Pressure peaking in pulsatile flow through arterial tree structures

Association of pulse wave velocity and pressure wave reflection with the ankle-brachial pressure index in Japanese men not suffering from peripheral artery disease

BACKGROUND AND AIMS

We examined the cross-sectional and longitudinal association of arterial stiffness and pressure wave reflection with the ankle-brachial pressure index (ABI) in middle-aged Japanese subjects free of peripheral artery disease (PAD).

METHODS

ABI, brachial-ankle pulse wave velocity (baPWV) and radial augmentation index (rAI) were measured annually during the 9-year observation period in 3066 men (42 ± 9 years old) with ABI ≥1.00 at baseline of the study period, and not taking any antihypertensive medication.

RESULTS

In the cross-sectional assessments, mediation analysis demonstrated that baPWV showed both direct and indirect (via the rAI) associations with ABI, and rAI showed both direct and indirect (via the heart-arm difference of systolic blood pressure) associations with the ankle-arm difference of systolic blood pressure, both at study baseline and end of study period. Mixed model linear regression analysis of the repeated-measurement data obtained over the 9-year observation period demonstrated that annual increase of baPWV (estimate = 0.73 × 10^-4, p < 0.01) and rAI (estimate = 0.33 × 10^-3, <0.01) was associated with ABI. When baPWV and rAI were entered into the same model, only baPWV showed a significant longitudinal association with ABI.

CONCLUSION

In middle-aged Japanese men free of PAD, arterial stiffness may contribute to ABI directly and via pressure wave reflection. Pressure wave reflection may contribute to ABI directly and, at least in part, via attenuation of peripheral pulse pressure amplification.

Coronary wave intensity patterns in stable coronary artery disease: influence of stenosis severity and collateral circulation

Objective

Wave intensity analysis is a method that allows separating pulse waves into components generated proximally and in the periphery of arterial trees, as well as characterising them as accelerating or decelerating. The early diastolic suction wave (eaDSW) is one of the most prominent wave events in the coronaries. The aim of this study was to determine whether (1) microvascular dilatation directly influences its energy, (2) stenosis severity can be assessed proximal to stenoses, (3) distal pulse wave entrapment exists in the presence of stenoses and (4) coronary collaterals influence wave entrapment.

Methods

In 43 coronary artery disease patients, Doppler flow velocity and pressure measurements were performed in a proximal coronary segment at rest, in a distal segment at rest, during adenosine-induced hyperaemia and during balloon occlusion. Wave energies were calculated as the area under the wave intensity curves.

Results

The eaDSW energy showed a significant increase during hyperaemia, but did not differ between proximal and distal segments. There was no significant correlation between eaDSW energy and coronary stenosis severity. Pulse wave entrapment could not be observed consistently in the distal segments. Consequently, the effect of coronary collaterals on pulse wave entrapment could not be studied.

Conclusions

Microvascular dilation in the coronary circulation increases distal eaDSW energy. However, it does not show any diagnostically useful variation between measurement sites, various stenosis degrees and amount of collateral flow. The assessment eaDSW and its reflections were not useful for the quantification of coronary stenosis severity or the collateral circulation in clinical practice.

Computational Assessment of Blood Flow Heterogeneity in Peritoneal Dialysis Patients' Cardiac Ventricles

Dialysis prolongs life but augments cardiovascular mortality. Imaging data suggests that dialysis increases myocardial blood flow (BF) heterogeneity, but its causes remain poorly understood. A biophysical model of human coronary vasculature was used to explain the imaging observations, and highlight causes of coronary BF heterogeneity. Post-dialysis CT images from patients under control, pharmacological stress (adenosine), therapy (cooled dialysate), and adenosine and cooled dialysate conditions were obtained. The data presented disparate phenotypes. To dissect vascular mechanisms, a 3D human vasculature model based on known experimental coronary morphometry and a space filling algorithm was implemented. Steady state simulations were performed to investigate the effects of altered aortic pressure and blood vessel diameters on myocardial BF heterogeneity. Imaging showed that stress and therapy potentially increased mean and total BF, while reducing heterogeneity. BF histograms of one patient showed multi-modality. Using the model, it was found that total coronary BF increased as coronary perfusion pressure was increased. BF heterogeneity was differentially affected by large or small vessel blocking. BF heterogeneity was found to be inversely related to small blood vessel diameters. Simulation of large artery stenosis indicates that BF became heterogeneous (increase relative dispersion) and gave multi-modal histograms. The total transmural BF as well as transmural BF heterogeneity reduced due to large artery stenosis, generating large patches of very low BF regions downstream. Blocking of arteries at various orders showed that blocking larger arteries results in multi-modal BF histograms and large patches of low BF, whereas smaller artery blocking results in augmented relative dispersion and fractal dimension. Transmural heterogeneity was also affected. Finally, the effects of augmented aortic pressure in the presence of blood vessel blocking shows differential effects on BF heterogeneity as well as transmural BF. Improved aortic blood pressure may improve total BF. Stress and therapy may be effective if they dilate small vessels. A potential cause for the observed complex BF distributions (multi-modal BF histograms) may indicate existing large vessel stenosis. The intuitive BF heterogeneity methods used can be readily used in clinical studies. Further development of the model and methods will permit personalized assessment of patient BF status.

