Reverse flow in the major infrarenal vessels--a capacitive phenomenon

Investigation on the differences of hemodynamics in normal common carotid, subclavian, and common femoral arteries using the vector flow technique

OBJECTIVES

To investigate the feasibility of the vector flow imaging (V Flow) technique to measure peripheral arterial hemodynamic parameters, including wall shear stress (WSS) and turbulence index (Tur) in healthy adults, and compare the results in different arteries.

MATERIALS AND METHODS

Fifty-two healthy adult volunteers were recruited in this study. The maximum and mean values of WSS, and the Tur values at early-systole, mid-systole, late-systole, and early diastole for total 156 normal peripheral arteries [common carotid arteries (CCA), subclavian arteries (SCA), and common femoral arteries (CFA)] were assessed using the V Flow technique.

RESULTS

The mean WSS values for CCA, SCA, and CFA were (1.66 ± 0.68) Pa, (0.62 ± 0.30) Pa, and (0.56 ± 0.27) Pa, respectively. The mean Tur values for CCA, SCA, and CFA were (0.46 ± 1.09%), (20.7 ± 9.06%), and (24.63 ± 17.66%), respectively. The CCA and SCA, as well as the CCA and CFA, showed statistically significant differences in the mean WSS and the mean Tur (P < 0.01). The mean Tur values had a negative correlation with the mean WSS; the correlation coefficient between log(Tur) and WSS is -0.69 (P < 0.05).

CONCLUSION

V Flow technique is a simple, practical, and feasible quantitative imaging approach for assessing WSS and Tur in peripheral arteries. It has the potential to be a useful tool for evaluating atherosclerotic plaques in peripheral arteries. The results provide a new quantitative foundation for future investigations into diverse arterial hemodynamic parameters.

The effect of varying degrees of stenosis on transition to turbulence in oscillatory flows

Many complications in physiology are associated with a deviation in flow in arteries due to a stenosis. The presence of stenosis may transition the flow to weak turbulence. The degree of stenosis as well as its configuration whether symmetric or non-symmetric to the parent artery influences whether the flow would stay laminar or transition to turbulence. Plenty of research efforts focus on investigating the role of varying degrees of stenosis in the onset of turbulence under steady and pulsatile flow conditions. None of the studies, however, have focused on investigating this under oscillatory flow conditions as flow reversal is a major occurrence in a number of physiologic flows, and is of particular relevance in cerebrospinal fluid flow research. Following up on the previous work in which a \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$75\%$$\end{document}75% stenosis was studied, this contribution is a detailed investigation of the role of degrees of stenosis on transition in an oscillatory flow. A cylindrical pipe with \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$25\%$$\end{document}25%, \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$50\%$$\end{document}50% and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$60\%$$\end{document}60% reductions in area in axisymmetric and eccentric configurations is studied for transition with 3 different pulsation frequencies of a purely oscillatory flow. Cycle averaged Reynolds numbers between 1800 and 2100 in steps of 100 are studied for each configuration resulting in 72 simulations each conducted on 76,800 CPU cores of a modern supercomputer. It is found that a higher degree of stenosis and eccentricity causes earlier transition to turbulence in oscillatory flow. The results further demonstrate that a higher frequency of oscillation results in larger hydrodynamic instability in the flow, which is more prominent in smaller degrees of stenosis.

Inlet and Outlet Boundary Conditions and Uncertainty Quantification in Volumetric Lattice Boltzmann Method for Image-Based Computational Hemodynamics

Inlet and outlet boundary conditions (BCs) play an important role in newly emerged image-based computational hemodynamics for blood flows in human arteries anatomically extracted from medical images. We developed physiological inlet and outlet BCs based on patients’ medical data and integrated them into the volumetric lattice Boltzmann method. The inlet BC is a pulsatile paraboloidal velocity profile, which fits the real arterial shape, constructed from the Doppler velocity waveform. The BC of each outlet is a pulsatile pressure calculated from the three-element Windkessel model, in which three physiological parameters are tuned by the corresponding Doppler velocity waveform. Both velocity and pressure BCs are introduced into the lattice Boltzmann equations through Guo’s non-equilibrium extrapolation scheme. Meanwhile, we performed uncertainty quantification for the impact of uncertainties on the computation results. An application study was conducted for six human aortorenal arterial systems. The computed pressure waveforms have good agreement with the medical measurement data. A systematic uncertainty quantification analysis demonstrates the reliability of the computed pressure with associated uncertainties in the Windkessel model. With the developed physiological BCs, the image-based computation hemodynamics is expected to provide a computation potential for the noninvasive evaluation of hemodynamic abnormalities in diseased human vessels.

Effect of Anastomosis Angles on Retrograde Perfusion and Hemodynamics of Hybrid Treatment for Thoracoabdominal Aortic Aneurysm

BACKGROUND

Hemodynamic effects on the retrograde visceral reconstruction (RVR) for thoracoabdominal aortic aneurysms treatment by anastomotic angle remains unclear. This study aims to qualitatively and quantitatively investigate the effects of different anastomotic angles on hemodynamics and patency.

METHODS

Three RVR models with 45°, 60° and 90° anastomotic angles were reconstructed respectively by manipulating apostoperative patient-specific model. The manipulated models of the RVRs were numerically simulated and analyzed in terms of hemodynamics including theinstant and cumulative patency, flow pattern and indicators based on wall shear stress (WSS).

RESULTS

Although a smaller anastomotic angle may decrease the patency rate of common iliac arteries, it can improve the visceral perfusion during a cardiac cycle. More importantly, RVR with the smallest anastomotic angle experienced a minimal low time-averaged wall shear stress, high oscillatory shear index and relative residence time in the anastomosis region, whereas the largest anastomotic angle can introduce more unfavorable WSS in the graft trunk. Furthermore, a spiral flow pattern was observed in the proximal graft trunk of all three models, where no high-risk shear distribution was detected in this region.

CONCLUSION

A smaller anastomotic angle may have more benefits of hemodynamic environment in RVR, especially the WSS distribution and flow pattern in the graft trunk. We may also suggest that additional stents or an extended cuff for the graft can be used to induce spiral flow intentionally, which can further improve local hemodynamic environment and long-term prognosis.

Influence of Iliac Stenotic Lesions on Blood Flow Patterns Near a Covered Endovascular Reconstruction of the Aortic Bifurcation (CERAB) Stent Configuration

PURPOSE

To investigate the effect of distal stenotic lesions on flow patterns near a covered endovascular reconstruction of the aortic bifurcation (CERAB) configuration used in the treatment of aortoiliac occlusive disease.

METHOD

Laser particle image velocimetry measurements were performed using in vitro models of the aortic bifurcation with and without a CERAB configuration in place. A hemodynamically nonsignificant stenosis (ΔP: 9 mm Hg), a hemodynamically significant (ΔP: 26 mm Hg) stenosis, and a total occlusion were simulated in the left iliac arteries. Velocity fields and time-averaged wall shear stress (TAWSS) were calculated.

RESULTS

Hemodynamically significant distal lesions did not influence the inflow patterns or TAWSS (0.5-0.6 Pa) in either model. However, hemodynamically significant distal stenotic lesions caused a 2-fold decrease in peak outflow velocities (control: 106 vs 56 cm/s, CERAB: 96 vs 54 cm/s) and a 3-fold decrease in TAWSS (control: 1.34 vs 0.44 Pa, CERAB: 0.75 vs 0.21 Pa). There was a 2-fold decrease in wall shear stress in the CERAB outflow compared with the control, independent of lesion severity.

