Early outcomes and computational fluid dynamic analyses of chimney reconstruction in the Norwood procedure†

Hemodynamics and Wall Mechanics of Vascular Graft Failure

Blood vessels are subjected to complex biomechanical loads, primarily from pressure-driven blood flow. Abnormal loading associated with vascular grafts, arising from altered hemodynamics or wall mechanics, can cause acute and progressive vascular failure and end-organ dysfunction. Perturbations to mechanobiological stimuli experienced by vascular cells contribute to remodeling of the vascular wall via activation of mechanosensitive signaling pathways and subsequent changes in gene expression and associated turnover of cells and extracellular matrix. In this review, we outline experimental and computational tools used to quantify metrics of biomechanical loading in vascular grafts and highlight those that show potential in predicting graft failure for diverse disease contexts. We include metrics derived from both fluid and solid mechanics that drive feedback loops between mechanobiological processes and changes in the biomechanical state that govern the natural history of vascular grafts. As illustrative examples, we consider application-specific coronary artery bypass grafts, peripheral vascular grafts, and tissue-engineered vascular grafts for congenital heart surgery as each of these involves unique circulatory environments, loading magnitudes, and graft materials.

Blood flow analysis with computational fluid dynamics and 4D-flow MRI for vascular diseases

Both computational fluid dynamics (CFD) and time-resolved, three-dimensional, phase-contrast, magnetic resonance imaging (4D-flow MRI) enable visualization of time-varying blood flow structures and quantification of blood flow in vascular diseases. However, they are totally different. CFD is a method to calculate blood flow by solving the governing equations of fluid mechanics, so the obtained flow field is somewhat virtual. On the other hand, 4D-flow MRI measures blood flow in vivo, thus the flow is real. Recently, with the development and enhancement of computers, medical imaging techniques, and related software, blood flow analysis has become more accessible to clinicians and its usefulness in vascular diseases has been demonstrated. In this review, we have outlined the methods and characteristics of CFD and 4D-flow MRI, respectively. We have discussed the differences in the characteristics between both methods; reviewed the milestones achieved by blood flow analysis in various vascular diseases; and discussed the usefulness, challenges, and limitations of blood flow analysis. We have discussed the difficulties and limitations of current blood flow analysis. We have also discussed our views on future directions.

Hemodynamic Parameters for Cardiovascular System in 4D Flow MRI: Mathematical Definition and Clinical Applications

Blood flow imaging becomes an emerging trend in cardiology with the recent progress in computer technology. It not only visualizes colorful flow velocity streamlines but also quantifies the mechanical stress on cardiovascular structures; thus, it can provide the detailed inspections of the pathophysiology of diseases and predict the prognosis of cardiovascular functions. Clinical applications include the comprehensive assessment of hemodynamics and cardiac functions in echocardiography vector flow mapping (VFM), 4D flow MRI, and surgical planning as a simulation medicine in computational fluid dynamics (CFD).For evaluation of the hemodynamics, novel mathematically derived parameters obtained using measured velocity distributions are essential. Among them, the traditional and typical parameters are wall shear stress (WSS) and its related parameters. These parameters indicate the mechanical damages to endothelial cells, resulting in degenerative intimal change in vascular diseases. Apart from WSS, there are abundant parameters that describe the strength of the vortical and/or helical flow patterns. For instance, vorticity, enstrophy, and circulation indicate the rotating flow strength or power of 2D vortical flows. In addition, helicity, which is defined as the cross-linking number of the vortex filaments, indicates the 3D helical flow strength and adequately describes the turbulent flow in the aortic root in cases with complicated anatomies. For the description of turbulence caused by the diseased flow, there exist two types of parameters based on completely different concepts, namely: energy loss (EL) and turbulent kinetic energy (TKE). EL is the dissipated energy with blood viscosity and evaluates the cardiac workload related to the prognosis of heart failure. TKE describes the fluctuation in kinetic energy during turbulence, which describes the severity of the diseases that cause jet flow. These parameters are based on intuitive and clear physiological concepts, and are suitable for in vivo flow measurements using inner velocity profiles.

Prediction for future occurrence of type A aortic dissection using computational fluid dynamics

OBJECTIVES

The actual underlying mechanisms of acute type A aortic dissection (AAAD) are not well understood. The present study aimed to elucidate the mechanism of AAAD using computational fluid dynamics (CFD) analysis.

