Peak Shear Rate Research Articles

Effect of acute sympathetic nervous system activation on flow-mediated dilation of brachial artery

We tested the hypothesis that flow-mediated dilation (FMD) of the brachial artery would be impaired by acute increases in sympathetic nervous system activity (SNA) in models where similar peak shear stress stimulus was achieved by varying the duration of forearm muscle ischemia. Eleven healthy young men were studied under four different conditions, each with its own control: lower body suction (LBS), cold pressor test (CPT), mental arithmetic task (MAT), and activation of muscle chemoreflex (MCR). The duration of ischemia before observation of FMD by ultrasound imaging was 5 min each for control, LBS, and CPT; 3 min for MAT; and 2-min for MCR. Peak shear rate was not different between control and any of the SNA conditions, although total shear in the first minute was reduced in MAT. MCR was the only condition in which brachial artery vasoconstriction was observed before forearm occlusion [4.38 (SD 0.53) vs. control 4.60 (SD 0.53) mm, P < 0.05]; however, diameter increased to the same absolute value as that of the control, so the percent FMD was greater for MCR [9.85 (SD 2.33) vs. control 5.29 (SD 1.50)%]. Blunting of the FMD response occurred only in the CPT model [1.51 (SD 1.20)%]. During SNA, the increase in plasma cortisol from baseline was significant only for MCR; the increase in plasma norepinephrine was significant for MCR, LBS, and CPT; and the increase in epinephrine was significant only for MCR. These results showed that the four models employed to achieve increases in SNA had different effects on baseline brachial artery diameter and that blunted FMD is not a general response to increased SNA.

Initial Stress-Kick Is Required for Fluid Shear Stress-Induced Rate Dependent Activation of Bone Cells

The shear stress induced by the loading-mediated flow of interstitial fluid through the lacuno-canalicular network is a likely stimulus for bone cell adaptive responses. Furthermore, the magnitude of the cellular response is related to the rate of mechanical loading rather than its magnitude. Thus, bone cells might be very sensitive to sudden stress-kicks, as occuring e.g., during impact loading. There is evidence that cells change stiffness under stress, which might make them more sensitive to subsequent loading. We studied the influence of a stress-kick on the mechanosensitivity of MC3T3-E1 osteoblast-like cells under different peak shear rate conditions, as measured by nitric oxide production. MC3T3-E1 bone cells were treated with steady or pulsating fluid shear stress (PFSS) for 5 min with different peak rates (9.70, 17.5, and 22.0 Pa Hz) using varying frequencies (5 and 9 Hz), and amplitudes (0.70 and 0.31 Pa). PFSS treatment was done with or without fluid flow pretreatment phase, which removed the initial stress-kick by first applying a slow fluid flow increase. Nitric oxide production in response to fluid shear stress was rate dependent, but necessitated an initial stress-kick to occur. This suggests that high-rate stimuli condition bone cells to be more sensitive for high-frequency, low-amplitude loads.

Flow-mediated dilation in human brachial artery after different circulatory occlusion conditions.

Different magnitudes and durations of postocclusion reactive hyperemia were achieved by occluding different volumes of tissue with and without ischemic exercise to test the hypotheses that flow-mediated dilation (FMD) of the brachial artery would depend on the increase in peak flow rate or shear stress and that the position of the occlusion cuff would affect the response. The brachial artery FMD response was observed by high-frequency ultrasound imaging with curve fitting to minimize the effects of random measurement error in eight healthy, young, nonsmoking men. Reactive hyperemia was graded by 5-min occlusion distal to the measurement site at the wrist and the forearm and proximal to the site in the upper arm. Flow was further increased by exercise during occlusion at the wrist and forearm positions. For the two wrist occlusion conditions, flow increased eightfold and FMD was only 1 to 2% (P > 0.05). After the forearm and upper arm occlusions, blood flow was almost identical but FMD after forearm occlusions was 3.4% (P < 0.05), whereas it was significantly greater (6.6%, P < 0.05) and more prolonged after proximal occlusion. Forearm occlusion plus exercise caused a greater and more prolonged increase in blood flow, yet FMD (7.0%) was qualitatively and quantitatively similar to that after proximal occlusion. Overall, the magnitude of FMD was significantly correlated with peak forearm blood flow (r = 0.59, P < 0.001), peak shear rate (r = 0.49, P < 0.002), and total 5-min reactive hyperemia (r = 0.52, P < 0.001). The prolonged FMD after upper arm occlusion suggests that the mechanism for FMD differs with occlusion cuff position.

