The effects of extremely low shear stress on cellular proliferation and neointimal thickening in the failing bypass graft.

OBJECTIVE: Previous studies demonstrating a correlation between low shear stress (tau = 5-15 dyne/cm(2)) and experimental vein graft neointimal thickening (NIT) support the role of low tau in vein graft failure. However, a simple linear relationship between low tau and NIT would underestimate the degree of NIT evident in high-grade occlusive lesions of failing human vein grafts. In this study we used a new experimental model that maintains patency at low tau (< 2 dyne/cm(2)), to delineate possible deviations from linearity in the low tau --> NIT hypothesis. METHODS: Thirty-two New Zealand White rabbits underwent creation of a common carotid vein patch with a segment of ipsilateral external jugular vein. Very low tau was created in 13 patches by ligation of the distal common carotid artery, leaving the only outflow through a small muscular branch. Normal tau was created in 11 patches by leaving the common carotid artery outflow intact. High tau was created in eight patches by ligation of the contralateral common carotid artery. