Plaques vulnerable to rupture are characterized by a thin and stiff fibrous cap overlaying a soft lipid-rich necrotic core. The ability to measure local plaque stiffness directly to quantify plaque stress and predict rupture potential would be very attractive, but no current technology does so. This study seeks to validate the use of Brillouin microscopy to measure the Brillouin frequency shift, which is related to stiffness, within vulnerable plaques. The left carotid artery of an ApoE 2/2 mouse was instrumented with a cuff that induced vulnerable plaque development in nine weeks. Adjacent histological sections from the instrumented and control arteries were stained for either lipids or collagen content, or imaged with confocal Brillouin microscopy. Mean Brillouin frequency shift was 15.79 + 0.09 GHz in the plaque compared with 16.24 + 0.15 ( p , 0.002) and 17.16 + 0.56 GHz ( p , 0.002) in the media of the diseased and control vessel sections, respectively. In addition, frequency shift exhibited a strong inverse correlation with lipid area of 20.67 + 0.06 ( p , 0.01) and strong direct correlation with collagen area of 0.71 + 0.15 ( p , 0.05). This is the first study, to the best of our knowledge, to apply Brillouin spectroscopy to quantify atherosclerotic plaque stiffness, which motivates combining this technology with intravascular imaging to improve detection of vulnerable plaques in patients.

The optimal performance of the cardiovascular system, as well as the breakdown of this performance with disease, both involve complex biomechanical interactions between the heart, conduit vascular networks and microvascular beds. 'Wave analysis' refers to a group of techniques that provide valuable insight into these interactions by scrutinizing the shape of blood pressure and flow/velocity waveforms. The aim of this review paper is to provide a comprehensive introduction to wave analysis, with a focus on key concepts and practical application rather than mathematical derivations. We begin with an overview of invasive and non-invasive measurement techniques that can be used to obtain the signals required for wave analysis. We then review the most widely used wave analysis techniques-pulse wave analysis, wave separation and wave intensity analysis-and associated methods for estimating local wave speed or characteristic impedance that are required for decomposing waveforms into forward and backward wave components. This is followed by a discussion of the biomechanical phenomena that generate waves and the processes that modulate wave amplitude, both of which are critical for interpreting measured wave patterns. Finally, we provide a brief update on several emerging techniques/concepts in the wave analysis field, namely wave potential and the reservoir-excess pressure approach.

Exposure of endothelial cells to low and multidirectional blood flow is known to promote a pro-atherogenic phenotype. The mechanics of the vessel wall is another important mechano-stimulus within the endothelial cell environment, but no study has examined whether changes in the magnitude and direction of cell stretch can be pro-atherogenic. Herein, we developed a custom cell stretching device to replicate the in vivo stretch environment of the endothelial cell and examined whether low and multidirectional stretch promote nuclear translocation of NF-κB. A fluid–structure interaction model of the device demonstrated a nearly uniform strain within the region of cell attachment and a negligible magnitude of shear stress due to cyclical stretching of the cells in media. Compared to normal cyclical stretch, a low magnitude of cyclical stretch or no stretch caused increased expression of nuclear NF-κB (p = 0.09 and p < 0.001, respectively). Multidirectional stretch also promoted significant nuclear NF-κB expression, comparable to the no stretch condition, which was statistically higher than the low (p < 0.001) and normal (p < 0.001) stretch conditions. This is the first study to show that stretch conditions analogous to atherogenic blood flow profiles can similarly promote a pro-atherogenic endothelial cell phenotype, which supports a role for disturbed vessel wall mechanics as a pathological cell stimulus in the development of advanced atherosclerotic plaques.

With aging, a reduction in the stiffness gradient between elastic and muscular arteries is thought to reduce wave reflection in conduit arteries, leading to increased pulsatile pressure transmission into the microvasculature. This assumes that wave reflection limits pressure transmission in arteries. However, using a computational model, we showed that wave reflection promotes pulsatile pressure transmission, although it does limit hydraulic energy transmission. Increased microvascular pulse pressure with aging is instead related to decreasing arterial compliance.