Brachial artery shear stress is independent of gender or age and does not modify vessel wall mechanical properties

The objective of the present study was to obtain brachial artery (BA) baseline shear stress (SS) values in healthy volunteers and to relate this to gender and age. Peak and mean wall shear rate (SR) were noninvasively measured in 30 healthy subjects using an SR estimation system. Arterial diameter and wall characteristics were obtained with a wall tracking system. SS was estimated from SR and calculated whole blood viscosity. Intrasubject variability and the effects of age and gender were assessed. Intrasubject variability of BA peak and mean SR were 16.2% and 28.6%. Baseline peak (≈ 3.0 ± 0.7 Pa) and mean SS (≈ 0.5 ± 0.2 Pa) were not gender-dependent, nor were they influenced by age. No vessel wall parameter related to BA SS. No differences in BA SS were observed between the genders and no influence of age was apparent. Our results indicate that the BA adequately responds to chronic changes in blood flow. (E-mail:r.dammers@bf.unimaas.nl)

Intimal Hyperplasia and Wall Shear in Arterial Bypass Graft Distal Anastomoses: An In Vivo Model Study

The observation of intimal hyperplasia at bypass graft anastomoses has suggested a potential interaction between local hemodynamics and vascular wall response. Wall shear has been particularly implicated because of its known effects upon the endothelium of normal vessels and, thus, was examined as to its possible role in the development of intimal hyperplasia in arterial bypass graft distal anastomoses. Tapered (4-7 mm I.D.) e-PTFE synthetic grafts 6 cm long were placed as bilateral carotid artery bypasses in six adult, mongrel dogs weighing between 25 and 30 kg with distal anastomotic graft-to-artery diameter ratios (DR) of either 1.0 or 1.5. Immediately following implantation, simultaneous axial velocity measurements were made in the toe and artery floor regions in the plane of the anastomosis at radial increments of 0.35 mm, 0.70 mm, and 1.05 mm using a specially designed 20 MHz triple crystal ultrasonic wall shear rate transducer Mean, peak, and pulse amplitude wall shear rates (WSRs), their absolute values, the spatial and temporal wall shear stress gradients (WSSG), and the oscillatory shear index (OSI) were computed from these velocity measurements. All grafts were harvested after 12 weeks implantation and measurements of the degree of intimal hyperplasia (IH) were made along the toe region and the artery floor of the host artery in 1 mm increments. While some IH occurred along the toe region (8.35+/-23.1 microm) and was significantly different between DR groups (p<0.003), the greatest amount occurred along the artery floor (81.6+/-106.5 microm, mean +/- S.D.) (p < 0.001) although no significant differences were found between DR groups. Linear regressions were performed on the paired IH and mean, peak, and pulse amplitude WSR data as well as the absolute mean, peak, and pulse amplitude WSR data from all grafts. The mean and absolute mean WSRs showed a modest correlation with IH (r = -0.406 and -0.370, respectively) with further improvements seen (r = -0.482 and -0.445, respectively) when using an exponential relationship. The overall best correlation was seen against an exponential function of the OSI (r = 0.600). Although these correlation coefficients were not high, they were found to be statistically significant as evidenced by the large F-statistic obtained. Finally, it was observed that over 75 percent of the IH occurred at or below a mean WSR value of 100 s(-1) while approximately 92 percent of the IH occurred at or below a mean WSR equal to one-half that of the native artery. Therefore, while not being the only factor involved, wall shear (and in particular, oscillators wall shear) appears to provide a stimulus for the development of anastomotic intimal hyperplasia.

