedances. From comparison with pressures measured in the 3D model, discrepancies were observed only in the UB signal, suggesting that the cause should be sought in the portion of mock circuit going from the UB outlets to UB compliance. Namely, a manifold merging perpendicularly the brachiocephalic vessels, right after the three outlets, was deemed to add non-negligible energy dissipations to the 3D model contribution.
Hydraulic characterization of the in vitro manifold was performed using the in
silico results obtained from multiscale and lumped parameter modelling. Then, a
multiscale simulation was carried out including the non-linear parameter representing the manifold. The new results showed improved agreement with the experimental data, and a peculiar flow distribution in the UB branches when total flow entering the UB impedance was approaching zero: indeed, backward flow crossing the innominate branch and heading to the shunt originated from the other UB branches, passing through the manifold.
To get rid of this effect and get closer to in vivo conditions (i.e. IA flow cannot originate from the other brachiocephalic arteries), a further multiscale simulation was performed. The downstream UB impedance was split in three R-C blocks, each connected to one UB outlet, and all downstream resistances were assumed to be constant with flow rate, in order to mimic the in vivo peripheral circulation. Results demonstrated that, while allowing for compactness and flexibility of the mock circulatory system, the in vitro use of non-linear resistances (i.e. needle-pinch valves) to represent peripheral circulation, and of a manifold merging the three UB outlets to the same impedance, yields unrealistic local hemodynamics.
Although the multidomain approach shed light on the complex hemodynamics of stage 1 circulation, reproducing different scenarios, simulations remain computationally expensive, thus hardly applicable to clinical care. Since the hemodynamics in the investigated 3D model is mainly characterized by non-linear localized pressure drops, i.e. the CoA and the mBTS, a pure LPM of the whole circulation was developed.
Based on the in silico results of the multidomain model and on mathematical models proposed in the literature, the following expressions were derived to model the pressure drop across the CoA (ΔPst) and the shunt (ΔPsh):
∆Pst=ftKt 8ρ πD022 D02 D12-1 2 |Q|Q+fvKvπD4μ03Q+KuπD4ρl0st2dtdQ (i) ∆Psh=Dfγsh∙γ4Qsh2+Dshδ4Qsh+Dshε2∙dtdQsh (ii)
The first terms on the right-hand side of the equations account for the non-linear convective effects associated with the change in cross-sectional area and with turbulence. The second terms describe the linear viscous effects, whereas the third terms represent the pressure drop contribution to fluid acceleration.
Extended abstract
In Eq. i, Q indicates the instantaneous flow rate across the CoA; D0 and D1 are the diameters of the unobstructed tube and of the stenosis, respectively; lst is the length over which the pressure drop is measured; ρ and µ are the fluid density and viscosity, respectively; Kt, Kv and Ku are proportionality coefficients taken from the literature; ft and fv are corrective factors derived for this specific case.
In Eq. ii, Qsh indicates the instantaneous flow rate through the shunt; Dsh is the shunt diameter; γ, δ and ε are proportionality constants depending on geometrical features and fluid properties; fγ is a corrective factor for this specific case.
The CoA and shunt lumped parameters, described by Eqs. i and ii, were then connected to the LPM used for multidomain simulations, thus creating a pure LPM. This model was validated against in vitro results with a more severe CI and in the absence of CoA, showing its suitability for a clinical environment, as providing fast and reliable information for clinical care of single ventricle palliations. However, to effectively support clinicians in the decision-making process of single ventricle palliations (e.g. whether to treat an aortic coarctation based on the associated hemodynamics), the pure LPM should be implemented by means of a closed loop. This way, if downstream resistances are increased due to the presence of a CoA or a shunt, the heart will react accordingly.
Chapter 4 – Patient-specific numerical models of the
systemic-to-pulmonary shunt
This chapter presents three patient-specific CFD models of various shunt geometries including the conduit and the anastomoses. The first part of the chapter concerns the models setting, following a methodological approach. Namely, numerical sensitivity analyses of the stand-alone 3D models were performed under pulsatile flow conditions, varying the mesh density, the assumptions related to the flow regime and those related to the boundary conditions. In the second part of the chapter, the CFD results were used as a clinical support to evaluate consistency of the clinical data available for each patient. Finally, pressures drops and total energy dissipations of the three models were examined in order to compare the hydraulic features of different shunts, and possibly derive a generic mathematical expression, which can reasonably fits all models behaviors.
The 3D models (M1, M2 and M3) were reconstructed from magnetic resonance imaging (MRI) data, acquired from three candidates ready for a stage 2 procedure. M1 represented a CS, while M2 and M3 reproduced mBTSs, though with different locations of the anastomoses with respect to the IA and the PAs. Boundary conditions were essentially constructed, under some assumptions, from the available patient-specific flows, while an arbitrary outlet was prescribed with a reference pressure, thus avoiding to force the flow profile.
Concerning the mesh sensitivity analysis, the initial mesh of each model was iteratively adapted according to the pressure gradients within the 3D volume, obtained from CFD simulations with laminar flow model. For all models, the changes in mean pressure drops decreased significantly, when passing from the initial mesh to the final one (i.e. when changes were less than 4%). This suggested
Extended abstract
that grid adaptation is appropriate for an accurate description of the complex hemodynamics in these kind of models.
Once the proper mesh density has been selected, a CFD simulation adopting a LES turbulence model, for M3, was performed to investigate the effects of using different flow regime models to describe the hemodynamics within the 3D domain. The resulting pressure drop closely reflected that obtained with the laminar flow model, also producing a similar local fluid dynamics. This demonstrated that the hemodynamic solution of the problem is independent of the flow regime assumption, provided that a good quality mesh is used.
Since assumptions were necessary to prescribe the boundary conditions, a couple of additional simulations were performed to detect possible differences in the resulting pressure drop across the shunt, when varying the assumptions. Namely, M1 was used to evaluate the influence of imposing a flat velocity profile at two different outlet sections, whereas M2 was employed to assess the impact of applying different (arbitrary) time-tracings for the IA. Overall, pressure drops resulting from simulations with varied boundary conditions were qualitatively and quantitatively similar to those of the original simulations. Therefore, solution independence of assumptions on flow boundary conditions was proven, indicating that pressure drop across the shunt is mostly determined by the flow rate through the conduit, regardless of its origin or downstream velocity profiles.
In view of a surgical planning of stage 2 procedures, the above simulation results could help selecting reliable information about flows and pressures that characterize the upstream and, more importantly, downstream circulations. From comparison with the clinically measured pressure drops across the shunts, the predicted values revealed considerable discrepancies, whose causes were deemed to derive from various issues.
First of all, flow measurements could be affected by errors, especially the LPA and RPA signals acquired from MRI, as observed for M2, when comparing the time-varying Qsh with the echo-Doppler velocity tracing. Such discrepancy could be responsible for the wrong prediction of pressure drop.
Another possible cause could be represented by errors in reconstructing the shunt geometry. For example, in M3, the clinical pressure drop was overestimated. Since clinicians affirmed that, from the acquired images, the shunt did not reveal significant occlusions, if compared to the originally implanted conduit of 3.5 mm, attention was directed to possible downsizing errors. In fact, M3 geometry presented a narrower cross-section. Accuracy of reconstruction depends on the image resolution and on the arbitrary inclusion/exclusion of pixels. In this instance, the MRI slices had a resolution of about 0.6 mm (pixel size), which was not negligible with respect to the shunt diameter. Indeed, given the 4th power dependence of pressure drop on the diameter, even half pixel included per each M3 slice reconstructed would be enough to decrease pressure drops to the clinical value.
In M1 instance, the main advantage of simulating the hemodynamics in the shunt model was to realize that the catheter aortic pressure additionally measured
Extended abstract
under the flow rate condition reported by MRI. The catheter signal, which was recorded during patient’s sedation, was evidently indicative of a physiological state of the patient different from that experienced during the MRI exam. It was thus preferred to rely on the mean brachial pressure recorded during MRI, as an estimator of the ascending aortic pressure. It is worthwhile noting, however, that, for CS patients such as M1, using brachial pressure to evaluate the clinical shunt pressure drop may result in an inadequate approximation, since the shunt is anastomosed directly to the AoA. This may partially explain the 13% mismatch observed in M1 pressure drop.
From the above remarks, it could be ascertained: i) how diverse the causes of predicted vs. clinical discrepancies could be among customized shunt models, and
ii) how important is to understand whether clinical data are reliable and whether
estimations or reduced data sets still give meaningful simulation results, as far as clinical acceptabilityconcerns.
The last part of this study was the analysis of local hemodynamics in the three models investigated, with the ultimate goal of deriving a generic mathematical expression which can be valid for all shunt geometries.
For each model, pressure drops computed with previous simulations were divided in three contributions accounting for the pressure drop ∆Pp of the proximal anastomosis, the pressure drop ∆Pd of the distal anastomosis, and the pressure drop ∆Pc of the mere conduit. Then, they were plotted against flow through the shunt Qsh. The predominance of ∆Pc over ∆Pp obtained for all models could be due to the smooth edges of the customized anastomoses. Unlike expectations, ∆Pc(Qsh) curves of all models were best-fitted by quadratic polynomial functions, rather than linear functions, indicating the absence of fully developed flow crossing the conduit. Therefore, a Poiseuille-like mathematical relationship to describe the hydraulic resistance of the conduit should be avoided.
The unexpected small contribution, given by the three distal anastomoses to the total pressure drops across the shunts, suggested that energy dissipations, distinguishing between kinetic and potential contributions, might better explain this behavior. Hence, power losses of the two anastomoses and of the conduit were calculated for each model, distinguishing between kinetic and potential contributions.
In general, the proximal anastomoses and the conduits were characterized by an increase in velocity. The kinetic contributions were comparable (in mean absolute value) with their potential counterpart, meaning that most of the energy dissipations due to pressure drops at the proximal anastomosis were transferred to kinetic energy, leading to abrupt flow acceleration.
Distal power losses were mostly determined by kinetic losses, as velocity fell down for the cross-sectional area increasing from the shunt to the PAs. This clarified the unexpected behavior shown by the distal anastomosis in terms of pressure drops.
Inter-model differences in the hydraulic behavior were detected in terms of kinetic power losses of the conduits, compared with those of the proximal anastomosis. Unlike the other two models, M2 reported predominance of kinetic
Extended abstract
power loss of the conduit, due to a distal narrowing present in M2. This was confirmed by comparison with potential power loss of the conduit, which, for M2, was similar (in absolute value) to its kinetic counterpart. Therefore, it could be inferred that, in general, if a shunt narrowing is present, kinetic and potential contributions to power loss could compensate each other, thus producing relatively small dissipations in the conduit.
Finally, total power loss obtained from each model was divided by the corresponding shunt flow, and compared with the previously calculated pressure drop across the shunt. Their equivalence, found in M2 and M3, demonstrated that the total dissipations of the models were determined, essentially, by pressure contributions, since the energy recovery, caused by an increase in velocity occurring at the proximal anastomosis and along the conduit, canceled out at the distal anastomosis due to an abrupt decrease in velocity.
The results obtained revealed the hemodynamics complexity of in vivo shunts. Conduits of the same class (mBTS such as M2 and M3) exhibited different hydraulic behaviours. Kinetic contribution may play an important role in determining the total energy dissipation, depending on the shunt class (CS such as M1 vs. mBTS such as M2 and M3). Therefore, given the inter-patient variability of shunt shapes, anastomoses locations, and their irregular diameters due to narrowing of the internal lumen, modelling patient-specific shunts with a simple and generic mathematical description, such as with a pure lumped parameter model, would not be trivial. Probably, mathematical relationships for the individual anastomoses and the mere conduit, accounting for potential and kinetic contributions, would be more appropriate to describe the complex hemodynamics of the in vivo shunts. However, using patient-specific in silico simulations would allow to obtain customized pressure drop-flow relationships that can be used for lumped parameter representations of the specific 3D shunts, and potentially included in stand-alone LPMs of stage 1 circulations, as seen in Chapter 3.
Preface
The present Ph.D. thesis was developed in the context of a large international project funded by the Fondation Leducq (Paris), entitled ‘Multi-scale modeling of single ventricle hearts for clinical decision support’. This research project, still underway, handles the modeling of surgical procedures for the treatment of single ventricle congenital heart diseases.
In particular, the present work focused on the first stage (or stage 1) of a multi-staged palliative strategy, which usually starts within the first days of life of the diseased baby. Conventional stage 1 procedures involve the implantation of a so-called systemic-to-pulmonary shunt, that is a conduit interposing between the systemic and pulmonary arterial circulations.
Three were the main questions this study intended to answer:
1. Is it possible to simulate the hemodynamics in relatively small vessels, characterized by high velocity and pressure gradients, as well as in the whole palliated circulation which is often affected by concomitant diseases?
2. Is it possible to provide clinicians with accurate and fast-computing tools, that can be used within a clinical environment as a support for pre-operative surgical planning of second stage palliations?
