In this study, we analyzed blood flow through a model stenosis with Reynolds numbers ranging from 300 to 3,600 using both experimental and numerical methods. The jet produced at the throat was ...turbulent, leading to an axisymmetric region of slowly recirculating flow. For higher Reynolds numbers, this region became more disturbed and its length was reduced. The numerical predictions were confirmed by digital particle image velocimetry and used to describe the fluid dynamics mechanisms relevant to prior measurements of platelet deposition in canine blood flow (R.T. Schoephoerster et al., Atherosclerosis and Thrombosis 12:1806-1813, 1993). Actual deposition onto the wall was dependent on the wall shear stress distribution along the stenosis, increasing in areas of flow recirculation and reattachment. Platelet activation potential was analyzed under laminar and turbulent flow conditions in terms of the cumulative effect of the varying shear and elongational stresses, and the duration platelets are exposed to them along individual platelet paths. The cumulative product of shear rate and exposure time along a platelet path reached a value of 500, half the value needed for platelet activation under constant shear (J.M.. Ramstack et al., Journal of Biomechanics 12: 113-125, 1979).
Laminar and turbulent numerical simulations of steady flow in an aneurysm model were carried out over Reynolds numbers ranging from 300 to 3600. The numerical simulations are validated with Digital ...particle Image Velocimetry (DPIV) measurements, and used to study the fluid dynamic mechanisms that characterize aneurysm deterioration, by correlating them to in vitro blood platelet deposition results. It is shown that the recirculation zone formed inside the aneurysm cavity creates conditions that promote thrombus formation and the viability of rupture. Wall shear stress values in the recirculation zone are around one order of magnitude less than in the entrance zone. The point of reattachment at the distal end of the aneurysm is characterized by a pronounced wall shear stress peak. As the Reynolds number increases in laminar flow, the center of the recirculation region migrates toward the distal end of the aneurysm, increasing the pressure at the reattachment point. Under fully turbulent flow conditions (Re = 3600) the recirculation zone inside the aneurysm shrinks considerably. The wall shear stress values are almost one order of magnitude larger than those for the laminar cases. The fluid dynamics mechanisms inferred from the numerical simulation were correlated with measurements of blood platelet deposition, offering useful explanations for the different morphologies of the platelet deposition curves.
Cerebrovascular damage leading to subsequent reductions in local cerebral blood flow (lCBF) may represent an important secondary injury mechanism following traumatic brain injury (TBI). We determined ...whether patterns of 111-indium-labeled platelet accumulation were spatially related to alterations in lCBF determined autoradiographically 30 min after TBI. Sprague-Dawley rats (n = 8), anesthetized with halothane and maintained on a 70:30 (vol/vol) mixture of nitrous oxide/oxygen and 0.5% halothane, underwent parasagittal fluid percussion brain injury (1.7-2.2 atm). 111-Indium-tropolone-labeled platelets were injected 30 min prior to TBI while 14C-iodoantipyrine was infused 30 min after trauma. Sham-operated animals (n = 7) underwent similar surgical procedures but were not injured. In autoradiographic images of the indium-labeled platelets, focal sites of platelet accumulation within the traumatized hemisphere were restricted to the pial surface (five of eight rats), the external capsule underlying the lateral parietal cortex (five of eight rats), and within cerebrospinal fluid (CSF) compartments (six of eight rats). In contrast, mild-to-moderate reductions in lCBF, not restricted to sites of platelet accumulation, were seen throughout the traumatized hemisphere. Flow reductions were most severe in coronal sections underlying the impact site. For example, within the lateral parietal cortex and hippocampus, lCBF was significantly reduced p <0.01; analysis of variance (ANOVA) from 1.71 +/- 0.34 (mean +/- SD) and 0.78 +/- 0.12 ml/g/min, respectively, versus 0.72 +/- 0.17 and 0.41 +/- 0.06 ml/g/min within the traumatized hemisphere. Significant flow reductions were also seen in remote cortical and subcortical areas, including the right frontal cortex and striatum. These results indicate that focal platelet accumulation and widespread hemodynamic depression are both early consequences of TBI. Therapeutic strategies directed at these early microvascular consequences of TBI may be neuroprotective by attenuating secondary ischemic processes.