METHODS

We performed CFD analysis using patient-specific computed tomography imaging in 3 healthy control cases and 3 patients with AAAD. From computed tomography images, we made a healthy control model or pre-dissection model for CFD analysis. Pulsatile cardiac flow during one cardiac cycle was simulated, and a three-dimensional flow streamline was visualized to evaluate flow velocity, wall shear stress and oscillatory shear index (OSI).

RESULTS

In healthy controls, the transvalvular aortic flow was parallel to the ascending aorta. There was no spotty high OSI area at the ascending aorta. In pre-dissection patients, accelerated transvalvular aortic flow was towards the posterolateral ascending aorta. The vortex flow was observed on the side of the lesser curvature in mid-systole and expanded throughout the entire ascending aorta during diastole. Systolic wall shear stress was high due to the accelerated aortic blood flow on the side of the greater curvature of the ascending aorta. On the side of the lesser curvature, high OSI areas were observed around the vortex flow. In all pre-dissection cases, a spotty high OSI area was in close proximity to the actual primary entry site of the future AAAD.

CONCLUSIONS

The pre-onset high OSI area with vortex flow is closely associated with the future primary entry site. Therefore, we can elucidate the mechanism of AAAD with CFD analysis.

Chimney reconstruction provides a wider subaortic space and reduces the risk of pulmonary artery compression in the Norwood-type aortic arch reconstruction without patch supplementation

OBJECTIVES

Pulmonary artery (PA) compression by the neoaorta is a serious complication after the Norwood-type palliation (NP) for hypoplastic left heart syndrome. Either excess patch tailoring or limited use of autologous tissue may cause narrowing of the subaortic space. The chimney technique could theoretically provide a wide subaortic space.

METHODS

Twenty-nine patients with both pre- and post-NP computed tomography data available of the 37 consecutive patients who underwent NP in our institution were reviewed. Arch height, arch width, sinus of Valsalva diameter, area under the neoaortic arch and arch angle were measured. These patients were divided according to the neo-arch reconstruction technique, chimney reconstruction technique (CR) or conventional direct reconstruction technique (DR).

RESULTS

Median age and weight at NP were 2.1 months and 3.5 kg, respectively. Twenty-two patients underwent previous bilateral PA banding. During NP, 17 CR and 12 DR were performed. Four patients in the DR group developed PA compression. No neoaortic arch dilatation was found in either group. Post-NP arch width, area under the neo-arch and the arch angle were significantly larger in the CR group. Pre-NP arch height was significantly smaller in DR patients with PA compression than in those without.

CONCLUSIONS

The chimney technique decreased the risk of PA compression and provided a wider subaortic space and a less acute arch angle. This technique had no short-term effect on the neoaortic root. Small preoperative arch height is a potential risk factor for PA compression in DR, and the chimney technique could be an effective solution.

The effect of a valved small conduit on systemic ventricle–pulmonary artery shunt in the Norwood-type palliation

OBJECTIVES

The aim of this study was to clarify the impact of valved systemic ventricle–pulmonary artery (SV–PA) shunt on outcomes after stage-1 Norwood-type palliation (NP) compared with the modified Blalock–Taussig shunt.

METHODS

Consecutive patients who underwent NP between 2003 and 2019 were enrolled. SV–PA shunts using the expanded polytetrafluoroethylene valved conduit were implanted in 18 patients (valved SV–PA group), and another 18 patients underwent modified Blalock–Taussig shunt during NP (modified Blalock–Taussig shunt group). All valved conduits were made in our institution in advance.

RESULTS

No differences in baseline characteristics were found between the groups, except for shunt size. During a median 2.9 (interquartile range 0.4–6.4, maximum 14.2) years of follow-up, 8 (22.2%) patients died across both groups. There were no statistically significant differences in early mortality (5.5% vs 11.1%, P = 0.55) and overall survival rates at 5 years (80.8% vs 71.4%, P = 0.48) in the valved SV–PA and modified Blalock–Taussig shunt groups. No statistically significant difference was observed in the frequency of interventions between the groups (31% vs 33%, P = 1.0). At the time of the bidirectional Glenn procedure, the systemic ventricular end-diastolic volume index was significantly lower (84 ± 24 vs 106 ± 31 ml/m2, P = 0.05) and the ejection fraction was significantly greater (62 ± 8% vs 55 ± 9%, P = 0.03) in the valved SV–PA group. There was no statistically significant difference in the pulmonary artery index (228 ± 85 vs 226 ± 60 mm2/m2, P = 0.92).

CONCLUSIONS

A valved SV–PA shunt using an expanded polytetrafluoroethylene valved conduit was associated with preserved ventricular function after NP and did not impair pulmonary artery growth by controlling pulmonary regurgitation.