CONCLUSION

In the CERAB technique, adequate distal runoff is identified as an important parameter to ensure patency. This in vitro study showed that distal stenotic lesions influence aortic bifurcation outflow patterns and TAWSS more extensively in the CERAB configuration. Distal stenotic lesions could therefore increase the risk of disease progression and loss of stent patency. In vivo studies are necessary to confirm these observations.

Partial renal coverage in endovascular aneurysm repair causes unfavorable renal flow patterns in an infrarenal aneurysm model

OBJECTIVE

To achieve an optimal sealing zone during endovascular aneurysm repair, the intended positioning of the proximal end of the endograft fabric should be as close as possible to the most caudal edge of the renal arteries. Some endografts exhibit a small offset between the radiopaque markers and the proximal fabric edge. Unintended partial renal artery coverage may thus occur. This study investigated the consequences of partial coverage on renal flow patterns and wall shear stress (WSS).

METHODS

In vitro models of an abdominal aortic aneurysm were used to visualize pulsatile flow using two-dimensional particle image velocimetry under physiologic resting conditions. One model served as control and two models were stented with an Endurant endograft (Medtronic Inc, Minneapolis, Minn), one without and one with partial renal artery coverage with 1.3 mm of stent fabric extending beyond the marker (16% area coverage). The magnitude and oscillation of WSS, relative residence time, and backflow in the renal artery were analyzed.

RESULTS

In both stented models, a region along the caudal renal artery wall presented with low and oscillating WSS, not present in the control model. A region with very low WSS (<0.1 Pa) was present in the model with partial coverage over a length of 7 mm compared with a length of 2 mm in the model without renal coverage. Average renal backflow area percentage in the renal artery incrementally increased from control (0.9%) to the stented model without (6.4%) and with renal coverage (18.8%).

CONCLUSIONS

In this flow model, partial renal coverage after endovascular aneurysm repair causes low and marked oscillations in WSS, potentially promoting atherosclerosis and subsequent renal artery stenosis. Awareness of the device-dependent offset between the fabric edge and the radiopaque markers is therefore important in endovascular practice.

A computational simulation of the effect of hybrid treatment for thoracoabdominal aortic aneurysm on the hemodynamics of abdominal aorta

Hybrid visceral-renal debranching procedures with endovascular repair have been proposed as an appealing technique to treat conventional thoracoabdominal aortic aneurysm (TAAA). This approach, however, still remained controversial because of the non-physiological blood flow direction of its retrograde visceral revascularization (RVR) which is generally constructed from the aortic bifurcation or common iliac artery. The current study carried out the numerical simulation to investigate the effect of RVR on the hemodynamics of abdominal aorta. The results indicated that the inflow sites for the RVR have great impact on the hemodynamic performance. When RVR was from the distal aorta, the perfusion to visceral organs were adequate but the flow flux to the iliac artery significantly decreased and a complex disturbed flow field developed at the distal aorta, which endangered the aorta at high risk of aneurysm development. When RVR was from the right iliac artery, the abdominal aorta was not troubled with low WSS or disturbed flow, but the inadequate perfusion to the visceral organs reached up to 40% and low WSS and flow velocity predominated appeared at the right iliac artery and the grafts, which may result in the stenosis in grafts and aneurysm growth on the host iliac artery.

