Liesbeth Taelman

Noninvasive pulmonary artery wave intensity analysis in pulmonary hypertension

Pulmonary wave reflections are a potential hemodynamic biomarker for pulmonary hypertension (PH) and can be analyzed using wave intensity analysis (WIA). In this study we used pulmonary vessel area and flow obtained using cardiac magnetic resonance (CMR) to implement WIA noninvasively. We hypothesized that this method could detect differences in reflections in PH patients compared with healthy controls and could also differentiate certain PH subtypes. Twenty patients with PH (35% CTEPH and 75% female) and 10 healthy controls (60% female) were recruited. Right and left pulmonary artery (LPA and RPA) flow and area curves were acquired using self-gated golden-angle, spiral, phase-contrast CMR with a 10.5-ms temporal resolution. These data were used to perform WIA on patients and controls. The presence of a proximal clot in CTEPH patients was determined from contemporaneous computed tomography/angiographic data. A backwards-traveling compression wave (BCW) was present in both LPA and RPA of all PH patients but was absent in all controls (P = 6e−8). The area under the BCW was associated with a sensitivity of 100% [95% confidence interval (CI) 63–100%] and specificity of 91% (95% CI 75–98%) for the presence of a clot in the proximal PAs of patients with CTEPH. In conclusion, WIA metrics were significantly different between patients and controls; in particular, the presence of an early BCW was specifically associated with PH. The magnitude of the area under the BCW showed discriminatory capacity for the presence of proximal PA clot in patients with CTEPH. We believe that these results demonstrate that WIA could be used in the noninvasive assessment of PH.

The aortic reservoir-wave as a paradigm for arterial haemodynamics: insights from three-dimensional fluid–structure interaction simulations in a model of aortic coarctation

Background: The reservoir-wave paradigm considers aortic pressure as the superposition of a ‘reservoir pressure’, directly related to changes in reservoir volume, and an ‘excess’ component ascribed to wave dynamics. The change in reservoir pressure is assumed to be proportional to the difference between aortic inflow and outflow (i.e. aortic volume changes), an assumption that is virtually impossible to validate in vivo. The aim of this study is therefore to apply the reservoir-wave paradigm to aortic pressure and flow waves obtained from three-dimensional fluid-structure interaction simulations in a model of a normal aorta, aortic coarctation (narrowed descending aorta) and stented coarctation (stiff segment in descending aorta). Method and results: We found no unequivocal relation between the intraaortic volume and the reservoir pressure for any of the simulated cases. When plotted in a pressure-volume diagram, hysteresis loops are found that are looped in a clockwise way indicating that the reservoir pressure is lower than the pressure associated with the change in volume. The reservoir-wave analysis leads to very high excess pressures, especially for the coarctation models, but to surprisingly little changes of the reservoir component despite the impediment of the buffer capacity of the aorta. Conclusion: With the observation that reservoir pressure is not related to the volume in the aortic reservoir in systole, an intrinsic assumption in the wave-reservoir concept is invalidated and, consequently, also the assumption that the excess pressure is the component of pressure that can be attributed to wave travel and reflection.

Modeling Hemodynamics in Vascular Networks Using a Geometrical Multiscale Approach: Numerical Aspects

On the one hand the heterogeneity of the circulatory system requires the use of different models in its different compartments, featuring different assumptions on the spatial degrees of freedom. On the other hand, the mutual interactions between its compartments imply that these models should preferably not be considered separately. These requirements have led to the concept of geometrical multiscale modeling, where the main idea is to couple 3D models with reduced 1D and/or 0D models. As such detailed information on the flow field in a specific region of interest can be obtained while accounting for the global circulation. However, the combination of models with different mathematical features gives rise to many difficulties such as the assignment of boundary conditions at the interface between two models and the development of robust coupling algorithms, as the subproblems are usually solved in a partitioned way. This review aims to give an overview of the most important aspects concerning 3D–1D–0D coupled models. In addition, some applications are presented in order to illustrate the potentialities of these coupled models.

Unstructured hexahedral mesh generation of complex vascular trees using a multi-block grid-based approach

The trend towards realistic numerical models of (pathologic) patient-specific vascular structures brings along larger computational domains and more complex geometries, increasing both the computation time and the operator time. Hexahedral grids effectively lower the computational run time and the required computational infrastructure, but at high cost in terms of operator time and minimal cell quality, especially when the computational analyses are targeting complex geometries such as aneurysm necks, severe stenoses and bifurcations. Moreover, such grids generally do not allow local refinements. As an attempt to overcome these limitations, a novel approach to hexahedral meshing is proposed in this paper, which combines the automated generation of multi-block structures with a grid-based method. The robustness of the novel approach is tested on common complex geometries, such as tree-like structures (including trifurcations), stenoses, and aneurysms. Additionally, the performance of the generated grid is assessed using two numerical examples. In the first example, a grid sensitivity analysis is performed for blood flow simulated in an abdominal mouse aorta and compared to tetrahedral grids with a prismatic boundary layer. In the second example, the fluid–structure interaction in a model of an aorta with aortic coarctation is simulated and the effect of local grid refinement is analyzed.