Dynamic blood flow and wall shear stress in pulmonary hypertensive disease

This study provides new model of pulsatile flow in the pulmonary circulation in health and pulmonary hypertensive disease. Structural vascular remodeling typical of pulmonary hypertensive disease was implemented in the model by progressively altering the mechanical properties of the arterial geometry and progressively increasing the inlet pulse pressure (PP). The transmission of PP throughout the tree was shown to increase in advanced stages of disease, creating the potential for a `vicious-cycle' of damage to vasculature. Wall shear stress (WSS) was shown to be highest in the terminal arteries of the model and increased significantly with disease. A further trend observed in WSS results was that high WSS values began to `climb' the arterial tree towards the proximal vessels as disease progressed. This suggests a link between WSS and distal remodeling in pulmonary hypertensive disease, which initiates in the small muscular arteries and arterioles and spreads into larger arteries as the disease progresses.

Effects of changes in the physical properties of the central elastic artery on haemodynamic characteristics during ageing

The change in blood pressure during the ageing process depends on the changes in the physical properties of the arterial system. The conventional method of studying ageing, i.e. observing a focus group of people, requires long periods of time and also makes it difficult to separate the ageing effect from the effects of complex geriatric diseases. Therefore, a computer simulation was used to analyse how physical property changes in the central elastic artery during the ageing process affect the blood pressure and input impedance of the blood vessels. An increase in the arterial stiffness and an increase in the wall thickness increased the systolic pressure and the pulse pressure. An increase in the lumen diameter decreased the mean pressure. The pulse wave velocity and shape were changed in the central elastic artery group but were not changed outside this group. Also, the input impedance at a low frequency was increased in the central elastic artery group but was not changed outside this group. In this study, the way in which changes in specific physical properties of the artery affect the haemodynamic characteristics during ageing was analysed.

Biomechanics of coronary artery and bypass graft disease: potential new approaches

The contribution of biomechanical factors to the incidence and distribution of coronary artery and bypass graft disease is underrecognized. This review examined the literature to determine which factors were relevant and the evidence for their importance. It identified two primary biomechanical factors that predispose to disease: (1) low-wall shear stress and (2) high-wall mechanical stress or strain. A range of secondary biomechanical factors have also been identified and include: vessel geometry; vessel movement; vessel wall characteristics and the presence of reflection waves. Potential surgical approaches for minimizing these effects are discussed.

Wave intensity amplification and attenuation in non-linear flow: implications for the calculation of local reflection coefficients

Local reflection coefficients (R) provide important insights into the influence of wave reflection on vascular haemodynamics. Using the relatively new time-domain method of wave intensity analysis, R has been calculated as the ratio of the peak intensities (R(PI)) or areas (R(CI)) of incident and reflected waves, or as the ratio of the changes in pressure caused by these waves (R(DeltaP)). While these methods have not yet been compared, it is likely that elastic non-linearities present in large arteries will lead to changes in the size of waves as they propagate and thus errors in the calculation of R(PI) and R(CI). To test this proposition, R(PI), R(CI) and R(DeltaP) were calculated in a non-linear computer model of a single vessel with various degrees of elastic non-linearity, determined by wave speed and pulse amplitude (DeltaP(+)), and a terminal admittance to produce reflections. Results obtained from this model demonstrated that under linear flow conditions (i.e. as DeltaP(+)-->0), R(DeltaP) is equivalent to the square-root of R(PI) and R(CI) (denoted by R(PI)(p) and R(CI)(p)). However for non-linear flow, pressure-increasing (compression) waves undergo amplification while pressure-reducing (expansion) waves undergo attenuation as they propagate. Consequently, significant errors related to the degree of elastic non-linearity arise in R(PI) and R(CI), and also R(PI)(p) and R(CI)(p), with greater errors associated with larger reflections. Conversely, R(Delta)(P) is unaffected by the degree of non-linearity and is thus more accurate than R(PI) and R(CI).

System for the analysis and visualization of large 3D anatomical trees

Modern micro-CT and multi-detector helical CT scanners can produce high-resolution 3D digital images of various anatomical trees. The large size and complexity of these trees make it essentially impossible to define them interactively. Automatic approaches have been proposed for a few specific problems, but none of these approaches guarantee extracting geometrically accurate multi-generational tree structures. This paper proposes an interactive system for defining and visualizing large anatomical trees and for subsequent quantitative data mining. The system consists of a large number of tools for automatic image analysis, semi-automatic and interactive tree editing, and an assortment of visualization tools. Results are presented for a variety of 3D high-resolution images.

The Prognostic Value of Pulmonary Vascular Capacitance Determined by Doppler Echocardiography in Patients with Pulmonary Arterial Hypertension

OBJECTIVES

We sought to determine if a novel measurement of pulmonary vascular (PV) capacitance (PVCAP) by Doppler echocardiography predicts all-cause mortality in patients with primary pulmonary arterial (PA) hypertension (PPAH).

BACKGROUND

The prognosis of patients with PPAH is variable and has been difficult to predict using clinical or hemodynamic parameters. PVCAP is a measure of the workload on the right ventricle (RV) and we recently have shown that PVCAP determined by cardiac catheterization is a strong predictor of survival. This same hemodynamic information to calculate PVCAP can be derived from Doppler echocardiography. Therefore, the purpose of this study was to determine if PVCAP from noninvasive Doppler echocardiography would be a useful measure of survival in patients with PPAH.

METHODS

We analyzed clinical and hemodynamic variables on all patients with PPAH who had a right heart catheterization and echocardiogram from January to December 1999. Because capacitance is directly proportional to stroke volume and inversely proportional to PA pulse pressure, PVCAP was defined as stroke volume/pulse pressure. PVCAP was derived noninvasively from a comprehensive 2-dimensional and Doppler echocardiogram. Using the peak systolic tricuspid regurgitation velocity and the end-diastolic pulmonary regurgitation velocity, the modified Bernoulli equation was used to calculate the PA systolic and diastolic pressures, respectively. Stroke volume was obtained using the volumetric flow through the left ventricular outflow tract. PVCAP was then analyzed as a predictor of mortality, adjusting for other known modifiers of risk.

RESULTS

In all, 54 patients (13 men) were studied with a mean age of 44 +/- 11 years, ejection fraction of 62 +/- 11%, and RV systolic pressure of 90 +/- 21 mm Hg. In all, 24% were in World Health Organization (WHO) class II, 52% in class III, and 24% in class IV. During follow-up of 1498 +/- 108 days, 12 patients died. The strongest noninvasive predictor of mortality was PVCAP (risk ratio 3.0/mL/mm Hg decrease in PVCAP, 95% confidence interval 1.2-8.0, P = .0212). WHO class, RV index of myocardial performance, RV systolic pressure, and RV ejection time were weaker predictors. PVCAP was also a stronger predictor of mortality than invasively determined PV resistance, right atrial pressure, and mean PA pressure. In multivariate analysis, PVCAP was the only noninvasive predictor of mortality. In quartile analysis the lowest PVCAP quartile had a 4-year mortality of 39% whereas the highest PVCAP had a mortality of 7%.

CONCLUSION

The novel measure of PVCAP, as determined by Doppler echocardiography, is a strong noninvasive predictor of mortality in patients with PPAH and adds prognostic value to conventional risk markers.

Pulsatile blood flow in the entire coronary arterial tree: theory and experiment

The pulsatility of coronary circulation can be accurately simulated on the basis of the measured branching pattern, vascular geometry, and material properties of the coronary vasculature. A Womersley-type mathematical model is developed to analyze pulsatile blood flow in diastole in the absence of vessel tone in the entire coronary arterial tree on the basis of previously measured morphometric data. The model incorporates a constitutive equation of pressure and cross-section area relation based on our previous experimental data. The formulation enables the prediction of the impedance, the pressure distribution, and the pulsatile flow distribution throughout the entire coronary arterial tree. The model is validated by experimental measurements in six diastolic arrested, vasodilated porcine hearts. The agreement between theory and experiment is excellent. Furthermore, the present pulse wave results at low frequency agree very well with previously published steady-state model. Finally, the phase angle of flow is seen to decrease along the trunk of the major coronary artery and primary branches toward the capillary vessels. This study represents the first, most extensive validated analysis of Womersley-type pulse wave transmission in the entire coronary arterial tree down to the first segment of capillaries. The present model will serve to quantitatively test various hypotheses in the coronary circulation under pulsatile flow conditions.

Relationship of pulmonary arterial capacitance and mortality in idiopathic pulmonary arterial hypertension

OBJECTIVES

The purpose of this study was to determine if pulmonary vascular capacitance predicts survival in patients with idiopathic pulmonary arterial hypertension (IPAH).