Aortic Blood Flow Reversal Determines Renal Function

Aortic stiffness determines the glomerular filtration rate (GFR) and predicts the progressive decline of the GFR. However, the underlying pathophysiological mechanism remains obscure. Recent evidence has shown a close link between aortic stiffness and the bidirectional (systolic forward and early diastolic reverse) flow characteristics. We hypothesized that the aortic stiffening–induced renal dysfunction is attributable to altered central flow dynamics. In 222 patients with hypertension, Doppler velocity waveforms were recorded at the proximal descending aorta to calculate the reverse/forward flow ratio. Tonometric waveforms were recorded to measure the carotid-femoral (aortic) and carotid-radial (peripheral) pulse wave velocities, to estimate the aortic pressure from the radial waveforms, and to compute the aortic characteristic impedance. In addition, renal hemodynamics was evaluated by duplex ultrasound. The estimated GFR was inversely correlated with the aortic pulse wave velocity, reverse/forward flow ratio, pulse pressure, and characteristic impedance, whereas it was not correlated with the peripheral pulse wave velocity or mean arterial pressure. The association between aortic pulse wave velocity and estimated GFR was independent of age, diabetes mellitus, hypercholesterolemia, and antihypertensive medication. However, further adjustment for the aortic reverse/forward flow ratio and pulse pressure substantially weakened this association, and instead, the reverse/forward flow ratio emerged as the strongest determinant of estimated GFR ( P =0.001). A higher aortic reverse/forward flow ratio was also associated with lower intrarenal forward flow velocities. These results suggest that an increase in aortic flow reversal (ie, retrograde flow from the descending thoracic aorta toward the aortic arch), caused by aortic stiffening and impedance mismatch, reduces antegrade flow into the kidney and thereby deteriorates renal function.

Supraceliac and Infrarenal Aortic Flow in Patients with Abdominal Aortic Aneurysms: Mean Flows, Waveforms, and Allometric Scaling Relationships

PURPOSE

Hemodynamic forces are thought to play a critical role in abdominal aortic aneurysm (AAA) growth. In silico and in vitro simulations can be used to study these forces, but require accurate aortic geometries and boundary conditions. Many AAA simulations use patient-specific geometries, but utilize inlet boundary conditions taken from a single, unrelated, healthy young adult.

METHODS

In this study, we imaged 43 AAA patients using a 1.5 T MR scanner. A 24-frame cardiac-gated one-component phase-contrast magnetic resonance imaging sequence was used to measure volumetric flow at the supraceliac (SC) and infrarenal (IR) aorta, where flow information is typically needed for simulation. For the first 36 patients, individual waveforms were interpolated to a 12-mode Fourier curve, peak-aligned, and averaged. Allometric scaling equations were derived from log-log plots of mean SC and IR flow vs. body mass, height, body surface area (BSA), and fat-free body mass. The data from the last seven patients were used to validate our model.

RESULTS

Both the SC and IR averaged waveforms had the biphasic shapes characteristic of older adults, and mean SC and IR flows over the cardiac cycle were 51.2 ± 10.3 and 17.5 ± 5.44 mL/s, respectively. Linear regression of the log-log plots revealed that BSA was most strongly predictive of mean SC (R² = 0.29) and IR flow (R² = 0.19), with the highest combined R². When averaged, the measured and predicted waveforms for the last seven patients agreed well.

CONCLUSIONS

We present a method to estimate SC and IR mean flows and waveforms for AAA simulation.

Quantification of hemodynamics in abdominal aortic aneurysms during rest and exercise using magnetic resonance imaging and computational fluid dynamics

Abdominal aortic aneurysms (AAAs) affect 5-7% of older Americans. We hypothesize that exercise may slow AAA growth by decreasing inflammatory burden, peripheral resistance, and adverse hemodynamic conditions such as low, oscillatory shear stress. In this study, we use magnetic resonance imaging and computational fluid dynamics to describe hemodynamics in eight AAAs during rest and exercise using patient-specific geometric models, flow waveforms, and pressures as well as appropriately resolved finite-element meshes. We report mean wall shear stress (MWSS) and oscillatory shear index (OSI) at four aortic locations (supraceliac, infrarenal, mid-aneurysm, and suprabifurcation) and turbulent kinetic energy over the entire computational domain on meshes containing more than an order of magnitude more elements than previously reported results (mean: 9.0-million elements; SD: 2.3 M; range: 5.7-12.0 M). MWSS was lowest in the aneurysm during rest 2.5 dyn/cm(2) (SD: 2.1; range: 0.9-6.5), and MWSS increased and OSI decreased at all four locations during exercise. Mild turbulence existed at rest, while moderate aneurysmal turbulence was present during exercise. During both rest and exercise, aortic turbulence was virtually zero superior to the AAA for seven out of eight patients. We postulate that the increased MWSS, decreased OSI, and moderate turbulence present during exercise may attenuate AAA growth.

