Ex Vivo Regional Mechanical Characterization of Porcine Pulmonary Arteries.

BACKGROUND: Regional mechanical characterization of pulmonary arteries can be useful in the development of computational models of pulmonary arterial mechanics. OBJECTIVE: We performed a biomechanical and microstructural characterization study of porcine pulmonary arteries, inclusive of the main, left, and right pulmonary arteries (MPA, LPA, and RPA, respectively). METHODS: The specimens were initially stored at -20°C and allowed to thaw for 12-24 hours prior to testing. Each artery was further subdivided into proximal, middle, and distal regions, leading to ten location-based experimental groups. Planar equibiaxial tensile testing was performed to evaluate the mechanical behavior of the specimens, from which we calculated the stress at the maximum strain (S 55), tensile modulus (TM), anisotropy index (AI), and strain energy in terms of area under the stress-strain curve (AUC). Histological quantification was performed to evaluate the area fraction of elastin and collagen content, intima-media thickness (IMT), and adventitial thickness (AT). The constitutive material behavior of each group was represented by a five-constant Holzapfel-Gasser-Ogden model. RESULTS: The specimens exhibited non-linear stress-strain characteristics across all groups. The MPA exhibited the highest mean wall stress and TM in the longitudinal and circumferential directions, while the bifurcation region yielded the highest values of AI and AUC. All regions revealed a higher stiffness in the longitudinal direction compared to the circumferential direction, suggesting a degree of anisotropy that is believed to be within the margin of experimental uncertainty. Collagen content was found to be the highest in the MPA and decreased significantly at the bifurcation, LPA and RPA. Elastin content did not yield such significant differences amongst the ten groups. The MPA had the highest IMT, which decreased concomitantly to the distal LPA and RPA. No significant differences were found in the AT amongst the ten groups. CONCLUSION: The mechanical properties of porcine pulmonary arteries exhibit strong regional dissimilarities, which can be used to inform future studies of high fidelity finite element models.

Experimental and computational investigation of the patient-specific abdominal aortic aneurysm pressure field.

The objective of the present manuscript is three-fold: (i) to study the detailed pressure field inside a patient-specific abdominal aortic aneurysm (AAA) model experimentally and numerically and discuss its clinical relevance, (ii) to validate a number of possible numerical model options and their ability to predict the experimental pressure field and (iii) to compare the spatial pressure drop in the AAA before and after the formation of intraluminal thrombus (ILT) for a late disease development timeline. A finite volume method was used to solve the governing equations of fluid flow to simulate the flow dynamics in a numerical model of the AAA. Following our patient-specific anatomical rapid prototyping technique, physical models of the aneurysm were created with seven ports for pressure measurement along the blood flow path. A flow loop operating with a blood analogue fluid was used to replicate the patient-specific flow conditions, acquired with phase-contrast magnetic resonance imaging, and measure pressure in the flow model. The Navier-Stokes equations and two turbulent models were implemented numerically to compare the pressure estimations with experimental measurements. The relative pressure difference from experiments obtained with the best performing model (unsteady laminar simulation) was ∼1.1% for the AAA model without ILT and ∼15.4% for the AAA model with ILT (using Reynolds Stress Model). Future investigations should include validation of the 3D velocity field and wall shear stresses within the AAA sac predicted by the three numerical models.

Impedance-based outflow boundary conditions for human carotid haemodynamics.

In this study, we develop structured tree outflow boundary conditions for modelling the human carotid haemodynamics. The model geometry was reconstructed through computerised tomography scan. Unsteady-state computational fluid dynamic analyses were performed under different conditions using a commercial software package ADINA R&D, Inc., (Watertown, MA, USA) in order to assess the impact of the boundary conditions on the flow variables. In particular, the results showed that the peripheral vessels massively impact the pressure while the flow is relatively unaffected. As an example of application of these outflow conditions, an unsteady fluid-structure interaction (FSI) simulation was carried out and the dependence of the wall shear stress (WSS) on the arterial wall compliance in the carotid bifurcation was studied. In particular, a comparison between FSI and rigid-wall models was conducted. Results showed that the WSS distributions were substantially affected by the diameter variation of the arterial wall. In particular, even similar WSS distributions were found for both cases, and differences in the computed WSS values were also found.

Considerations for numerical modeling of the pulmonary circulation--a review with a focus on pulmonary hypertension.

