Chapter 11 - Fast closed-loop CFD model for patient-specific aortic-dissection management

To explore the diagnostic performance of 4-dimensional phase-contrast magnetic resonance imaging (4D PC-MRI) in evaluating aortic dissection in different clinical scenarios. The study group comprised 32 patients with a known aortic dissection who each underwent computed tomography angiography (CTA), and then 4D PC-MRI with a 1.5-T MR scanner. The 4D PC-MRI images were compared with the CTA images to evaluate the aortic size, branch identification, and iliac and femoral arterial access. The patients were divided into three groups: (I) patients diagnosed with Type B aortic dissection but did not undergo intervention (n=8); (II) patients with residual aortic dissection after open repair of Type A dissection (n=7); (III) patients who underwent endovascular aortic repair with or without open surgery (n=17). Without radiation or contrast media injection, 4D PC-MRI provided similar aortic images for patients in Group 1 and most of those in Group 2. In Group 3, stainless steel stents affected image quality in three patients. High-quality 4D PC-MRI images were obtained for the remaining 14 patients in Group 3, who had non-stainless steel stents, and provided major aortic information comparable to that provided by CTA with contrast media. The hemodynamic parameters of true and false lumens were evaluated between three patients with Type B aortic dissections and three patients who underwent thoracic endovascular aortic repair for their aortic dissection. The stroke volume was higher in the true lumen of the patients with stent-grafts than in the patients with Type B aortic dissection without intervention. The regurgitant fraction, an indicator of nonlaminar flow, was higher in the false lumens than in the true lumens. All 32 patients in this study tolerated 4D PC-MRI without adverse events. 4D PC-MRI is radiation- and contrast media-free option for imaging aortic dissection. It not only provided images comparable in quality to those obtained with CTA but also provided information on hemodynamic parameters, including endoleak detection after thoracic endovascular aortic repair. 4D PC-MRI was safe and accurate in evaluating chronic Type B aortic dissection and residual aortic dissection after surgery for acute Type A aortic dissection. Therefore, it could be a potential tool in treating pathology in aortic dissection, especially for patients with malperfusion syndrome of visceral vessels and in young patients with renal function impairment. However, certain endograft materials, especially stainless steel, may prevent the further application of 4D PC-MRI and should be avoided.

Human tolerance to hypergravity is of increasing importance given the rising number of spaceflight participants. In this paper, we present a mathematical model of the human circulatory system that can predict an individual’s tolerance to hypergravity. We adopt a steady-state approach, where simplicity enables us to predict an individual’s tolerance to high gravitational acceleration (G-tolerance) with lower computational cost. Moreover, while our parameter set can be personalized to individual parameters, the model only requires cardiac output, three anthropometric measurements, vital signs, and sex to make a satisfactory prediction. Key features of the model include compartmentalization of the upper and lower circulation to allow for gravitational acceleration in the vertical direction to be varied, a model of venous collapse of the systemic veins, and feedback control to simulate regulation of heart rate and reserve volume. We also provide a case study of parameter calibration to predict G-tolerance for a single subject based on physiologic measurements before and during centrifuge-induced hypergravity. Simulation results were consistent with physiological expectations and subject report of G-related symptoms. These findings suggest that our model of the circulatory system has the potential to predict G-tolerance and serve as a clinical decision support tool to risk stratify subjects prior to spaceflight.