In vivo assessment of blood flow patterns in abdominal aorta of mice with MRI: implications for AAA localization

Abdominal aortic aneurysms (AAA) localize in the infrarenal aorta in humans, while they are found in the suprarenal aorta in mouse models. It has been shown previously that humans experience a reversal of flow during early diastole in the infrarenal aorta during each cardiac cycle. This flow reversal causes oscillatory wall shear stress (OWSS) to be present in the infrarenal aorta of humans. OWSS has been linked to a variety of proatherogenic and proinflammatory factors. The presence of reverse flow in the mouse aorta is unknown. In this study we investigated blood flow in mice, using phase-contrast magnetic resonance (PCMR) imaging. We measured blood flow in the suprarenal and infrarenal abdominal aorta of 18 wild-type C57BL/6J mice and 15 apolipoprotein E (apoE)-/- mice. Although OWSS was not directly evaluated, results indicate that, unlike humans, there is no reversal of flow in the infrarenal aorta of wild-type or apoE-/- mice. Distensibility of the mouse aortic wall in both the suprarenal and infrarenal segments is higher than reported values for the human aorta. We conclude that normal mice do not experience the reverse flow in the infrarenal aorta that is observed in humans.

Doppler ultrasound signals simulation from vessels with various stenosis degrees

As a non-invasive method, the Doppler ultrasound technique is used to detect the vessel stenosis. To search for characteristics of Doppler ultrasound signals sensitive to the stenosis, a computer simulation approach is proposed in this paper to generate Doppler ultrasound signals from vessels with various stenosis degrees. The blood flow velocity distribution in a stenosed vessel is firstly calculated using the transient finite element method (FEM). Then the power spectral density of Doppler signals is estimated using the overall-distribution nonparametric estimation method. Finally Doppler signals are generated using the cosine-superposed method. The proposed approach is proved to be useful for simulating Doppler ultrasound signals from vessels with various stenosis degrees. It is also shown that characteristics of Doppler ultrasound signals may be used to estimate the vessel's stenosis degree.

Wall shear rates differ between the normal carotid, femoral, and brachial arteries: an in vivo MRI study

PURPOSE

To investigate wall shear rates in vivo in the common carotid, brachial, and superficial femoral arteries using very high resolution magnetic resonance imaging (MRI) phase contrast measurements.

MATERIALS AND METHODS

Mean, maximum, and minimum wall shear rate and an oscillatory shear index were measured for 20 volunteers, aged 23.3 +/- 1.9 years, in the three arteries, using phase contrast imaging with 0.0625 mm2 resolution and three-dimensional paraboloid fitting.

RESULTS

The superficial femoral artery had the lowest mean (130.3 +/- 13.1 second(-1)), maximum (735.8 +/- 32.4 second(-1)), and minimum (-224.5 +/- 17.0 second(-1)) wall shear rate, as well as the highest oscillatory shear index (0.21 +/- 0.02). All values were significantly different (P < 0.05) from both the brachial artery and the common carotid artery values. The highest mean (333.3 +/- 13.6 second(-1)) and minimum (117.9 +/- 24.5 second(-1)) wall shear rates and the lowest oscillatory shear index (0 +/- 0) were found in the common carotid artery.

CONCLUSION

It is possible to measure wall shear rate in vivo in different arteries using MRI with very high resolution. The findings exhibit the in vivo environment of wall shear rates and suggest a nonuniform distribution of wall shear rates throughout the arterial system.

In vivo validation of a one-dimensional finite-element method for predicting blood flow in cardiovascular bypass grafts

Current practice in vascular surgery utilizes only diagnostic and empirical data to plan treatments and does not enable quantitative a priori prediction of the outcomes of interventions. We have previously described a new approach to vascular surgery planning based on solving the governing equations of blood flow in patient-specific models. A one-dimensional finite-element method was used to simulate blood flow in eight porcine thoraco-thoraco aortic bypass models. The predicted flow rate was compared to in vivo data obtained using cine phase-contrast magnet resonance imaging. The mean absolute difference between computed and measured flow distribution in the stenosed aorta was found to be 4.2% with the maximum difference of 10.6% anda minimum difference of 0.4%. Furthermore, the sensitivity of the flow rate and distribution with respect to stenosis and branch losses were quantified.

Flow changes in the aorta associated with the deployment of a AAA stent graft

The purpose of this study was to investigate the hemodynamic implications of a proximal shift in the aortic bifurcation that results from abdominal aortic aneurysm (AAA) stent graft deployment. A flow model was constructed in which an anatomically accurate model of the aorta was subjected to physiologic pulsatile flow. The model included the celiac, superior mesenteric, left and right renal arteries. The aortic bifurcation, leading to the right and left iliac arteries was included, as well as the lumbar curvature. Flow simulations were performed under resting and mild exercise conditions with and without a Cordis AAA stent graft deployed. Flow patterns were visualized with dye injection and recorded onto video. The flow rates through the iliac and renal arteries were continuously monitored using ultrasonic flowmeters. Flow visualization revealed that flow disturbances at the level of the renal arteries were slightly increased with the deployment of the stent graft. The orientation of the endolegs within the aorta had no perceptible effect on these disturbances. Under mild exercise conditions, very little flow disturbance was observed. In conclusion, there are slight changes in flow disturbance near the renal arteries due to stent graft deployment, but these changes would not be expected to have significant clinical implications.

Development of velocity profiles and retrograde flow in the porcine abdominal aorta under different haemodynamic conditions

Low and/or oscillating wall shear stresses are related to the development of atherosclerosis and this oscillation is influenced by changes in basic haemodynamics (exercise). The objective of this study was to provide in vivo data on the development of velocity profiles and oscillating blood velocities in the abdominal aorta under varying haemodynamic conditions. Six anaesthetized, 90-kg pigs were used in the study. Abdominal aortic velocity profiles across the anterior-posterior diameter were acquired at different axial positions using 10 MHz pulsed Doppler ultrasound. Measurements were obtained under normal conditions and during cardiac pacing up to 170 beats/min. Velocity profiles were obtained during heart rates ranging between 58 and 169 beats/min, and during flow rates ranging between 0.57 and 2.89 l/min. Main outcome measures included minimum velocities, frequency index, shape of velocity profiles (velocity distribution index), Reynolds' numbers, and Womersley's frequency parameter. Velocity profiles were blunted, with lowest velocities at the distal posterior vessel wall. Multiple regression analysis showed the development of velocity profiles to be inversely correlated with the pulsatility index, Womersley's frequency parameter and the mean Reynolds' number (r = 0.89, p < 0.0005). Minimum velocities were negatively correlated with the PI, Womersley's frequency parameter and positively with the mean Reynolds' number (r = 0.94, p < 10(-8)). Retrograde velocities (and hence oscillating wall shear stresses) were present at mean Reynolds' number < 1000. The oscillation of blood velocities at the wall in the porcine abdominal aorta was highly dependent on general haemodynamics (i.e. flow, heart rate and vessel diameter as expressed in the Reynolds' numbers and Womersley's frequency parameters). The velocity profiles in the abdominal aorta were found to be far from parabolic. These findings have important implications for the understanding and future modelling of the complex haemodynamics in the abdominal aorta and their relation to the development of atherosclerotic disease.

Effect of exercise on hemodynamic conditions in the abdominal aorta

PURPOSE

The beneficial effect of exercise in the retardation of the progression of cardiovascular disease is hypothesized to be caused, at least in part, by the elimination of adverse hemodynamic conditions, including flow recirculation and low wall shear stress. In vitro and in vivo investigations have provided qualitative and limited quantitative information on flow patterns in the abdominal aorta and on the effect of exercise on the elimination of adverse hemodynamic conditions. We used computational fluid mechanics methods to examine the effects of simulated exercise on hemodynamic conditions in an idealized model of the human abdominal aorta.

METHODS

A three-dimensional computer model of a healthy human abdominal aorta was created to simulate pulsatile aortic blood flow under conditions of rest and graded exercise. Flow velocity patterns and wall shear stress were computed in the lesion-prone infrarenal aorta, and the effects of exercise were determined.

RESULTS

A recirculation zone was observed to form along the posterior wall of the aorta immediately distal to the renal vessels under resting conditions. Low time-averaged wall shear stress was present in this location, along the posterior wall opposite the superior mesenteric artery and along the anterior wall between the superior and inferior mesenteric arteries. Shear stress temporal oscillations, as measured with an oscillatory shear index, were elevated in these regions. Under simulated light exercise conditions, a region of low wall shear stress and high oscillatory shear index remained along the posterior wall immediately distal to the renal arteries. Under simulated moderate exercise conditions, all the regions of low wall shear stress and high oscillatory shear index were eliminated.

CONCLUSION

This numeric investigation provided detailed quantitative data on the effect of exercise on hemodynamic conditions in the abdominal aorta. Our results indicated that moderate levels of lower limb exercise are necessary to eliminate the flow reversal and regions of low wall shear stress in the abdominal aorta that exist under resting conditions. The lack of flow reversal and increased wall shear stress during exercise suggest a mechanism by which exercise may promote arterial health, namely with the elimination of adverse hemodynamic conditions.

Determination of wall shear stress in the aorta with the use of MR phase velocity mapping

MR phase velocity mapping was used to calculate wall shear stress (WSS) in the suprarenal and infrarenal abdominal aorta, two sites with very different proclivities for development of a atherosclerosis. For the eight subjects studied, the average value of the mean (time averaged over the cardiac cycle) WSS in the suprarenal aorta was 10.4 dynes/cm2 at the posterior wall and 8.6 at the anterior wall. In the infrarenal aorta, WSS values were 4.7 at the posterior wall and 6.1 at the anterior wall. Peak WSS over the cardiac cycle was 48 and 54 at the anterior and posterior walls of the suprarenal aorta, respectively, and 33 and 30 at the anterior and posterior walls of the infrarenal aorta, respectively. Wide variation was found in both mean and peak WSS values among subjects. However, for 28 of 32 locations examined, mean and peak WSS were higher in the suprarenal aorta than in the infrarenal aorta. Because atherosclerosis is more likely to form in the infrarenal aorta than in the suprarenal aorta, this study supports the hypothesis that low WSS is a localizing factor for atherosclerosis, and high WSS may act as a deterrent against formation of atherosclerosis.

Renal artery velocity mapping with MR imaging

An MR phase imaging sequence with a very short echo time was used to assess blood velocity and flow at the renal artery bifurcation. Cardiac-gated MR imaging data were obtained in six healthy subjects in sagittal planes adjacent to the abdominal aorta and transverse planes above and below the renal artery bifurcation. Average renal artery flow rate was 23.8 +/- 9 mL/sec. A strong individual variability was found for the velocity profiles in the abdominal aorta during end-systolic regurgitation. Flow rate was also determined in three patients with reduced renal artery blood flow. Two patients received therapy with percutaneous transluminal angioplasty. The successful outcome was documented with MR imaging. A reliable assessment of renal artery flow with MR phase imaging is feasible. Measurement of the velocity profiles yields valuable insights in the complicated flow regime at the renal artery bifurcation.