Shear thickening in steady and superposition flows effect of particle interaction forces

The phenomenon of shear thickening has been studied in sterically stabilized colloidal suspensions. Temperature has been used to vary the solvent quality of the suspending medium for the grafted polymer in the stabilizer layer. A decrease in temperature causes the polymer coat to shrink. This results in a lower viscosity in the shear thinning region but also in a lower critical shear rate and shear stress at the onset of shear thickening. The critical shear stress increases with increasing particle volume fraction, at least for the volume fractions studied here. In large amplitude oscillatory flow, strain hardening for both storage and loss moduli sets in at the same peak shear rate that causes shear thickening in steady state flow. Parallel superposition also has been used to probe the effect of the stabilizer layer. The frequency dependence of the superposition moduli changes drastically once shear thickening sets in. Surprisingly, this produces a simple and constant shape for the moduli-frequency curves with relaxation times that become nearly independent of the shear rate.

Heart motion-adapted MR velocity mapping of blood velocity distribution downstream of aortic valve prostheses: initial experience.

To investigate blood flow velocities and shear rates at two distances downstream of an artificial aortic valve in patients. Blood velocity was quantified downstream of the valve prosthesis (for replacement after aortic valve stenosis or combined stenosis and regurgitation) in 10 patients by using a magnetic resonance (MR) cine velocity mapping method in which the imaging section position is adapted according to the excursion of the valvular plane of the heart. Two acquisitions were performed to display the blood velocity distributions one-fourth valve diameter and one valve diameter downstream of the valve and to quantify blood volumes and shear rates. The velocity profiles measured during flow acceleration one-fourth valve diameter downstream were characterized by a distinct pattern of two lateral jets and one central jet of antegrade flow. High shear rates were found along the leaflet tips. The profiles obtained one valve diameter downstream were skewed, with varying velocity patterns among patients. Peak shear rates were found close to the vessel wall. With correction for through-plane motion of the valve, the mean apparent regurgitant fraction (+/- SD) was 14% +/- 6; the mean regurgitant fraction without correction was 9% +/- 5. The described noninvasive procedure for velocity mapping enables measurements close to the valve and thus evaluation of blood flow patterns with respect to valve design in humans.

Simulation of coating flows with slip effects

This work is concerned with the numerical prediction of wire coating flows. Both annular tube-tooling and pressure-tooling type extrusion–drag flows are investigated for viscous fluids. The effects of slip at die walls are analysed and free surfaces are computed. Flow conditions around the die exit are considered, contrasting imposition of no-slip and various instances of slip models for die wall conditions. Numerical solutions are computed by means of a time marching Taylor–Galerkin/pressure–correction finite element scheme, that demonstrates how slip conditions on die walls mitigate stress singularities at the die exit. For pressure-tooling and with appropriate handling of slip, reduction in shear rate at the die exit may be achieved. Maximum shear rates for tube-tooling are about one quarter of those encountered in pressure-tooling. Equivalently, extension rates peak at land entry, and tube-tooling values are one third of those observed for pressure-tooling. With slip and tube-tooling, peak shear values at die exit may be almost completely eliminated. Nevertheless, in contrast to the pressure-tooling scenario, this produces larger peak shear rates upstream within the land region than would otherwise be the case for no-slip. Copyright © 2000 John Wiley & Sons, Ltd.