Aortic dissections, which split the aorta into a true lumen (TL) and a false lumen (FL), represent a serious medical condition, affecting otherwise healthy young people with an incidence between 5,000–10,000 cases per year in the United States and 3000 in Europe [1]. A recent study of the outcome of acute type III/ Stanford B aortic dissections (dissections confined to the descending aorta, B-AD) revealed that the long-term prognosis after hospital discharge of patients with B-AD is heterogeneous, with reported survival rates ranging from 56 to 92% at 1 year and from 48 to 82% at 5 years [2]. A partially thrombosed FL, results in a 2.7-fold increase in risk of death [3]. In a recent ex-vitro study, Tasi et al. investigated a chronic aortic dissection in three models [4]. The largest FL diastolic pressure was observed for the model simulating patients with partial false lumen thrombosis and occlusion of the distal entry tear. This study demonstrated that pressure differences between TL and FL are dependent on the geometry of the particular aortic dissection and the location and size of entry tears. A computational fluid dynamics (CFD) study on the effects of entry and exit tear coverage in B-AD based on a patient-derived geometry reported similar results [5]: In particular, occlusion of the exit tear caused increase FL pressure. Simulating thoracic endovascular repair (TEVAR) by occluding the entrance tear depressurized the FL. The capability of CFD to virtually simulate surgical interventions makes it an appealing method for use in pre-surgical planning. For general acceptance however, validation of the simulated results is needed. Catheter measurements of the pressures in the TL and FL are feasible but not very practicable as insertion of a catheter in the FL through the entry or exit tear bears unjustifiable risk to the patient. More recently, 7D phase contrast magnetic resonance imaging (pcMRI) methods (3 spatial directions, 3 velocity directions and time equal 7 dimensions) have been introduced that allow the acquisition of the 3D velocity field at several time points in the cardiac cycle, thereby providing information that can be directly compared with the velocity field simulated with CFD. Due to the large duration of the image acquisition, compromises in temporal and spatial resolution are made which need to be considered when performing such a comparison. Here we present a method based on interpolating the simulated velocity field onto a structured grid employing direct interpolation and spatial Fourier Fast Transformation (FFT) to replicate artifacts as they are present in the 7D pcMRI data. The interpolated velocity components are the then qualitatively compared using image correlation analysis.

Theoretic Impact of Infarct Compliance on Left Ventricular Function

To characterize the interaction of the heart with the pulmonary and systemic circulations, engineers developed a minimal closed‐loop cardiovascular model, predicting cardiac output and pressures from 18 critical parameters. Despite the need for numerical solutions, it has potential to be used by clinical investigators to study the mechanical basis of altered hemodynamic status and by physiologists to study homeostatic mechanisms. To capture the complex interplay between hemodynamics and structural adaptation, we modified the standard minimal model to make structural parameters adapt to local hemodynamics. First, we linearized all pressure‐volume and pressure‐flow relationships. Then we made the simplifying assumption that end‐systolic pressure approximates mean arterial pressure. Finally, we assumed that each parameter adapts as a linear function of regional pressure. Yielding cardiac output and mean pressures, our model reproduces basic adaptive behaviors such as cardiomegaly in response to decreases in contractility and diastolic dysfunction in response to decreases in sensitivity of the adaptive process. More importantly, by judicious use of simplifying assumptions, all solutions can be expressed algebraically, which makes this fundamental model accessible to physiologists, clinical investigators, and students.

A model of human cardiovascular system is proposed to describe the main heart rhythm, influence of autonomous regulation on frequency and strength of heart contractions and resistance of arterial vessels; process of formation of arterial pressure during systolic and diastolic phases; influence of respiration; synchronization between loops of autonomous regulation. The proposed model is used to simulate the dynamics of heart rate and arterial pressure during passive transition from supine to upright position. Results of mathematical modeling are compared to original experimental data.

A transmission line model of the human foetal circulatory system

The purpose of this paper is to present a model of the circulatory system, with central baroreflex control that has been developed in the Biomedical Engineering Laboratory of the University of Santa Catarina, Brazil. This model was conceived to match some concepts of circulatory system optimization. The main objective is to characterize the relative influences of baroreflex and muscular pump on the first seconds of exercise.