BACKGROUND

The prognosis of patients with IPAH is difficult to predict, despite knowledge of clinical and hemodynamic parameters previously identified as predictors.

METHODS

We proposed a capacitance index of stroke volume divided by pulmonary pulse pressure (SV/PP) and prospectively gathered data on IPAH patients who underwent a right heart catheterization. SV/PP was analyzed as a predictor of mortality after adjusting for other modifiers of risk.

RESULTS

During 4-year follow-up of 104 patients, 21 patients died. When compared with conventional markers, SV/PP was the strongest univariate predictor of mortality (hazard ratio 17.0 per ml.mm Hg(-1) decrease, 95% confidence interval 13.0 to 22.0; p < 0.0001). In successive bivariate analysis, SV/PP was the only predictor of mortality. In quartile analysis, the lowest SV/PP quartile had a 4-year mortality of 61%; the highest SV/PP had no deaths.

CONCLUSIONS

The capacitance index (SV/PP) is a strong independent predictor of mortality in patients with IPAH.

"Smart" baroreception along the aortic arch, with reference to essential hypertension

Beat-to-beat regulation of heart rate is dependent upon sensing of local stretching or local "disortion" by aortic baroreceptors. Distortions of the aortic wall are due mainly to left ventricular output and to reflected waves arising from the arterial tree. Distortions are generally believed to be useful in cardiac control since stretch receptors or aortic baroreceptors embedded in the adventitia of the aortic wall, transduce the distortions to cardiovascular neural reflex pathways responsible for beat-to-beat regulation of heart rate. Aortic neuroanatomy studies have also found a continuous strip of mechanosensory neurites spread along the aortic inner arch. Although their purpose is now unknown, such a combined sensing capacity would allow measurement of the space and time dependence of inner arch wall distortions due, among other things, to traveling waves associated with pulsatile flow in an elastic tube. We call this sensing capability--"smart baroreception." In this paper we use an arterial tree model to show that the cumulative effects of wave reflections, from many sites far downstream, have a surprisingly pronounced effect on the pressure distribution in the root segment of the tree. By this mechanism global hemodynamics can be focused by wave reflections back to the aortic arch, where they can rapidly impact cardiac control via smart baroreception. Such sensing is likely important to maintain efficient heart function. However, alterations in the arterial tree due to aging and other natural processes can lead in such a system to altered cardiac control and essential hypertension.

Effects of stent stiffness on local haemodynamics with particular reference to wave reflections

The placement of a rigid stent within an elastic vessel produces wave reflection sites at the entrance to and exit from the stent. The net haemodynamic effects of these reflections depend critically on the degree of stiffness of the stent and on its length and position within the diseased vessel, variables that have been found to affect the clinical performance of a stent. Here these effects are examined analytically, using a segmented tube model. The results indicate that the presence of the stent within the larger diseased vessel has the effect of producing higher pressure at the vessel entrance than that at exit. This pressure difference, when superimposed on the underlying pressure distribution within the vessel, has the net effect of actually aiding rather than impeding the flow, but the extent of this depends on the length and position of the stent. A short stent placed near the entrance of the diseased vessel may be favoured clinically for producing the least perturbation in the underlying haemodynamics and thus reducing the chance of restenosis, while a long stent placed near the exit may be favoured for producing a positive pressure difference and thus aiding the flow.

The Role of usable Length in the Compliance Measurement of Vascular Prostheses

In this study we investigated the dependence of the mechanical properties and in particular of the radial compliance of a vascular prosthesis as a function of its usable length. Radial compliance was measured at 60 bpm and in the pressure range 80–120 mmHg. Starting from compliance measurements a simple model was used to calculate the pulse wave velocity and the reflection coefficients between 6 mm and 8 mm grafts (knitted and woven) with iliac and subclavean artery of similar diameter. The results provide an indication of the influence of usable length on the compliance and diameter mismatch at the anasthomosis between graft and host artery.

Mechanics of blood supply to the heart: wave reflection effects in a right coronary artery

Mechanics of blood flow in the coronary circulation have in the past been based largely on models in which the detailed architecture of the coronary network is not included because of lack of data: properties of individual vessels do not appear individually in the model but are represented collectively by the elements of a single electric circuit. Recent data from the human heart make it possible, for the first time, to examine the dynamics of flow in the coronary network based on detailed, measured vascular architecture. In particular, admittance values along the full course of the right coronary artery are computed based on actual lengths and diameters of the many thousands of branches which make up the distribution system of this vessel. The results indicate that effects of wave reflections on this flow are far more significant than those generally suspected to occur in coronary blood flow and that they are actually the reverse of the well known wave reflection effects in the aorta.