Mechanisms of hemolysis with mitral prosthetic regurgitation study using transesophageal echocardiography and fluid dynamic simulation

Objectives. The aims of this study were to define the hydrodynamic mechanisms involved in the occurrence of hemolysis in prosthetic mitral valve regurgitation and to reproduce them in a numeric simulation model in order to estimate peak shear stress.Background. Although in vitro studies have demonstrated that shear stresses >3,000 dynes/cm2are associated with significant erythrocyte destruction, it is not known whether these values can occur in vivo in conditions of abnormal prosthetic regurgitant flow.Methods. We studied 27 patients undergoing reperattion for significant mitral prosthetic regurgitation, 16 with and 11 without hemolysis. We classified the origin and geometry of the regurgitant jets by using transesophageal echocardiography. By using the physical and morphologic characteristics defined, several hydrodynamic patterns were simulated numerically to determine shear rates.Results. Eight (50%) of the 16 patients with hemolysis had paravalvular leaks and the other 8 had a jet with central origin, in contrast to 2 (18%) and 9 (82%), respectively, of the 11 patients without hemolysis (p = 0.12, power 0.38). Patients with hemolysis had patterns of flow fragmentation (n = 2), collision (n = 11) or rapid acceleration (n = 3), whereas those without hemolysis had either free jets (n = 7) or slow deceleration (n = 4) (p < 0.001, power 0.99). Numeric simulation demonstrated peak shear rates of 6,000, 4,500, 4,500, 925 anda50 dynes/cm2in these five models, respectively.Conclusions. The distinct patterns of regurgitant flow seen in these patients with mitral prosthetic hemolysis were associated with rapid acceleration and deceleration or high peak shear rates, or both. The nature of the flow disturbance produced by the prosthetic regurgitant lesion and the resultant increase in shear stress are more important than the site of origin of the flow disturbance in producing clinical hemolysis.

Noninvasive Determination of Shear-Rate Distribution Across the Arterial Lumen

In vitro experiments have shown that the shear stress exerted by flowing blood on the endothelial surface affects the morphology of the vascular wall and the release of vasoactive substances and growth factors by that wall. It is believed that the caliber of a vessel adjusts to the local shear stress to maintain a specific value of the shear stress. The local shear stress follows from local shear rate by multiplying shear rate by the local blood viscosity. The present article describes a method in which ultrasound techniques are used to assess transcutaneously the time-dependent wall shear rate in vivo in arteries. This method is applied to the assessment of wall shear rate in the common carotid artery of volunteers, presumed to be healthy, in two age categories (young age group, 20 to 30 years old, n = 8; old age group, 60 to 70 years old, n = 6). Although the peak shear rate in the young age group is markedly higher than in the old age group, the mean shear rate averaged over a cardiac cycle has the same value of 210 s-1 for both groups, corroborating earlier observations that mean shear rate and, hence, mean shear stress are maintained at a particular value. Conversion of the observed shear rates to shear stresses, assuming a blood viscosity of 3.5 mPa.s for both age groups, gives shear stresses of approximately 0.7 Pa. This is a factor of two lower than the shear stresses estimated from the relation between volume flow and artery caliber (1.5 Pa).

Role of EDRF in the Regulation of Shear Rate in Large Coronary Arteries in Conscious Dogs

To determine whether dilation of large coronary arteries normalizes shear during increased flow following brief occlusion, six dogs were instrumented to measure aortic and left ventricular pressures, left circumflex coronary artery external diameter, and coronary blood flow. The coronary artery was occluded for 15 or 30 s. Data were obtained before and after blockade of EDRF synthesis with nitro-L-arginine. Internal coronary artery diameter and wall shear were calculated on a moment-to-moment basis and the area under the flow curve was measured. Peak flow and shear rate were unaffected by NLA or by the occlusion duration. Flow curve area increased with the duration of occlusion. Internal and external diameters increased significantly for 15 s occlusions before NLA (by 4 ± 1% in external diameter and by 11 ± 4% in internal diameter) and for 30 s occlusions before NLA (by 5 ± 1% in external diameter and by 14 ± 5% in internal diameter) but not after NLA. Adenosine infusions of 0.05, 0.10, 0.50, and 1.0 μmol/kg/min were also used to dilate the coronary arteries. With each infusion, flow, shear and diameter were allowed to reach steady state. Steady state shear was reduced only slightly and did not approach the baseline state. We conclude that increased shear rate causes an increase in coronary artery diameter which is EDRF dependent. Increased coronary artery diameter during reactive hyperemia and adenosine infusions did not normalize wall shear.