3. Is it possible to reproduce the patient-specific clinical scenario, and develop an effective mathematical model that can adapt to all stage 1 patients?
To answer these questions, different modelling methodologies were used. After exploring the literature studies on modelling of stage 1 univentricular circulations and of associated cardiovascular congenital defects (Chapter 1), parametric multidomain models with idealized geometry were developed to compare different stage 1 techniques (Chapter 2).
In Chapter 3, a multiscale model of stage 1 circulation with a patient-specific aortic arch, including a coarctation of the aorta, was implemented, based on experimental data. During the validation process, useful feedback for a more effective in vitro modeling of stage 1 circulation were pointed out. Additionally, a pure lumped parameter model of the circulation was developed for simple and fast clinical use.
Finally, in Chapter 4, three-dimensional customized models of various shunts were created. Local hemodynamics was investigated in terms of pressure drops and energy dissipations, in order to verify the possibility to describe the hydraulic behaviour of the patient-specific shunts with a simple and generic mathematical model.
Chapter 1
Stage 1 palliations for single
ventricle heart diseases: surgical
and modeling remarks
Single ventricle heart diseases include a wide range of cardiovascular abnormalities leading to only one functioning ventricle which supports both systemic and pulmonary circulations. Although they represent a small percentage of congenital cardiovascular defects, they are certainly fatal if left untreated. In the last decades, various surgical procedures have been developed to treat univentricular patients in a multi-staged fashion, with the ultimate goal of bypassing the nonfunctional ventricle, thus creating an ‘in series’ circulation. This Chapter focuses on the first stage palliation, characterized by a shunt interposition between the systemic and pulmonary arterial circulations, that, as a consequence, work in parallel. Stage 1 surgical options, along with their common objectives and specific features, are described. The more recent hybrid approach is also presented, discussing the pros and cons with respect to the traditional techniques. Furthermore, aortic coarctation is introduced from the clinical point of view, as a frequently concomitant vascular defect. Finally, a brief digression is made on mathematical modeling for the study of cardiovascular hemodynamics; in particular, the state-of-the-art methods applied to stage 1 palliation and aortic coarctation are illustrated.
1.1
Introduction
Single ventricle heart diseases encompass a wide range of abnormalities in cardiac anatomy leading to only one fully developed, functioning ventricle, which supports both systemic and pulmonary circulations. They represent about 1.4% of congenital cardiovascular defects affecting children in the United States. If left untreated, they are certainly fatal and, even after treatment, have a mortality rate much higher than the average mortality risk (25% vs. 4.8%) for surgery of all types of congenital heart diseases [Roger et al. 2012].
The most common form of single ventricle malformation is the hypoplastic left heart syndrome (HLHS) (Fig. 1.1). As a consequence, in the fetus the ascending aorta (AoA) is underdeveloped and systemic blood flow is fully, or mostly, provided by the right ventricle through the patent ductus arteriosus (DA). The fetus is generally well and unaffected by the anatomical abnormality, but the natural changes from the fetal to the newborn physiology are life threatening. In fact, the decrease in pulmonary vascular resistance (PVR) shortly after birth results in a volume shift from the systemic to the pulmonary circulation, and, as the DA closes, systemic perfusion is further impaired [Norwood 1991].
Figure 1.1 Anatomy of the heart with hypoplastic left heart syndrome [downloaded from
In the last decades, various surgical procedures have been developed to treat infants with such complex cardiovascular defects, and have been improved to limit mortality. The ultimate goal of surgeries is to bypass the dysfunctional, or missing, ventricle by connecting the systemic venous return to the pulmonary arterial system, thus creating a circulation in series powered by a single ventricle [Fontan & Baudet 1971]. However, at birth the lungs are immature and the PVR is naturally high, precluding a series circulation in the neonatal period [Norwood 1991]. Thus, multi-staged management is necessary. This is typically performed in three stages over the first few years of life, to ensure adequate systemic oxygen delivery without severe volume overloading of the ventricle. Stage 1 surgery occurs as a neonate in order to provide an unrestrictive systemic outflow tract with a balanced pulmonary flow. This is allowed by reconstructing the AoA and placing a systemic-to-pulmonary shunt (shunt), as in the Norwood procedure (Fig. 1.2-a), or alternatively stenting the DA and banding the pulmonary arteries (PAs), depending on the patient’s anatomy and on the practice of the clinical centre [Corsini et al. 2013].