The restenosis or occlusion that frequently follows balloon angioplasty is poorly understood. Thus, the pathophysiological response to angioplasty of the common carotid artery in 38 heparinized ...normal pigs was investigated by quantification of the 111In-labeled platelet deposition and histological and electron microscopic examination from 1 hour to 60 days after angioplasty. At 1 hour, the following findings were noted: complete endothelial denudation in all arteries, marked platelet deposition (44.7 +/- 20.7 X 10(6)/cm2), mural thrombus in seven of 10 pigs, and a medial tear extending through the internal elastic lamina in nine of 18 arteries. All nine arteries with tears had associated mural thrombus and severe platelet deposition (76 X 10(6)/cm2); in contrast, the nine arteries without a tear had no mural thrombus and much lower platelet deposition (6 X 10(6)/cm2). Necrosis of medial smooth muscle cells was evident at 24 hours. Platelet deposition remained high at 24 hours (40.5 +/- 20.6 X 10(6)/cm2), but was markedly reduced at 4 days (4.4 +/- 1.5 X 10(6)/cm2), coincident with partial regrowth of endothelium or periluminal lining cells. No significant platelet deposition was noted at 7 days, when the endothelial cell type of regrowth was largely complete. Intimal proliferation of smooth muscle cells was mild and patchy at 7 days, significantly greater and more uniform at 14 days, and unchanged at 30 and 60 days after angioplasty. Complete thrombotic occlusion occurred in four (11%) of the 38 pigs. A significant stenosis present at 30 days after angioplasty was shown by histological examination to be due to organization of mural thrombus.
An important aspect of blood-material interactions is the activation, adhesion, and subsequent aggregation of blood platelets on the artificial surface, all of which are directly affected by local ...fluid dynamics. The objective of this work was to directly correlate changing local fluid dynamic conditions produced by various vessel geometries, including stenosis, aneurysm, and separate contraction and expansion geometries, with quantitative in vitro measurements of regional platelet deposition. We directly measured platelet deposition as a function of axial position along four Lexan flow chambers with axisymmetric models of these geometries using 111In-labeled platelets. Platelet deposition was maximum in observed areas of flow recirculation and reattachment and minimum in locations of high shear and separation. For the stenosis geometry, two distinct regions of increased platelet deposition were apparent, one proximal to and one distal to the stenosis throat. An approximately linear increase in platelet densities was produced in the aneurysm region, increasing in the direction of flow. Through a comparison of platelet deposition with local fluid streamline orientation, we have shown that platelet deposition is increased in certain areas due to the enhanced convective transport of platelets and blood cells to the vessel wall along locally curved streamlines with velocity components perpendicular to the vessel wall.
Mild-to-moderate reductions in local cerebral blood flow (ICBF) have been reported to occur in rats after moderate (1.7-2.2 atm) fluid percussion brain injury. The purpose of this study was to ...determine whether evidence for severe ischemia (i.e., mean ICBF < 0.25 ml/g/min) could be demonstrated after severe brain injury. In addition, patterns of indium-labeled platelet accumulation and histopathological outcome were correlated with the hemodynamic alterations.
Sprague-Dawley rats (n = 23), anesthetized with halothane and maintained on a 70:30 mixture of nitrous oxide:oxygen and 0.5% halothane, underwent normothermic (37 degrees C) parasagittal fluid percussion brain injury (2.4-2.6 atm). Indium-111-tropolone-labeled platelets were injected 30 minutes before traumatic brain injury (TBI), while 14C-iodoantipyrine was infused 30 minutes after trauma for ICBF determination. Sham-operated animals (n = 8) underwent similar surgical procedures but were not injured. For histopathological analysis, traumatized rats (n = 5) were perfusion-fixed 3 days after TBI.