Age-related changes in aortic biomechanics can impact the brain by reducing blood flow and increasing pulsatile energy transmission. Clinical studies have shown that impaired cardiac function in patients with heart failure is associated with cognitive impairment. Although previous studies have attempted to elucidate the complex relationship between age-associated aortic stiffening and pulsatility transmission to the cerebral network, they have not adequately addressed the effect of interactions between aortic stiffness and left ventricle (LV) contractility (neither on energy transmission nor on brain perfusion). In this study, we use a well-established and validated one-dimensional blood flow and pulse wave computational model of the circulatory system to address how age-related changes in cardiac function and vasculature affect the underlying mechanisms involved in the LV-aorta-brain hemodynamic coupling. Our results reveal how LV contractility affects pulsatile energy transmission to the brain, even with preserved cardiac output. Our model demonstrates the existence of an optimal heart rate (near the normal human heart rate) that minimizes pulsatile energy transmission to the brain at different contractility levels. Our findings further suggest that the reduction in cerebral blood flow at low levels of LV contractility is more prominent in the setting of age-related aortic stiffening. Maintaining optimal blood flow to the brain requires either an increase in contractility or an increase in heart rate. The former consistently leads to higher pulsatile power transmission, and the latter can either increase or decrease subsequent pulsatile power transmission to the brain.NEW & NOTEWORTHY We investigated the impact of major aging mechanisms of the arterial system and cardiac function on brain hemodynamics. Our findings suggest that aging has a significant impact on heart-aorta-brain coupling through changes in both arterial stiffening and left ventricle (LV) contractility. Understanding the underlying physical mechanisms involved here can potentially be a key step for developing more effective therapeutic strategies that can mitigate the contributions of abnormal LV-arterial coupling toward neurodegenerative diseases and dementia.

A model of the arterial system is discussed which considers determination of the changes in density of energy dissipation of the pulse with the system entrance being the forcing of blood from the heart to the aorta. The effects of boundary conditions and branching upon dissipation density are analyzed on the basis of the model.

Time-varying elastance models can simulate only the pressure and volume signals in the heart chambers while the diagnosis of clinical cases and evaluation of different treatment techniques require more information. In this study, an extended model utilizing the geometric dimensions of the heart chambers was developed to describe the cardiac function. The new cardiac model was evaluated by simulating a healthy and dilated cardiomyopathy (DCM) condition for adults and children. The left ventricular ejection fraction, end-diastolic volume, end-diastolic diameter and diastolic sphericity index were 53.60%, 125 mL, 5.08 cm and 1.82 in the healthy adult cardiovascular system model and 23.70%, 173 mL, 6.60 cm and 1.40 in the DCM adult cardiovascular system model. In the healthy child cardiovascular system model, the left ventricular ejection fraction, end-diastolic volume, end-diastolic diameter and diastolic sphericity index were 59.70%, 92 mL, 4.10 cm and 2.26 respectively and 30.70%, 125 mL, 4.94 cm and 1.87 in the DCM child cardiovascular system model. The developed cardiovascular system model simulates the hemodynamic variables and clinical diagnostic indicators within the physiological range for healthy and DCM conditions proving the feasibility of this new model to evaluate clinical cases in adults and children.