Figure 1.2 Surgical palliations to treat hypoplastic left heart syndrome: a) stage 1
Norwood, b) stage 2 Glenn, c) stage 3 Fontan [reprinted from Radiology, 247, Gaca et al. 2008, Repair of congenital heart disease: a primer–part 1, pp. 617-631, with permission from Elsevier]. SVC (IVC) = superior (inferior) vena cava; RPA = right pulmonary artery.
In the first few months, the PVR decreases and lung surface area increases [Bardo et al. 2001], thus physiological conditions for stage 2 surgery are accomplished. At about 6 months, the shunt is removed (alternatively, the ductus/stent complex is resected and the PA bands are removed) [Pizarro et al. 2008], and the superior vena cava (SVC) is attached to the pulmonary arterial vasculature to provide pulmonary flow. The bidirectional cavo-pulmonary anastomosis may be done via a Glenn operation (SVC to right pulmonary artery – RPA) (Fig. 1.2-b) or hemi-Fontan procedure (SVC remains connected to the right
Shunt Neo-aorta SVC RPA IVC a) b) c)
atrium with a homograft patch redirecting its flow to the PAs). During the first 3/4 years of life the proportion of systemic venous return from the lower body progressively increases; therefore at stage 3, the Fontan procedure or total cavo-pulmonary connection (Fig. 1.2-c), also the inferior vena cava (IVC) flow is rerouted to the pulmonary vasculature. There is wide variation in anatomy and physiology between individuals requiring single ventricle surgery, with several alternative procedures at each stage. The resulting different local hemodynamics may differently affect global parameters such as pulmonary flow distribution and systemic oxygen delivery [Corsini et al. 2013]. As these effects are difficult to predict clinically, mathematical and computational models can be useful to explore various anatomical configurations, providing the surgeon with important guidelines. The present Chapter will focus on stage 1 palliation, including its variants in terms of procedure, anatomy and physiology, and will analyze the state of the art of the modeling approaches.
1.2
Surgical stage 1 procedures
In newborns with a univentricular circulation, a shunt has been proved to be an excellent palliation. Surgically connecting the systemic arterial circulation with the PAs, the placement of a shunt creates a complex, non-physiologic parallel circuit (Fig. 1.3), with the single ventricular chamber pumping blood to both the systemic and pulmonary circulations [Pennati et al. 2010]. However, this arrangement is preparatory for the next two surgical procedures that will be performed within the first few months and years of life, respectively. If properly sized, the shunt allows to regulate the pulmonary flow for maturation of the pulmonary vasculature, in order to avoid stenosis development and minimize the volume load on the single ventricle. Furthermore, the parallel arrangement in HLHS patients is achieved with aortic arch reconstruction to guarantee unobstructed flow from the functioning ventricle, with growth potential preventing from further aortic operation [Norwood 1991].
After the introduction of the first shunt in 1945 in patients with tetralogy of Fallot, i.e. the most common cyanotic congenital heart disease, by Blalock and Taussig [1945], various modifications of the original procedure have been developed and performed to treat other cardiac defects, e.g. HLHS. The original operation consists in an end-to-side anastomosis of the subclavian artery to the PA, while the surgical variants include the modified Blalock-Taussig shunt (mBTS), the central shunt (CS), the Waterston and Potts shunts, and the Sano modification (Fig. 1.4). These techniques connect different vessels in different ways.
Figure 1.3 Physiologic (left) and univentricular (right) circulations. LV/RV/SV:
left/right/single ventricle; PC/SC: pulmonary/systemic circulation; sh: systemic-to-pulmonary shunt; Qp/Qs: systemic-to-pulmonary/systemic flow.
The mBTS is a Gore-Tex conduit joining the innominate (or right subclavian) artery to the RPA. The CS, instead, connects the AoA to the main PA, aiming to a more equal distribution of flow between the two PAs. Additionally, it is suitable for very small babies in whom the subclavian artery or branch PAs are too small to insert the shunt. The Waterston and Potts shunts are direct, side-to-side aorto-pulmonary anastomoses: the former is between the AoA and the RPA; the latter is between the descending aorta (DAo) and the left pulmonary artery (LPA). These shunts are now rarely performed, since both share the problem of distortion of the branch PA and difficulty in accurately sizing the anastomotic ‘window’.
In the last decade, a modification interposing a conduit between the right ventricle and a PA, known as ‘right ventricle-pulmonary artery shunt’ (RVPAS), has been introduced by Sano and colleagues [2003] to avoid the adverse hemodynamic consequences of the diastolic run-off related to the previous techniques.
