Carotid arterial plaque stress analysis using fluid-structure interactive simulation based on in-vivo magnetic resonance images of four patients

The rupture of atherosclerotic plaques is known to be associated with the stresses that act on or within the arterial wall. The extreme wall tensile stress (WTS) is usually recognized as a primary trigger for the rupture of vulnerable plaque. The present study used the in-vivo high-resolution multi-spectral magnetic resonance imaging (MRI) for carotid arterial plaque morphology reconstruction. Image segmentation of different plaque components was based on the multi-spectral MRI and co-registered with different sequences for the patient. Stress analysis was performed on totally four subjects with different plaque burden by fluid-structure interaction (FSI) simulations. Wall shear stress distributions are highly related to the degree of stenosis, while the level of its magnitude is much lower than the WTS in the fibrous cap. WTS is higher in the luminal wall and lower at the outer wall, with the lowest stress at the lipid region. Local stress concentrations are well confined in the thinner fibrous cap region, and usually locating in the plaque shoulder; the introduction of relative stress variation during a cycle in the fibrous cap can be a potential indicator for plaque fatigue process in the thin fibrous cap. According to stress analysis of the four subjects, a risk assessment in terms of mechanical factors could be made, which may be helpful in clinical practice. However, more subjects with patient specific analysis are desirable for plaque-stability study.

In vivo non-invasive high resolution MR-based method for the determination of the elastic modulus of arterial vessels

The mechanical properties of arterial walls have long been recognized to play an essential role in the development and progression of cardiovascular disease (CVD). Early detection of variations in the elastic modulus of arteries would help in monitoring patients at high cardiovascular risk stratifying them according to risk. An in vivo, non-invasive, high resolution MR-phase-contrast based method for the estimation of the time-dependent elastic modulus of healthy arteries was developed, validated in vitro by means of a thin walled silicon rubber tube integrated into an existing MR-compatible flow simulator and used on healthy volunteers. A comparison of the elastic modulus of the silicon tube measured from the MRI-based technique with direct measurements confirmed the method's capability. The repeatability of the method was assessed. Viscoelastic and inertial effects characterizing the dynamic response of arteries in vivo emerged from the comparison of the pressure waveform and the area variation curve over a period. For all the volunteers who took part in the study the elastic modulus was found to be in the range 50-250 kPa, to increase during the rising part of the cycle, and to decrease with decreasing pressure during the downstroke of systole and subsequent diastole.

Measurement of stenotic carotid arterial compliance with MRI

Arterial compliance has been shown to correlate well with overall cardiovascular outcome and it may also be a potential risk factor for the development of atheromatous disease. This study assesses the utility of 2-D phase contrast Magnetic Resonance (MR) imaging with intra-sequence blood pressure measurement to determine carotid compliance and distensibility. 20 patients underwent 2-D phase contrast MR imaging and also ultrasound-based wall tracking measurements. Values for carotid compliance and distensibility were derived from the two different modalities and compared. Linear regression analysis was utilised to determine the extent of correlation between MR and ultrasound derived parameters. In those variables that could be directly compared, an agreement analysis was undertaken. MR measures of compliance showed a good correlation with measures based on ultrasound wall-tracking (r=0.61, 95% CI 0.34 to 0.81 p=0.0003). Vessels that had undergone carotid endarterectomy previously were significantly less compliant than either diseased or normal contralateral vessels (p=0.04). Agreement studies showed a relatively poor intra-class correlation coefficient (ICC) between diameter-based measures of compliance through either MR or ultrasound (ICC=0.14). MRI based assessment of local carotid compliance appears to be both robust and technically feasible in most subjects. Measures of compliance correlate well with ultrasound-based values and correlate best when cross-sectional area change is used rather than derived diameter changes. If validated by further larger studies, 2-D phase contrast imaging with intra-sequence blood pressure monitoring and off-line radial artery tonometry may provide a useful tool in further assessment of patients with carotid atheroma.