Both in academic research and in clinical settings, virtual simulation of the cardiovascular system can be used to rapidly assess complex multivariable interactions between blood vessels, blood flow, and the heart. Moreover, metrics that can only be predicted with computational simulations (e.g., mechanical wall stress, oscillatory shear index, etc.) can be used to assess disease progression, for presurgical planning, and for interventional outcomes. Because the pulmonary vasculature is susceptible to a wide range of pathologies that directly impact and are affected by the hemodynamics (e.g., pulmonary hypertension), the ability to develop numerical models of pulmonary blood flow can be invaluable to the clinical scientist. Pulmonary hypertension is a devastating disease that can directly benefit from computational hemodynamics when used for diagnosis and basic research. In the present work, we provide a clinical overview of pulmonary hypertension with a focus on the hemodynamics, current treatments, and their limitations. Even with a rich history in computational modeling of the human circulation, hemodynamics in the pulmonary vasculature remains largely unexplored. Thus, we review the tasks involved in developing a computational model of pulmonary blood flow, namely vasculature reconstruction, meshing, and boundary conditions. We also address how inconsistencies between models can result in drastically different flow solutions and suggest avenues for future research opportunities. In its current state, the interpretation of this modeling technology can be subjective in a research environment and impractical for clinical practice. Therefore, considerations must be taken into account to make modeling reliable and reproducible in a laboratory setting and amenable to the vascular clinic. Finally, we discuss relevant existing models and how they have been used to gain insight into cardiopulmonary physiology and pathology.

CFD analysis of the human airways under impedance-based boundary conditions: application to healthy, diseased and stented trachea.

A computational fluid dynamics model of a healthy, a stenotic and a post-operatory stented human trachea was developed to study the respiration under physiological boundary conditions. For this, outflow pressure waveforms were computed from patient-specific spirometries by means of a method that allows to compute the peripheral impedance of the truncated bronchial generation, modelling the lungs as fractal networks. Intratracheal flow pattern was analysed under different scenarios. First, results obtained using different outflow conditions were compared for the healthy trachea in order to assess the importance of using impedance-based conditions. The resulted intratracheal pressures were affected by the different boundary conditions, while the resulted velocity field was unaffected. Impedance conditions were finally applied to the diseased and the stented trachea. The proposed impedance method represents an attractive tool to compute physiological pressure conditions that are not possible to extract in vivo. This method can be applied to healthy, pre- and post-operatory tracheas showing the possibility of predicting, through numerical simulation, the flow and the pressure field before and after surgery.

FSI analysis of a human trachea before and after prosthesis implantation.

In this work we analyzed the response of a stenotic trachea after a stent implantation. An endotracheal stent is the common treatment for tracheal diseases such as stenosis, chronic cough, or dispnoea episodes. Medical treatment and surgical techniques are still challenging due to the difficulties in overcoming potential complications after prosthesis implantation. A finite element model of a diseased and stented trachea was developed starting from a patient specific computerized tomography (CT) scan. The tracheal wall was modeled as a fiber reinforced hyperelastic material in which we modeled the anisotropy due to the orientation of the collagen fibers. Deformations of the tracheal cartilage rings and of the muscular membrane, as well as the maximum principal stresses, are analyzed using a fluid solid interaction (FSI) approach. For this reason, as boundary conditions, impedance-based pressure waveforms were computed modeling the nonreconstructed vessels as a binary fractal network. The results showed that the presence of the stent prevents tracheal muscle deflections and indicated a local recirculatory flow on the stent top surface which may play a role in the process of mucous accumulation. The present work gives new insight into clinical procedures, predicting their mechanical consequences. This tool could be used in the future as preoperative planning software to help the thoracic surgeons in deciding the optimal prosthesis type as well as its size and positioning.

The association of clinical variables and filter design with carotid artery stenting thirty-day outcome.

OBJECTIVE: Patient and device selection are important for the success of carotid artery stenting (CAS). We hypothesize that distal protection filter (DPF) design characteristics that minimize blood flow resistance and maximize capture efficiency are associated with the absence of transient ischemic attack (TIA), stroke and neurologic-related death after 30 days. METHODS: Records from 208 patients were reviewed retrospectively. Filter design characteristics were quantified previously in our laboratory. The association between risk factors and design characteristics with 30-day outcome was quantified using univariate analysis. RESULTS: The 30-day all-cause stroke and death rate was 8.7% (asymptomatic: 7.7%, symptomatic: 10.6%). Five DPFs were used in the study: Accunet (41.3%), Angioguard (33.2%), FilterWire (24%), Emboshield (1%), and Spider (.5%). Diabetes (P = .04) and prior carotid endarterectomy (CEA, P = .03) were associated with adverse outcome. Prior stroke (P = .01) and prior CEA (P = .04) were significant for peri-procedural stroke. Design characteristics such as capture efficiency were associated with favorable outcomes. CONCLUSIONS: Patients with prior CEA or stroke are more likely to have unfavorable CAS outcomes after 30 days. Filters with high capture efficiency may yield the best clinical results. Analysis of the effect of design characteristics on CAS outcome should aid the design of future devices.

A relation between near-wall particle-hemodynamics and onset of thrombus formation in abdominal aortic aneurysms.

