CT-based fractional flow reserve: development and expanded application

Computations of fractional flow reserve, based on CT coronary angiography and computational fluid dynamics (CT-based FFR) to assess the severity of coronary artery stenosis, was introduced around a decade ago and is now one of the most successful applications of computational fluid dynamic modelling in clinical practice. Although the mathematical modelling framework behind this approach and the clinical operational model vary, its clinical efficacy has been demonstrated well in general. In this review, technical elements behind CT-based FFR computation are summarised with some key assumptions and challenges. Examples of these challenges include the complexity of the model (such as blood viscosity and vessel wall compliance modelling), whose impact has been debated in the research. Efforts made to address the practical challenge of processing time are also reviewed. Then, further application areas – myocardial bridge, renal stenosis and lower limb stenosis – are discussed along with specific challenges expected in these areas.

Role of Arterial Pressure, Wall Stiffness, Pulse Pressure and Waveform in Arterial Wall Stress/Strain and Its Clinical Implications

Biomechanical stress applied to the intima of arteries has long been suspected as a factor in the initiation and localisation of atherosclerotic plaque, and it is implicated in the separation of plaque from the underlying arterial wall giving rise to the acute clinical consequences of thrombosis, dissection and embolism. The factors underlying transmural stress were investigated in-vitro using fresh porcine abdominal aortas on an experimental rig in which pulse pressure, pulse waveform, fluid viscosity, pulse rate, vessel wall compliance and systolic and diastolic blood pressure could be varied at will. Vessel wall compliance was progressively reduced by exposure of the artery to formaldehyde vapour for increased periods of time, a saline-treated artery being used as control. Centripetal transmural stress (CTS) and strain were studied by direct observation of the displacement of a compliant false intima (FI) using real-time B and M mode ultrasound, and by measuring the differential pressure between the space beneath the FI and the adjacent vessel lumen. CTS was found to be directly related to pulse pressure (r = 0.907, p < 0.001) and inversely related to vessel wall compliance. It was independently affected by ranked peak pressure waveform (R = 0.93, p < 0.01) being higher with sharp peak pressure and lower when the waveform was rounded, and it peaked in early diastole in untreated vessels, and both in diastole and peak systole in ones stiffened by formaldehyde vapour. Mean arterial pressure exerted a profound effect via its effect on vessel wall stiffness, which was found to rise 7-fold across the mean arterial pressure range 50-130 mmHg and continued to increase in a logarithmic fashion as the upper physiological range of mean arterial pressure was exceeded. There are two potential clinical implications: in mitigating the postulated biomechanical aspects atherogenesis and atherosclerotic plaque detachment, maintaining large vessel wall compliance is important, and the main factor determining this in a healthy artery is mean arterial pressure; if the arterial wall has already become stiffened as a result of disease, and in the absence of critical stenosis, the findings suggest that the appropriate therapeutic targets are modification of pulse pressure and pulse waveform profile. Simply reducing the diastolic pressure in elderly patients may be unwise if the result is a widened pulse pressure and increased transmural strain. The distribution of atheroma at points of focal mechanical strain in the vessel wall may be explicable if the stress induced by an excessive pulse pressure provokes the inflammatory changes seen in repetitive strain injury. Investigation of inflammatory signalling in the vessel wall provoked by repeated mechanical stress may represent a productive area for future research.

Hemodynamic Parameters After Prone Positioning of COVID-19 Patients

Aim of the study. To examine the effect of prone positioning on hemodynamics in patients with COVID-19.Materials and methods. The study enrolled 84 patients of both sexes with community-acquired multisegmental viral and bacterial pneumonia associated with COVID-19, who were divided into groups according to the type of respiratory support. The tests were performed using the integrated hardware and software system for noninvasive central hemodynamic assessment by volumetric compression oscillometry.Results. We found that the pulse blood pressure velocity decreased from 281 [242.0; 314.0] to 252 [209; 304] mm Hg/s in patients with severe COVID-19 on oxygen support (p=0.005); volume ejection rate decreased from 251 [200; 294] to 226 [186; 260] ml/s (P=0.03); actual/estimated normalized vascular resistance ratio dropped from 0.549 [0.400; 0.700] to 0.450 [0.300; 0.600] (P=0.002), while the arterial wall compliance increased from 1.37 [1.28; 1.67] to 1.45[1.10; 1.60] ml/mm Hg (P=0.009). Prone positioning of patients on noninvasive lung ventilation associated with a reduction of linear blood flow rate from 40.0 [34.0; 42.0] to 42.5 [42.5; 47.25] cm/s (7=0.04) and arterial wall compliance from 1.4 [1.24; 1.50] to 1.32 [1.14; 1.49] ml/mm Hg (7=0.03). Prone positioning of patients on invasive lung ventilation did not result in significant hemodynamic changes.Conclusion. The greatest hemodynamic changes during prone positioning were found in patients on oxygen respiratory support, whereas the least significant alterations were seen in patients on invasive ventilatory support.

