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.

Differential impact of local stiffening and narrowing on hemodynamics in repaired aortic coarctation: an FSI study.

Even after successful treatment of aortic coarctation, a high risk of cardiovascular morbidity and mortality remains. Uncertainty exists on the factors contributing to this increased risk among which are the presence of (1) a residual narrowing leading to an additional resistance and (2) a less distensible zone disturbing the buffer function of the aorta. As the many interfering factors and adaptive physiological mechanisms present in vivo prohibit the study of the isolated impact of these individual factors, a numerical fluid-structure interaction model is developed to predict central hemodynamics in coarctation treatment. The overall impact of a stiffening on the hemodynamics is limited, with a small increase in systolic pressure (up to 8 mmHg) proximal to the stiffening which is amplified with increasing stiffening and length. A residual narrowing, on the other hand, affects the hemodynamics significantly. For a short segment (10 mm), the combination of a stiffening and narrowing (coarctation index 0.5) causes an increase in systolic pressure of 58 mmHg, with 31 mmHg due to narrowing and an additional 27 mmHg due to stiffening. For a longer segment (25 mm), an increase in systolic pressure of 50 mmHg is found, of which only 9 mmHg is due to stiffening.

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.

Wave reflection leads to over- and underestimation of local wave speed by the PU- and QA-loop methods: theoretical basis and solution to the problem.

Single-point methods such as the PU- and QA-loop methods are used to estimate local pulse wave velocity (PWVPU and PWVQA) in arteries from a combination of pressure (P), flow (Q), velocity (U) or cross-sectional area (A) waveforms. Available data indicate that the PU-loop method tends to overestimate PWV, while the QA-loop method tends to underestimate. Wave reflection has been suggested as a factor playing a role in the agreement between different methods. In this work, we first provide a theoretical basis to (i) demonstrate the interference of wave reflection with the PU-loop method for both solitary sinusoidal waves as well as physiological waveforms; (ii) develop an operator-independent method to correct for the presence of reflections. Fluid-structure interaction simulations in a tube and carotid artery model with known mechanical properties confirm the theory. For the carotid artery model, PWVPU severely overestimates PWV, while PWVQA underestimates PWV. Correction (leading to an estimate termed PWV1-5) seems to eliminate the impact of reflections. Finally, methods are applied in vivo. Compared to PWVPU and PWVQA, PWV1-5 leads to significantly better correlations of carotid PWV with PWV derived from carotid distensibility based on the Bramwell-Hill equation (with r(2) improving from about 0.25 to 0.91). We conclude that neither the PU-loop nor the QA-loop method provides reliable estimates of local PWV in settings where wave reflections are present-even when the PU- or QA-loops show a linear segment. They offer no alternative for the Bramwell-Hill based approach and their application should therefore be discouraged, especially for the carotid artery, although caution is probably warranted in general.

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.

Comparison of non-invasive methods for measurement of local pulse wave velocity using FSI-simulations and in vivo data.

In the search for better predictors of cardiovascular events, pulse wave velocity (PWV) has gained considerable interest. We compared three single-location methods to locally estimate PWV based on simultaneous measurements of pressure (P), velocity (U) or arterial diameter (D): the PU, ln(D)U and QA-method. First, the performance of these methods was analyzed using 3D fluid-structure interaction simulations (FSI) in a tube and patient-specific carotid artery. We demonstrated that the outcome was dependent on whether the methods were tested in the ideal conditions of a 3D axisymmetrical and reflection-free tube or in the more realistic setup of a carotid artery. The three single-location PWV methods performed similarly in the tube (4.29 m/s for PU, 4.44 m/s for ln(D)U and 4.38 m/s for QA) while the carotid data showed that the PU-method dramatically overestimates PWV (9.16 m/s), and the ln(D)U and QA-method underestimate (3.86 and 3.84 m/s, respectively). The erroneously high wavespeeds from the PU-method were attributed to wave reflections, which was confirmed by measurements in 37 healthy adults. This in vivo study showed similar discrepancies between the 3 single-location techniques as present in the carotid simulations, with the difference between the PU- and ln(D)U-method related to the magnitude of wave reflection.