Figure 1.4 Different surgical techniques to construct a systemic-to-pulmonary shunt
[reprinted from Progress in Pediatric Cardiology, 30, Pennati et al. 2010, Modeling of systemic-to-pulmonary shunts in newborns with a univentricular circulation: State of the art
and future directions, pp. 23-29, with permission from Elsevier]. BTS: original Blalock-Taussig shunt; mBTS: modified Blalock-Blalock-Taussig shunt; CS: central shunt; RVPAS: right
ventricle-pulmonary artery shunt.
With previous shunts, indeed, pulmonary blood flow is forward throughout the cardiac cycle, potentially leading to pulmonary overcirculation and decreased coronary blood flow, which further exacerbates the right ventricular overload. A clinical study, conducted on 440 infants who received either the mBTS or the RVPAS (50% respectively) at 15 North American centers, showed no significant difference neither in right ventricular size and function, nor in cardiac transplantation-free survival between the two groups [Ohye et al. 2010]. Nevertheless, controversy remains about the potential salutary effects of the RVPAS, as the drawbacks of performing an incision in the systemic right ventricle might not be apparent for many years [Bove et al. 2008].
The features of the optimal shunt are related to the surgical technique, to the physical properties of the shunt itself, and to the hemodynamics affecting both the surgical region and the global cardiovascular system [Pennati et al. 2010]. Concerning the first two aspects, the shunt should be technically simple to implant and easily removable from the circulation when cavopulmonary connection is performed. Furthermore, it should avoid PA distortion or stenosis to facilitate
stage 2 procedure. From the hemodynamics point of view, the optimal shunt
should provide even distribution of both cardiac output to the pulmonary and systemic circulations, and pulmonary blood flow to the right and left lungs. In fact, an unbalance between systemic and pulmonary flows is contributory to the mortality rate following surgery for HLHS. On top of this, proper systemic oxygen delivery and coronary perfusion are required for minimal ventricular volume overload. Ultimately, the shunt should avoid pulmonary hypertension and maintain intermediate-term patency, which is strongly associated with the local flow conditions, since clotting and intimal hyperplasia at the anastomotic sites, due to local shear stresses, have been identified as potential causes of shunt obstruction [Girdhar & Bluestein 2008].
Several factors, including geometrical and patient related characteristics, influence shunt hemodynamics, thus clinical outcomes. Some of them are the diameter, length and shape of the conduit, as well as the location and size of the proximal and distal anastomoses. Therefore, a thorough understanding of shunt hemodynamics by means of mathematical modeling approaches can assist surgeons not only in appropriate design, but also in post-operative management.
1.3
Hybrid stage 1 procedure
Introduced in 1993 [Gibbs et al. 1993], the hybrid approach, also known as ‘hybrid Norwood’ (HN), for the management of HLHS has been developed as an alternative strategy to the surgical stage 1 palliations. It involves a less invasive procedure combining a surgical technique, i.e. branch pulmonary artery banding (PAB), and interventional cardiology techniques, i.e. DA stenting and balloon atrial septostomy [Galantowicz et al. 2008] (Fig. 1.5). The former technique is to maintain systemic perfusion; the latter aim to limit pulmonary blood flow while promoting unobstructed pulmonary venous return. The circulatory arrangement
achieved with the hybrid approach is different from that of the surgical palliation: besides the distinct way to regulate pulmonary flow (PAB vs. shunt), in the HN circulation the ventricular outflow tract is not reconstructed. Therefore, in case of concomitant aortic atresia or severe aortic stenosis, systemic and cerebral perfusion would rely on retrograde flow through the aortic arch.
One of the main reasons why the HN initially gained popularity is the possibility to avoid cardiopulmonary bypass and cardiac or total circulatory arrest, thereby postponing morbidity and mortality risks of a major open heart surgery. Encouraged by improved techniques and increasing experience to overcome the early ‘learning curve’, some institutions have adopted the hybrid approach as the definitive strategy for stage 1 HLHS palliation [Caldarone et al. 2007, Galantowicz et al. 2008]. Although newborns considered at ‘high risk’ for the surgical procedure due to low birth weight and/or associated co-morbidities or genetic abnormalities (e.g. aortic atresia, intact or restrictive atrial septum) may benefit from the HN [Bacha et al. 2006, Venugopal et al. 2010], this rests on the unproven assumptions that avoiding neonatal cardiopulmonary bypass and postponing aortic arch reconstruction until later in infancy would improve overall survival and cardiac functional outcomes [Caldarone et al. 2007].