Blood flow in single surface arterioles and venules on the mouse somatosensory cortex measured with videomicroscopy, fluorescent dextrans, nonoccluding fluorescent beads, and computer-assisted image analysis.

Cortical surface vessels were monitored through closed cranial windows with an epifluorescence microscope and SIT or ICCD cameras. Fluorescent dextrans or 1.3 microns latex beads were injected into the contralateral jugular vein for plasma labeling and for vascular transits. For close arterial transits, these tracers or physiological saline were injected into the ipsilateral external carotid artery. AVTTs were calculated from intensity differences of tracers between a branch of the MCA and a vein draining the same cortical region over time. AVTTs for saline dilutions of RBCs were significantly shorter (0.73 times) than for dextrans. Both dextrans and beads distributed with plasma. With FITC-dextran, inner diameters of arterioles and venules averaged 6 microns larger than hemoglobin under green light. This difference was likely due to the segregation of red blood cells and plasma during flow. Velocities of individual fluorescent beads were measured in pial vessels by strobe epi-illumination. Plots of bead velocities against radial position in arterioles were blunted parabolas. Peak shear rates in the marginal layer next to the vessel walls were determined directly from bead tracks in arterioles (D = 21-71 microns) and were 1.32 times the Poiseuille estimate. The calculated peak wall shear stress was 39 +/- 14 dyn/cm2 (mean +/- SD) for these arterioles but was probably severalfold greater in the smallest terminal pial arterioles. Vmax near the axes of arterioles increased with D+0.5. The calculated peak wall shear rate was highest in small arterioles and decreased with D-0.5. The calculated flow Q increased with D+2.5. These methods permit direct, simultaneous, dynamic measurements on multiple identified cerebral microvessels.

In Vitro Analysis of the Physical Characteristics of Femoral Bypass Cannulae

The physical dimensions and flow characteristics of arterial and venous femoral cannulae were evaluated. An in vitro femoral bypass simulator was employed to measure pressure drop across various O.D. arterial and venous cannulae from Cook Inc., C.R. Bard Inc., D.L.P. Inc., Shiley, Inc. and BioMedicus Inc. 30°C, 40% hematocrit bovine blood was employed with a Biomedicus BP-80 vortex pump. Pressure drop, peak flow velocity, shear rate, and Reynold's numbers were statistically compared. The maximum "safe" flow rate for each cannula was determined. The blood flow rate that yielded a pressure drop greater than 200 mmHg for the arterial cannulae, and the blood flow rate achieved at 300 mmHg Bio-pump inlet pressure for the venous cannulae are: Arterial Cannula Blood Flow L/min @ 200 mmHg Venous Cannula Blood Flow 1/min @300 mmHg 16 Fr. Biomedicus 3.5 18 Fr. Biomedicus 3.0 20 Fr. Canyon 4.0 20 Fr. Canyon 4.0 19 Fr. Biomedicus 5.0 18 Fr. Bard 4.5 20 Fr. Cook 6.0 20 Fr. Bard 6.5 17 Fr. CR Bard 6.0 21 Fr. DLP 6.5 20 Fr. Shiley &gt;6.5 26 Fr. Shiley &gt;6.5 20 Fr. C.R. Bard &gt;7.0 24 Fr. Biomedicus &gt;7.5 21 Fr. DLP &gt;7.0 29 Fr. Cook &gt;8.0 For the smallest Fr. O.D. to facilitate insertion and to yield the maximum safe (turbulence and friction) blood flow rate, the 20 Fr. C.R. Bard arterial cannula in tandem with the 20 Fr. C.R. Bard, or 21 Fr. DLP venous cannula, appear to be the optimal choice for full-flow femoral bypass.