In autoradiographic images of indium-labeled platelets, abnormal platelet accumulation that was most pronounced overlying the pial surface was commonly associated with severe reductions in ICBF within underlying cortical regions 30 minutes after TBI. For example, within the lateral parietal cortex, ICBF was significantly reduced from 1.67 +/- 0.11 ml/g per minute (mean +/- standard error of the mean) in sham-operated animals to 0.23 +/- 0.03 ml/g per minute within the traumatized group. In addition to focal severe ischemia, moderate reductions in ICBF were detected throughout the traumatized hemisphere, including the frontal and occipital cortices, hippocampus, thalamus, and striatum. Mild decreases in ICBF were also observed throughout the contralateral cerebral cortex. At 3 days after severe TBI, histopathology demonstrated intracerebral and subarachnoid hemorrhage associated with cerebral contusion and selective neuronal necrosis.
These data indicate that multiple cerebrovascular abnormalities, including subarachnoid hemorrhage, focal platelet accumulation, and severe ischemia, are important early events in the pathogenesis of cortical contusion formation after TBI. Injury severity is expected to be a critical factor in determining what therapeutic strategies are attempted in the clinical setting.
The relative importance of hemodynamic factors in the pathogenesis of thrombotic or embolic stroke is unclear. Of particular therapeutic interest are those substances that facilitate vasodilation and ...the clearance of platelet aggregates in the compromised microvasculature. A likely contributor to these functions is nitric oxide because it is known to inhibit platelet aggregability and promote vascular relaxation. To investigate the involvement of nitric oxide in the hemodynamic changes after experimental ischemia, photochemically induced nonocclusive common carotid artery thrombosis (CCAT) was studied. CCAT is a rat model of unilateral carotid artery stenosis and platelet embolization to the brain. This study characterized the acute hemodynamic consequences of CCAT and the resultant pattern of platelet deposits with and without nitric oxide synthase inhibition by nitro-L-arginine methyl ester (L-NAME). In addition, the subacute local cerebral blood flow changes were studied at 24 hours. Right CCAT was produced in 30 male Wistar rats injected with (111)In-labeled platelets. Between 5 and 15 minutes after thrombosis, rats were treated with either 15 mg/kg of L-NAME (intravenously) or saline vehicle. Hemodynamic changes were studied 30 to 45 minutes after thrombosis using 14Ciodoantipyrine autoradiography. Eight coronal levels were analyzed, and cortical and subcortical regions of interest were defined. Significant increases were observed in total platelets in the ipsilateral hemisphere after L-NAME treatment, and in the distribution of platelets in the anterior frontal and occipital cortices with nitric oxide synthase inhibition, encompassing the anterior and posterior border zone areas of the ipsilateral cortex. Otherwise, foci of labeled platelets were detected throughout the ipsilateral and contralateral hemispheres. Mean local cerebral blood flow images (n = 5) revealed a moderate bilateral global reduction in flow acutely, which normalized in the untreated thrombosed group by 24 hours. In contrast, the L-NAME-treated groups (sham and experimental) had lasting, widespread reductions in flow of approximately 25%. Pairwise comparisons between groups showed that CCAT/L-NAME was significantly different from shams in the corpus callosum and different from L-NAME shams in the internal capsule (P < 0.05) These hemodynamic and platelet accumulation changes may partially account for the aggravation of cognitive and sensorimotor deficits previously reported in this model of thromboembolic stroke.
Although patients with urinary incontinence have been treated successfully by periurethral injection of polytef paste, this study in continent animals demonstrates migration of polytef particles from ...the injection site. We injected polytef paste periurethrally into female dogs and male monkeys. Particles were found at 50 to 70 days in pelvic nodes in six of seven animals and lungs in four of seven (the kidneys and brain were not studied); and at 10 1/2 months in pelvic nodes, lungs, and brain in seven of seven; kidneys in four of seven; and spleen in two of seven. X-ray microanalysis confirmed that the particles were polytef. At 10 1/2 months, polytef granulomas were found at all injection sites and some sites of distant migration. Since these granulomas signify chronic foreign-body reaction, we believe that until the long-term effects in humans are known, polytef paste should not be used in children or young adults with normal life expectancy.