Figure 1.5 Hybrid procedure for stage 1 palliation: surgical banding of the branch
pulmonary arteries, stenting of the ductus arteriosus and balloon atrial septostomy [reprinted from The Annals of Thoracic Surgery, 85, Galantowicz et al. 2008, Hybrid approach for hypoplastic left heart syndrome: intermediate results after the learning curve,
In the last decade several groups have reported the clinical outcomes of the hybrid palliation performed in single ventricle neonates, investigating the effects on hemodynamic parameters. Li and colleagues [2007] asserted that the early postoperative hybrid patient has less favorable overall circulatory performance, in terms of hemodynamic stability and oxygen transport, with respect to postoperative surgical patients. Thus, they suggested that, in hybrid candidates with known pre-operative myocardial dysfunction, PAB may not completely protect against pulmonary overcirculation, rather a decrease of the systemic vascular resistance (SVR) and an increase of myocardial contractility should be achieved through inotropic and vasodilatory support, which is routinely used in surgical patients. Other teams investigated PA growth in both hybrid and surgical patients, as this physiological process is very important for stage 1 patients to become suitable for stage 2 and stage 3 procedures [Honjo et al. 2009, Santoro et al. 2009]. Potential problems are associated with either excessively loose or tight bilateral PAB: the former may result in mid-term pulmonary hypertension, whereas the latter may provide inadequate pulmonary blood flow, impeding vascular growth. However, both studies found that the HN is as effective as the conventional procedures in promoting a global PA growth, enabling an even distribution of the pulmonary blood flow. This feature ensured a balanced vascular development between left and right in the hybrid group, balance that was rarely found in the surgical group probably because of aortic arch reconstruction causing LPA distortion.
Despite the current survival greater than 70% after conventional stage 1 surgery [Venugopal et al. 2010], a general interest in the hybrid strategy for ‘high risk’ patients persists among clinicians, but conflicting opinions about whether the HN provides major benefits are still present.
1.4
Aortic coarctation
Coarctation of the aorta (CoA) is a discrete narrowing of the proximal DAo at the juxtaductal (i.e. adjacent to the DA) region (Fig. 1.6). CoA can present in various shapes: from the neonate with heart failure to the asymptomatic child or adult with hypertension. In particular, it is very common in HLHS patients, occurring both in newborns with an incidence higher than 80%, and in the intervening years between the staged surgeries with an incidence ranging from 0 to 37%. In fact, interim neoaortic arch obstruction (NAO) is a serious complication affecting morbidity and mortality after the stage 1 palliation, as even mild degrees of obstruction can result in decreased cardiac output, ventricular dysfunction and atrioventricular valve regurgitation [Bautista-Hernandez et al. 2007, Bendaly et al. 2012]. However, no standard approach to repair this defect has been widely accepted as the primary modality of treatment.
Figure 1.6 Anatomy of the heart with aortic coarctation [downloaded from
http://www.lpch.org/].
Percutaneous balloon angioplasty (BA) was first described in 1982 to relieve CoA surgically created in newborn lambs [Lock et al. 1982], and in 1987 to remove NAO in three patients with HLHS [Saul et al. 1987]. Since that time the procedure has gained popularity by pediatric cardiologists, with several groups reporting acute success rates of 89–100%. Although recurrent obstruction within the first year after initial angioplasty is not uncommon (incidence of about 18%), BA is still deemed as an effective palliation by these cardiologists [Zeltser et al. 2005, Moszura et al. 2009, Bendaly et al. 2012]. On the other hand, encouraged by a reduction of the residual gradient and aortic wall abnormalities when compared with BA, other groups have started to prefer stenting as the best endovascular procedure for CoA repair. Various surgical techniques have been utilized as well, including end-to-end resection, patch repair and tube grafts, but they are no longer routinely performed because of the reported aneurysm formation following all types of repair [Tanous et al. 2009].
With regard to native CoA, coarctectomy is usually performed at the time of
stage 1 palliation not only to remove the narrowing but also because it has been
found to significantly reduce the incidence of NAO. Given its frequent recurrence after stage 1, interest in understanding the influence of the technique used for arch reconstruction on NAO development has been shown in the clinical field
Coarctation of the aorta