Speeding Up Fluid-Structure Interaction Simulation of the Blood Flow in a Flexible Artery Using Sub-Cycling: Stability and Accuracy

In the cardiovascular system, the distensibility of the blood vessels is the driving mechanism of wave propagation. As the blood flow interacts mechanically with the flexible vessel walls, this phenomenon gives rise to complex fluid-structure interaction (FSI) problems. Several studies comparing rigid wall with FSI simulations in settings with large deformations demonstrate the importance of including the flexible wall modeling, in particular with respect to the simulation of wall shear stress. As both the equations governing the flow and the arterial deformation (and their interaction) need to be solved, FSI simulations are characterized by a high computational cost. To make them applicable to a broader range of cardiovascular problems, efforts to reduce the calculation time (such as the use of so-called ‘sub-cycling’) should be made.Copyright

Assessing the accuracy of non-invasive measuring methods of pulse wave velocity: an analysis based on fluid-structure interaction simulations in the carotid artery

Pulse wave velocity (PWV) is the propagation speed of pressure and flow waves in the arterial system induced by the contracting left ventricle. PWV is a measure of arterial stiffness, and has been shown to predict cardiovascular events. In a clinical setting, PWV is usually associated with carotid-femoral PWV, reflecting the propagation speed over the aorta. It is, however, also possible to assess local PWV at a given measuring location, which reflects the stiffness of the artery under investigation at that particular location. When locally assessing PWV, single-location techniques are commonly used, which rely on the fact that in uniform elastic tubes, the relationship between a change in pressure (dP) and velocity (dU) is constant in the absence of wave reflections. As such, when plotting the pressure P as a function of the velocity U in an artery, a PU-loop is obtained, where reflection-free instants emerge as a straight line (typically during early systole), with a slope given by ρPWV (ρ = blood density). The original method relied on pressure and velocity data (PU-method), but alternative methods have been introduced based on cross-sectional area (A) and flow (Q) (QA-method), or diameter (D) and velocity (U) (ln(D)U-method).Copyright

PREDICTING THE FUNCTIONAL IMPACT OF RESIDUAL AORTIC COARCTATION LESIONS USING FLUID-STRUCTURE INTERACTION SIMULATIONS

Aortic coarctation is a congenital disease, characterized by a narrowing of the upper descending aorta, obstructing the blood flow from the heart towards the lower part of the body. The treatment can be minimally invasive using a stent and/or a balloon catheter to dilate the coarctation zone, or the narrow section can be removed surgically. Even after a successful treatment, a high risk of cardiovascular morbidity and mortality remains. Two aspects contribute to this increased risk: (1) a residual narrowing, leading to an additional resistance in the arterial system and (2) a local stiffening after treatment, disturbing the buffer function of the aorta. Moreover, these residual narrowing and stiffening lead to an impedance mismatch and are a source of wave reflections that reach the heart fast, given the short distance to the heart.Copyright

Analysis of aortic wave travel and reflection using advanced modeling methods in simplified geometries

The analysis of wave travel and reflection has gained increased interest in the clinical community. The speed of the pressure pulse, PWV, (assessed from the time delay of the foot of the wave measured at two distant locations), is considered the gold standard method to assess the stiffness of arterial segments and has been shown to be of prognostic value for cardiovascular disease. Also, analysis of wave reflections has been suggested as diagnostic tool, a.o. to estimate the effect of the treatment of aortic coarctation (a congenital disease characterized by an obstructive narrowing of the upper descending aorta) on the load on the heart. The presence of a residual narrowing and/or local stiffening after treatment of aortic coarctation leads to an impedance mismatch and generates wave reflections that reach the heart very fast, given the short distance to the heart. The exact interplay between arterial stiffness, wave travel and reflection is, however, still relatively poorly understood due to the complexity of the arterial tree leading to scattered wave reflection, rather than discrete reflection.Copyright

Pulse propagation and wave reflection in arteries: new insights using advanced modeling methods

Objective: Non-invasive diagnostic devices for arterial stiffness often synthesize information related to the propagation and reflection of pressure pulses throughout the arterial system. These phenomena are, however, complex and still not fully understood. Methods: We return to the basics by studying the propagation and reflection of a short, isolated pressure pulse in a straight and tapered aorta (see Figure 1) mimicking the foot of the physiological pressure wave. Theoretical pulse wave velocity (PWVth) in the straight model was 4.88 m/s. It increased from 4.38 to 5.35 m/s (average 4.88 m/s) in the tapered model. Additional simulations included a local stiffening (≈ a nonobstructing repaired aortic coarctation). Full 3D numerical fluid-structure interaction simulations with a short time step (1ms) were used. Reflected waves at the distal end were suppressed, thus isolating the effects of reflection due to aortic tapering and the presence of the rigid segment. Results: In both control models, PWVff was higher than PWVth (5.50 and 5.60 m/s resp.). Interestingly, a so-called “precursor wave” appeared in the simulations (see e.g. at t≈0.025 s at the outlet), traveling back and forth within the arterial wall at a speed approximately 3 times higher than PWVth (16.5 m/s; Figure 1). Tapering amplifies the forward wave (Figure 1C). In the tapered model, the backward wave shows the same pattern induced by the precursor waves, but with an offset indicating continuous wave reflections. The stiff segment induced backward waves proximal to the rigid zone; distally, the forward traveling wave was reduced by approximately 9%. Conclusions: PWVff does not match the theoretical PWV and aortic tapering complicates the unequivocal interpretation of wave reflections. The co-appearance of a fast-traveling wave in the arterial wall warrants further (in vivo) investigations, but may open new perspectives for more direct characterization of the mechanical properties of arterial tissue.