The adenosine-regulating agent acadesine has been shown to reduce the incidence of myocardial infarction and stroke after cardiopulmonary bypass surgery. The present study examined the effect of ...acadesine on the accumulation of indium-labeled platelet emboli and infarct size after photothrombosis of the common carotid artery.
Rats were anesthetized with halothane and preloaded with 111In-tropolone-labeled platelets (50 to 80 microCi) 30 minutes before nonocclusive common carotid artery thrombosis induced by a rose bengal-mediated photochemical insult. Intravenous infusion of acadesine (0.5, 1, or 2 mg/kg per minute) or vehicle was begun 30 minutes before right common carotid artery thrombosis and continued for an additional 15 minutes. Rats were then killed and brains processed for the autoradiographic quantitation of labeled platelet aggregates. In a separate group of rats, infarct areas and volumes were determined in treated (acadesine 1 mg/kg per minute) (n = 9) and nontreated (n = 9) rats 7 days after thrombosis.
Although the ratio of right-to-left common carotid artery radioactivity was not affected by treatment, acadesine at 1 and 2 mg/kg per minute significantly decreased (P < .01) platelet deposition within the right cerebral cortex, hippocampus, and striatum. For example, within the frontoparietal cortex, numbers of platelet aggregates were 11.8 +/- 1.8 (mean +/- SEM), 6.1 +/- 1.4, 2.3 +/- 0.6, and 3.2 +/- 0.8 in rats infused with vehicle, 0.5, 1, and 2 mg/kg per minute acadesine, respectively. In addition, infarct volume was reduced by 48% in acadesine-treated (1 mg/kg per minute) rats, with a significant reduction in infarct area at the coronal level 3.7 mm anterior to bregma (P < .01).
These results support a prophylactic role for acadesine in reducing the accumulation of platelet emboli during vascular thrombosis and subsequent brain infarction. Acadesine treatment in patients at risk for embolic stroke could potentially lead to cerebral protection.
The aim of the study was to evaluate the effect of binding hydrophilic polyethylene oxide (PEO) onto Dacron fibers in the sewing ring of a mechanical heart valve (MHV), in terms of thrombogenicity of ...the prosthesis.
The study was performed in blinded fashion. Six Yorkshire-cross pigs (bodyweight 35-45 kg) were implanted with MHVs, in the mitral annulus, with the PEO-treated sewing ring. An additional five pigs implanted with identical MHVs, but with untreated sewing rings, served as controls. PEO of chain-length 10,000 Da was grafted to Dacron fibers using gamma irradiation. PEO-bonded Dacron fibers (diameter 100 microns) were used to weave the sewing ring, which was then assembled on a titanium stent (OD 25 mm). Autologous platelets were labeled with 111In-tropolone and injected intravenously (850-1250 microCi per injection) into the pigs on removal from cardiopulmonary bypass (CPB). At 20-24 h after surgery, platelet thrombi adherent to MHV components, and shed emboli trapped in the brain, lung, heart, kidneys and other organs/connective tissues were imaged using a gamma camera. The animals were killed and the amounts of thrombi adherent to MHV components and organ-trapped emboli quantified using an ionization chamber and gamma counter.
There was no statistically significant difference in the adhesion of 111In-labeled platelets to either control sewing rings (0.08 +/- 0.06% dose) or PEO-treated rings (0.19 +/- 0.21% dose). The thrombogenicity of MHV components in both animal groups was in the ascending order: Dacron ring > Teflon pledgets > polypropylene sutures > titanium housing > pyrolytic carbon. The number of platelet-emboli trapped in the organs was not significantly different between the two groups.
Simple modifications may not reduce platelet thrombosis or wound-healing of the sewing ring in the acute phase, at which time several complex processes are activating and inactivating platelets and coagulant factors during CPB and implantation of MHVs.
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