Mathematical model of arterial stenosis

A mathematical model for pulsatile flow in a partially occluded tube is presented. The problem has applications in studying the effects of blood flow characteristics on atherosclerotic development. The model brings out the importance of the pulsatility of blood flow on separation and the stress distribution. The results obtained show fairly good agreement with the available experimental results.

Chronic and acute stress in atherosclerosis : the role of endothelial function and arterial elasticity

Atherosclerosis is the main underlying pathology of coronary heart disease. Coronary heart disease is a serious health problem in Finland, and it is the leading cause of morbidity and mortality in industrialized countries. Psychological stress correlates with coronary heart disease events – myocardial infarction and sudden death, which are the most common clinical syndromes of atherosclerotic narrowing of arteries. The present series of studies examines the interaction between stress and endothelial function in relation to atherosclerosis. The study also aims to give new information on the mechanisms through which stress has its effect on atherosclerosis progression, focusing on possible relations between psychological stress and the functioning of the endothelium. Our project is based on data from one of the largest national epidemiological studies, the Cardiovascular Risk in Young Finns study, which has monitored the development of risk factors for coronary heart disease in 3596 young adults since 1980. The present study combines experimental stress research with epidemiology and uses an advanced method for examining atherosclerosis development in healthy subjects (intima-media thickness ultrasound measurement). The physiological parameters used were heart rate, respiratory sinus arrhythmia and pre-ejection period. Chronic stress was assessed by vital exhaustion. The ultrasound measurements that served as the indexes of preclinical atherosclerosis were carotid intima-media thickness, brachial flow-mediated dilatation and carotid artery compliance. The effects of cardiovascular risk factors found to be important were taken into account: serum cholesterols level, triglyceride level, serum insulin level and systolic and diastolic blood pressure. There were 69, 1596, 81 and 1721 participants in studies I-IV, respectively. The results showed that both chronic and acute stress may exert an effect on atherosclerosis in subjects with impaired endothelial responses. The findings are consistent with the idea that risk factors are more harmful if the endothelium is not working properly. Chronic stress was found to be a risk if it has resulted in ineffective cardiac stress reactivity or delayed recovery. Men were shown to be at increased risk for atherosclerotic progression in early life, which suggests men’s decreased stress coping ability in relation to stressful psychosocial coronary risk factors. Autonomic imbalance may be the common mechanism of the stress influence on atherosclerosis development. The results of the present study contain background information for the identification the first stages of atherosclerosis, and they may be useful for preventive medicine programs for young adults and could help to improve cardiovascular health in Finland as well as in other countries.

Mathematical model of an arterial stenosis, allowing for tethering

Closed-form solutions are presented for approximate equations governing the pulsatile flow of blood through models of mild axisymmetric arterial stenosis, taking into account the effect of arterial distensibility. Results indicate the existence of back-flow regions and the phenomenon of flow-reversal in the cross-sections. The effects of pulsatility of flow and elasticity of vessel wall for arterial blood flow through stenosed vessels are determined.

Neck length and mean arterial pressure in the sauropod dinosaurs

How blood was able to reach the heads of the long-necked sauropod dinosaurs has long been a matter of debate and several hypotheses have been presented. For example, it has been proposed that sauropods had exceptionally large hearts, multiple ‘normal’ sized hearts spaced at regular intervals up the neck or held their necks horizontal, or that the siphon effect was in operation. By means of an experimental model, we demonstrate that the siphon principle is able to explain how blood was able to adequately perfuse the sauropod brain. The return venous circulation may have been protected from complete collapse by a structure akin to the vertebral venous plexus. We derive an equation relating neck height and mean arterial pressure, which indicates that with a mean arterial pressure similar to that of the giraffe, the maximum safe vertical distance between heart and head would have been about 12 m. A hypothesis is presented that the maximum neck length in the fossil record is due to the siphon height limit. The equation indicates that to migrate over high ground, sauropods would have had to either significantly increase their mean arterial pressure or keep their necks below a certain height dependent on altitude.