Comparison of steady and pulsatile flow in a double branching arterial model

Data are presented to compare fluid flow parameters for steady flow with those for time-varying flow in a simplified two branch model which simulates the region of the abdominal aorta near the celiac and superior mesenteric branches of the dog. Measurements in the model included laser doppler anemometry velocity profiles during steady flow, sinusoidal flow with a superimposed mean flow (referred to as simple oscillatory flow) and arterial pulsatile flow. Shear rate measurements were made by an electrochemical technique during steady flow. Flow visualization studies were done during steady and pulsatile flow. Fluid flow effects in the simplified model during steady flow showed many similarities to the results from previous steady flow studies in a canine aortic cast. Shear rates in the region of the proximal (first, or celiac) branch were independent of flow rates in the distal (second, or mesenteric) branch, but the shear pattern within the proximal branch changed significantly as flow in the proximal branch increased. Shear rates on the proximal flow divider (leading edge into the distal branch) depended primarily on the flow rate to the proximal branch, but not on flow to the distal branch. At certain daughter branch flow ratios (approximately 2:1, proximal to distal), flow separation was promoted at the outer wall of the second branch, but flow separation did not occur in the first branch. In contrast to the canine aortic cast results, flow separation was never detected on the distal (mesenteric) flow divider of the simplified model. This observation reflects the subtle effects of geometry on flow since the mesenteric flow divider in the canine cast protrudes into the main flow whereas the distal flow divider in the simplified model does not. There were distinct differences in the flow phenomena between steady, simple oscillatory and arterial pulsatile flow. Peak shear rates during pulsatile flow were as much as 10–100 times greater than steady flow shear rates at comparable mean flow rates. Particularly noteworthy for the pulsatile flow with a Womersley parameter of sixteen were very blunt velocity profiles throughout systole, and the absence of flow separation or reversal in those regions of the model that exhibited flow separation during steady flow. The shape of the waveform influences the nature of the flow during time-varying flows. Future studies of fluid dynamics in model systems must consider the pulsatile nature of the flow if a true interpretation of arterial flow phenomena is to be made.

Action of shear on enzymes: studies with alcohol dehydrogenase.

Yeast alcohol dehydrogenase (ADH) solutions (approximately 1 mg/ml, pH 7) were sheared in a coaxial cylindrical viscometer. This was fitted with a lid sealing the contents from the atmosphere and preventing evaporation. At 30 degrees C after a total of 5 hr intermittent shearing at 683 sec-1 no losses of activity were observed. No losses were found after 5 hr continuous shearing and in a no-shear control. At 40 degrees C and 683 sec-1 there were only small activity losses in 5 hr. Shearing at 3440 sec-1 no measurable losses of activity were found with a 1.03 mg/ml solution in 5 hr at 30 degrees C, a 1.03 mg/ml solution in 8 hr at 5 degrees C, and with a 3.89 mg/ml solution in 3 hr at 5 degrees C. In all these cases, however, a white precipitate formed that was not observed in zero shear control experiments. The sheared 3.89 mg/ml solution was clarified by centrifugation. It was shown that there were no ADH aggregates in the supernatant and that the precipitate was less than 2% of the original protein. At 30 degrees C under adverse pH conditions (pH 8.8) there was no significant difference in activity losses of an approximately 1 mg/ml solution sheared at 65 and 744 sec-1. An approximately 0.5 mg/ml ADH solution, pH 7, was agitated in a small reactor with no free air-liquid interface. Peak shear rates near the impeller were estimated to be about 9000 sec-1. Only a small decrease in specific activity was observed until over 15 hr total running at 5 degrees C.