Calculating the work of breathing during mechanical ventilation.

Left figure: Passive patient esophageal pressure (Pes) in cmH2O on x-axis versus tidal volume in ml on y-axis. Green dashed line represents the chest wall compliance Right figure: same patient actively breathing on pressure support ventilation. (Pes) in cmH2O on x-axis versus tidal volume in ml on y-axis. Green dashed line represents the chest wall compliance. Red shaded area is the Campbell diagram representing the inspiratory work of breathing

Experiments and Modeling of a Compliant Wall Response to a Turbulent Boundary Layer with Dynamic Roughness Forcing

The response of a compliant surface in a turbulent boundary layer forced by a dynamic roughness is studied using experiments and resolvent analysis. Water tunnel experiments are carried out at a friction Reynolds number of Reτ≈410, with flow and surface measurements taken with 2D particle image velocimetry (PIV) and stereo digital image correlation (DIC). The narrow band dynamic roughness forcing enables analysis of the flow and surface responses coherent with the forcing frequency, and the corresponding Fourier modes are extracted and compared with resolvent modes. The resolvent modes capture the structures of the experimental Fourier modes and the resolvent with eddy viscosity improves the matching. The comparison of smooth and compliant wall resolvent modes predicts a virtual wall feature in the wall normal velocity of the compliant wall case. The virtual wall is revealed in experimental data using a conditional average informed by the resolvent prediction. Finally, the change to the resolvent modes due to the influence of wall compliance is studied by modeling the compliant wall boundary condition as a deterministic forcing to the smooth wall resolvent framework.

Numerical modeling in arterial hemodynamics incorporating fluid-structure interaction and microcirculation

Background The effects of arterial wall compliance on blood flow have been revealed using fluid-structure interaction in last decades. However, microcirculation is not considered in previous researches. In fact, microcirculation plays a key role in regulating blood flow. Therefore, it is very necessary to involve microcirculation in arterial hemodynamics. Objective The main purpose of the present study is to investigate how wall compliance affects the flow characteristics and to establish the comparisons of these flow variables with rigid wall when microcirculation is considered. Methods We present numerical modeling in arterial hemodynamics incorporating fluid-structure interaction and microcirculation. A novel outlet boundary condition is employed to prescribe microcirculation in an idealised model. Results The novel finding in this work is that wall compliance under the consideration of microcirculation leads to the increase of wall shear stress in contrast to rigid wall, contrary to the traditional result that wall compliance makes wall shear stress decrease when a constant or time dependent pressure is specified at an outlet. Conclusions This work provides the valuable study of hemodynamics under physiological and realistic boundary conditions and proves that wall compliance may have a positive impact on wall shear stress based on this model. This methodology in this paper could be used in real model simulations.

The effect of estimating chest wall compliance on the work of breathing during exercise as determined via the modified Campbell diagram

The modified Campbell diagram provides one of the most comprehensive assessments of the work of breathing (Wb) during exercise, wherein the resistive and elastic work of inspiration and expiration are quantified. Importantly, a necessary step in constructing the modified Campbell diagram is to obtain a value for chest wall compliance (CCW). To date, it remains unknown whether estimating or directly measuring CCW impacts on the Wb as determined by the modified Campbell diagram. Therefore, the purpose of this study was to evaluate whether the components of the Wb differ when the modified Campbell diagram is constructed using an estimated versus measured value of CCW. Forty-two participants (n = 26 men, 16 women) performed graded exercise to volitional exhaustion on a cycle ergometer. CCW was measured directly at rest via quasi-static relaxation. Estimated values of CCW were taken from prior literature. The measured value of CCW was greater than that obtained via estimation (214 ± 52 mL∙cmH2O-1 vs. 189 ± 18 mL∙cmH2O-1, p < 0.05). At modest to high minute ventilations (i.e., 50-200 L∙min-1), the inspiratory elastic Wb was greater, and expiratory resistive Wb was lower, when modified Campbell diagrams were constructed using estimated compared with measured values of CCW (p < 0.05). These differences were however small, and never exceeded ±5%. Thus, although our findings demonstrate that estimating CCW has a measurable impact on the determination of the Wb, its effect appears relatively small within a cohort of healthy adults during graded exercise.