ObjectiveIntravascular ultrasound (IVUS) is a useful adjunct to obtain precise intraluminal measurements during thoracic endovascular aortic repair (TEVAR), but IVUS examination is not routinely used by all surgeons or centers during TEVAR. We sought to identify patient- and system-related factors that influence the decision to use IVUS examination during TEVAR. MethodsA retrospective review of the Vascular Quality Initiative (VQI) database was performed to identify all patients undergoing TEVAR from 2015 to 2019. Multivariable logistic regression modeling with three-fold repeated cross-validation was performed to identify predictors of IVUS use during TEVAR. Association of IVUS use with contrast volume and radiation exposure was also assessed. ResultsA total of 12,414 patients undergoing TEVAR met the inclusion criteria. Of these, IVUS examination was used in 41.3% of cases (n = 5121). IVUS use was more common in younger patients with fewer comorbidities; however, IVUS examination was also more commonly deployed in symptomatic patients and those with a higher preoperative American Society of Anesthesiology classification. IVUS examination was use in 80% (n = 3385/4213) TEVAR procedures performed for type B aortic dissection, which accounted for 50% of total IVUS use and only 11% of cases during which IVUS examination was not used (n = 822/7293) (P < .01). In multivariable analysis, the strongest independent predictor of IVUS use was aortic dissection (odds ratio, 13.7; 95% confidence interval, 11.7-16.3; P < .001, with aortic aneurysm as the reference). Surgeon years of independent practice experience was not associated with IVUS use, but when accounting for clustering on physicians and geographic regions, these variables explained 15% of the variance observed in the final risk-adjusted model. After adjustment for confounding factors, IVUS use was associated with a significant decrease in fluoroscopy time and contrast volume (both P < .001). ConclusionsThe decision to use IVUS examination during TEVAR is most heavily influenced by aortic pathology. Although surgeon experience was not associated with the decision to use IVUS examination, there was substantial variation in IVUS examination use among individual surgeons and VQI regions. IVUS use was associated with decreased contrast administration and fluoroscopy use but did not appear to have an impact on survival or re-intervention. Although aortic dissection was strongly associated with IVUS use, a significant number of TEVAR for dissection were performed without IVUS examination. Further research is warranted to identify the barriers to IVUS use as well as the risks and benefits of IVUS use during TEVAR so that quality benchmarks can be established and resource use is improved.

Analysing requirements is a major challenge in the area of safety-critical software, where requirements quality is an important issue to build a dependable critical system. Most of the time, any project fails due to lack of understanding of user needs, missing functional and non-functional system requirements, inadequate methods and tools, and inconsistent system specification. This often results from the poor quality of system requirements. Based on our experience and knowledge, an environment model has been recognized to be a promising approach to support requirements engineering to validate a system specification. It is crucial to get an approval and feedback in early stage of system development to ensure completeness and correctness of requirements specification. In this paper, we propose a method for analysing system requirements using a closed-loop modelling technique. A closed-loop model is an integration of system model and environment model, where both the system and environment models are formalized using formal techniques. Formal verification of the closed-loop model helps to identify missing system requirements or new emergent behaviours, which are not covered earlier during the requirements elicitation process. Moreover, an environment model assists in the construction, clarification, and validation of the given system requirements.

Model reduction is a technique commonly used to reduce the computation time of structural dynamic models. The results of the reduced model can then be expanded to full space using the transformation matrix developed in the reduction process. This work focuses on expanding the mode shapes of system models that are comprised of reduced component models. Typically, this expansion requires the full space system model to be computed in order to obtain the system model transformation matrix needed to perform the expansion. Computing the full space system model to determine the expansion matrix does not save computation time and therefore defeats the purpose of using model reduction. This paper proposes using the expansion matrices of the individual components to expand the assembled system model modes. In this work, System Equivalent Reduction Expansion Process (SEREP) is used for reduction and expansion. The accuracy of the expanded system model is shown to be dependent on the modes retained in the reduced component models. Recent work on Variability Improvement of Key Inaccurate Node Groups (VIKING) has shown that over specifying the number of modes used in the reduction/expansion process that span the space of the system model modes significantly improves the results. The VIKING technique is the basis for the expansion process developed in this work. Multiple analytical cases are presented to show how the selection of component modes affects the expansion results. The analytical cases demonstrate that accurate system model expansion results can be obtained when a sufficient set of component modes that span the space of the system model modes are used.

A bond graph model of the cardiovascular system has been developed. With the model, changes in the blood pressure waveform are studied at different points in the systemic and pulmonary circulations, when exposed to a normal ventricular function. The waveforms thus obtained show a good model performance which reflects the actual waves, when compared with physiological human data. Wave reflection phenomena effects, capability in handling elements, addition of new compartments, and obtention of numerical data for spectral analysis are among the model features. These results and facilities offered by the intuitive and simple bond graph approach suggest its use, especially for life scientists, in the development of more detailed models for educational, research and clinical purposes.