A novel computational particle-hemodynamics analysis of key criteria for the onset of an intraluminal thrombus (ILT) in a patient-specific abdominal aortic aneurysm (AAA) is presented. The focus is on enhanced platelet and white blood cell residence times as well as their elevated surface-shear loads in near-wall regions of the AAA sac. The generalized results support the hypothesis that a patient's AAA geometry and associated particle-hemodynamics have the potential to entrap activated blood particles, which will play a role in the onset of ILT. Although the ILT history of only a single patient was considered, the modeling and simulation methodology provided allow for the development of an efficient computational tool to predict the onset of ILT formation in complex patient-specific cases.

FSI Analysis of a healthy and a stenotic human trachea under impedance-based boundary conditions.

In this work, a fluid-solid interaction (FSI) analysis of a healthy and a stenotic human trachea was studied to evaluate flow patterns, wall stresses, and deformations under physiological and pathological conditions. The two analyzed tracheal geometries, which include the first bifurcation after the carina, were obtained from computed tomography images of healthy and diseased patients, respectively. A finite element-based commercial software code was used to perform the simulations. The tracheal wall was modeled as a fiber reinforced hyperelastic solid material in which the anisotropy due to the orientation of the fibers was introduced. Impedance-based pressure waveforms were computed using a method developed for the cardiovascular system, where the resistance of the respiratory system was calculated taking into account the entire bronchial tree, modeled as binary fractal network. Intratracheal flow patterns and tracheal wall deformation were analyzed under different scenarios. The simulations show the possibility of predicting, with FSI computations, flow and wall behavior for healthy and pathological tracheas. The computational modeling procedure presented herein can be a useful tool capable of evaluating quantities that cannot be assessed in vivo, such as wall stresses, pressure drop, and flow patterns, and to derive parameters that could help clinical decisions and improve surgical outcomes.

The effect of asymmetry in abdominal aortic aneurysms under physiologically realistic pulsatile flow conditions.

In the abdominal segment of the human aorta under a patient's average resting conditions, pulsatile blood flow exhibits complex laminar patterns with secondary flows induced by adjacent branches and irregular vessel geometries. The flow dynamics becomes more complex when there is a pathological condition that causes changes in the normal structural composition of the vessel wall, for example, in the presence of an aneurysm. This work examines the hemodynamics of pulsatile blood flow in hypothetical three-dimensional models of abdominal aortic aneurysms (AAAs). Numerical predictions of blood flow patterns and hemodynamic stresses in AAAs are performed in single-aneurysm, asymmetric, rigid wall models using the finite element method. We characterize pulsatile flow dynamics in AAAs for average resting conditions by means of identifying regions of disturbed flow and quantifying the disturbance by evaluating flow-induced stresses at the aneurysm wall, specifically wall pressure and wall shear stress. Physiologically realistic abdominal aortic blood flow is simulated under pulsatile conditions for the range of time-average Reynolds numbers 50 < or = Rem < or = 300, corresponding to a range of peak Reynolds numbers 262.5 < or = Repeak < or = 1575. The vortex dynamics induced by pulsatile flow in AAAs is depicted by a sequence of four different flow phases in one period of the cardiac pulse. Peak wall shear stress and peak wall pressure are reported as a function of the time-average Reynolds number and aneurysm asymmetry. The effect of asymmetry in hypothetically shaped AAAs is to increase the maximum wall shear stress at peak flow and to induce the appearance of secondary flows in late diastole.

Blood flow in abdominal aortic aneurysms: pulsatile flow hemodynamics.

Numerical predictions of blood flow patterns and hemodynamic stresses in Abdominal Aortic Aneurysms (AAAs) are performed in a two-aneurysm, axisymmetric, rigid wall model using the spectral element method. Physiologically realistic aortic blood flow is simulated under pulsatile conditions for the range of time-averaged Reynolds numbers 50< or =Re(m)< or =300, corresponding to a range of peak Reynolds numbers 262.5< or =Re(peak) < or = 1575. The vortex dynamics induced by pulsatile flow in AAAs is characterized by a sequence of five different flow phases in one period of the flow cycle. Hemodynamic disturbance is evaluated for a modified set of indicator functions, which include wall pressure (p(w)), wall shear stress (tau(w)), and Wall Shear Stress Gradient (WSSG). At peak flow, the highest shear stress and WSSG levels are obtained downstream of both aneurysms, in a pattern similar to that of steady flow. Maximum values of wall shear stresses and wall shear stress gradients obtained at peak flow are evaluated as a function of the time-average Reynolds number resulting in a fourth order polynomial correlation. A comparison between predictions for steady and pulsatile flow is presented, illustrating the importance of considering time-dependent flow for the evaluation of hemodynamic indicators.