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Fluid flow is an important regulator of cell function and metabolism in many tissues. Fluid shear stresses have been used to level the mechanical stimuli applied in vitro with what occurs in vivo. However, these experiments often lack dynamic similarity, which is necessary to ensure the validity of the model. For interstitial fluid flow, the major requirement for dynamic similarity is the Reynolds number (Re), the ratio of inertial to viscous forces, is the same between the system and model. To study the necessity of dynamic similarity for cell mechanotransduction studies, we investigated the response of osteocyte-like MLO-Y4 cells to different Re flows at the same level of fluid shear stress. Osteocytes were chosen for this study as flows applied in vitro and in vivo have Re that are orders of magnitude different. We hypothesize that osteocytes' response to fluid flow is Re dependent. We observed that cells exposed to lower and higher Re flows developed rounded and triangular morphologies, respectively. Lower Re flows also reduced apoptosis rates compared to higher Re flows. Furthermore, MLO-Y4 cells exposed to higher Re flows had stronger calcium responses compared to lower Re flows. However, by also controlling for flow rate, the lower Re flows induced a stronger calcium response; while degradation of components of the osteocyte glycocalyx reversed this effect. This work suggests that osteocytes are highly sensitive to differences in Re, independent of just shear stresses, supporting the need for improved in vitro flow platforms that better recapitulate the physiological environment. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:663-671, 2018.

The arterial network in healthy young adults is thought to be structured to minimise wave reflection in conduit arteries, producing an ascending aortic pressure waveform with three key features: early systolic peak, negative systolic augmentation, and diastolic hump. One-dimensional computer models have provided significant insights into arterial haemodynamics, but no previous models of the young adult have exhibited these three features. Since the latter was likely to be related to unrepresentative or non-optimised impedance properties of the model arterial networks, we developed a new &lsquo;YoungAdult&rsquo; model that incorporated 1) a novel and more accurate empirical equation for approximating wave speeds, based on area and relative distance to elastic-muscular arterial transition points, 2) optimally-matched arterial junctions, and 3) an improved arterial network geometry that eliminated &lsquo;within-segment&rsquo; taper (which causes wave reflection in conduit arteries) whilst establishing &lsquo;impedance-preserving&rsquo; taper. These model properties led to wave reflection occurring predominantly at distal vascular beds, rather than in conduit arteries. The model predicted all three typical characteristics of an ascending aortic pressure waveform observed in young adults. When compared with non-invasively acquired pressure and velocity measurements (obtained via tonometry and Doppler ultrasound in 7 young adults), the model was also shown to reproduce the typical waveform morphology observed in the radial, brachial, carotid, temporal, femoral, and tibial arteries. The YoungAdult model provides support for the concept that the arterial tree impedance in healthy young adults is exquisitely optimised, and it provides an important baseline model for investigating cardiovascular changes in ageing and disease states.

Arterial ageing is thought to cause a diastolic-to-systolic shift in the return time (RT) of backward waves to central arteries. However, current methods of estimating RT-inflection point, zero crossing, and foot methods-depend on a single waveform feature and produce systolic RT throughout life. We propose a novel centroid method that accounts for the entire backward pressure waveform and develop a ground truth RT (GTRT), which can be used in computational models to test the accuracy of RT estimation methods. Linear wave tracking was implemented in a one-dimensional systemic arterial tree model and GTRT was calculated as the amplitude-weighted mean RT of backward waves at the ascending aorta. Using a virtual cohort of 1200 patients, the centroid RT was closest to GTRT compared to the zero crossing, inflection point, and foot methods; mean differences (limits of agreement) were -8 (-47,30), vs -42 (-136,52), -78 (-305,149), and -197 (-379,-15) ms, respectively. The sensitivity of the methods to changes in RT was also assessed in ten sheep. A balloon catheter in the descending thoracic aorta was used to generate a backward-running pulse that arrived at the ascending aorta at different times during diastole or systole, allowing the "bulk" RT of the backward-running wave ensemble to be manipulated. Only the centroid method was sensitive to both diastolic and systolic changes in RT. We conclude that the accuracy and robustness of the centroid method make it most suitable for evaluating the diastolic-tosystolic shift in RT of backward waves with ageing.