Two-dimensional velocity measurements in a pulsatile flow model of the normal abdominal aorta simulating different hemodynamic conditions

The infrarenal abdominal aorta and aortic bifurcation are frequent sites of atherosclerosis. The local hemodynamics are considered to be an atherogenetic factor, and a detailed description of the flow fields in this region of the arterial tree is therefore essential. The aim of this study was to provide quantitative two-dimensional data on the velocity fields in the abdominal aorta, using a realistic flow model of the abdominal aorta and its main branches, under various physiologic flow conditions (i.e. rest and exercise). Velocities in the suprarenal abdominal aorta were antegrade, with very little retrograde and radial velocity components present. In the infrarenal abdominal aorta, velocity profiles were not fully developed, and large-scale retrograde flow was present during part of diastole for the rest condition. For the exercise conditions small-scale retrograde velocities were present during diastole, especially at the distal posterior vessel wall, but not at the distal anterior vessel wall. For the rest and medium exercise conditions, secondary flows were created in the distal abdominal aorta during diastole, most prominent near the posterior wall. Calculated wall shear stress directions revealed the presence of both oscillatory and multidirectional wall shear stresses mainly in parts of the infrarenal abdominal aorta, and were found to correlate well with the published data on the distribution of early atherosclerotic lesions. This quantitative study demonstrates the necessity of carefully modeling both the anatomy and the physiology in order to understand the complex hemodynamics present in the abdominal aorta.

Flow pattern analysis in the abdominal aorta with velocity-encoded cine MR imaging

The sites of deposition of atherosclerotic plaque on the aortic wall are considered to be influenced by secondary and retrograde flow patterns that cause regions of altered shear stress. To detect secondary flow patterns and areas of retrograde flow in the abdominal aorta, velocity-encoded cine (VEC) magnetic resonance (MR) imaging was performed at five different levels of the abdominal aorta in nine healthy volunteers. Net retrograde flow (expressed as a percentage of antegrade flow) increased from proximal to distal levels and was maximal (13.8% +/- 11.8) just distal to the origin of the renal arteries. An increase in the duration of retrograde flow over the cardiac cycle was observed from proximal to distal levels. Whereas retrograde flow was present at end systole and early diastole in each volunteer at every level, the duration and amount of retrograde flow during diastole showed high interindividual variation. Such differences suggest the possibility of variable vascular geometric risk factors in the population for the development of atherosclerotic plaque. The location of retrograde flow in the abdominal aorta demonstrated in vivo with VEC MR imaging was close to that obtained with in vitro flow visualization studies in models of the abdominal aorta.

Pulsatile flow visualization in a model of the human abdominal aorta and aortic bifurcation

The infrarenal abdominal aorta and aortic bifurcation are frequent sites of atherosclerosis. The local hemodynamics are considered to be atherogenetic factors; a detailed description of these flow fields is, therefore, essential to understand their relationship to atherosclerosis. The aim of this study was, therefore, to provide such detailed information using a flow visualization technique in an anatomically realistic flow model of the abdominal aorta and its main branches in which the complex pulsatile flow waveforms and flow rates were simulated for two physiologic flow conditions (rest and exercise). At rest, the particle path lines in the suprarenal abdominal aorta were straight with no visible signs of flow reversal. Vortices were initiated opposite to the main branches. In the infrarenal aorta, large flow separation zones formed at the posterior aortic wall and at the lateral walls in the aortic bifurcation during systolic deceleration, and flow reversal was present during diastole. Under exercise conditions, the particle path lines were straight, and only slight flow reversal was seen. This study emphasizes, that rather than being a straight tube with forward-moving fluid, the abdominal aorta has to be considered as a complex part of the arterial tree. Distinct local hemodynamic qualities of importance for explaining atherogenesis were pointed out. At rest, the suprarenal abdominal aorta had much less complicated flow characteristics than the infrarenal abdominal aorta where the distal, posterior vessel wall and the lateral walls of the bifurcation were sites of flow patterns thought to be associated with atherosclerosis. During exercise, the infrarenal flow patterns changed dramatically away from the flow patterns associated with the induction of atherosclerosis.