Arterial stiffness and cardiovascular risk factors

Background: As the human body ages, the arteries gradually lose their elasticity and become stiffer. Although inevitable, this process is influenced by hereditary and environmental factors. Interestingly, many classic cardiovascular risk factors affect the arterial stiffness. During the last decade, accelerated arterial stiffening has been recognized as an important cardiovascular risk factor associated with increased mortality as well as with several chronic disorders. Objectives: This thesis examines the role of arterial stiffness in relation to variations in a physiological feature in healthy individuals. In addition, the effect on arterial stiffness of an acute transitory disease and the effect of a chronic disease are studied. Furthermore, the thesis analyzes the prognostic value of a marker of arterial stiffness in individuals with chronic disease. Finally, a potential method of reducing arterial stiffness is evaluated. Material and study design: The first study examines pulse wave reflection and pulse wave velocity in relation to muscle fibre distribution in healthy middle-aged men. In the second study, pulse wave reflection in women with current or previous preeclampsia is compared to a healthy control group. The effect of aging on the different blood pressure indices in patients with type 1 diabetes is examined in the third study, whereas the fourth paper studies the relation between these blood pressure indices and mortality in type 2 diabetes. The fifth study evaluates how intake of a fermented milk product containing bioactive peptides affects pulse wave reflection in individuals with mild hypertension. Results and conclusions: Muscle fibre type distribution is not an independent determinant of arterial stiffness in middle-aged males. Pulse wave reflection is increased in pregnant women with preeclampsia, but not in previously preeclamptic non-pregnant women. Patients with type 1 diabetes have a higher and more rapidly increasing pulse pressure, which suggests accelerated arterial stiffening. In elderly type 2 diabetic patients, very high and very low levels of pulse pressure are associated with higher mortality. Intake of milk-derived bioactive peptides reduces pulse wave reflection in hypertensive males but not in hypertensive females.

Macromolecular Transport Through Porous Arterial Walls

Arteries are heterogeneous, composite structures that undergo large cyclic deformations during blood transport. Presence, build-up and consequent rupture of blockages in blood vessels, called atherosclerotic plaques, lead to disruption in the blood flow that can eventually be fatal. Abnormal lipid profile and hypertension are the main risk factors for plaque progression. Treatments span from pharmacological methods, to minimally invasive balloon angioplasty and stent procedures, and finally to surgical alternatives. There is a need to understand arterial disease progression and devise methods to detect, control, treat and manage arterial disease through early intervention. Local delivery through drug eluting stents also provide an attractive option for maintaining vessel integrity and restoring blood flow while releasing controlled amount of drug to reduce and alleviate symptoms. Development of drug eluting stents is hence interesting albeit challenging because it requires an integration of knowledge of mechanical properties with material transport of drug through the arterial wall to produce a desired biochemical effect. Although experimental models are useful in studying such complex multivariate phenomena, numerical models of mass transport in the vessel have proved immensely useful to understand and delineate complex interactions between chemical species, physical parameters and biological variables. The goals of this review are to summarize literature based on studies of mass transport involving low density lipoproteins in the arterial wall. We also discuss numerical models of drug elution from stents in layered and porous arterial walls that provide a unique platform that can be exploited for the design of novel drug eluting stents.

Blood pressure evaluation using sphygmomanometry assisted by arterial pulse waveform detection by fiber Bragg grating pulse device

We report a blood pressure evaluation methodology by recording the radial arterial pulse waveform in real time using a fiber Bragg grating pulse device (FBGPD). Here, the pressure responses of the arterial pulse in the form of beat-to-beat pulse amplitude and arterial diametrical variations are monitored. Particularly, the unique signatures of pulse pressure variations have been recorded in the arterial pulse waveform, which indicate the systolic and diastolic blood pressure while the patient is subjected to the sphygmomanometric blood pressure examination. The proposed method of blood pressure evaluation using FBGPD has been validated with the auscultatory method of detecting the acoustic pulses (Korotkoff sounds) by an electronic stethoscope. (C) 2013 Society of Photo-Optical Instrumentation Engineers (SPIE)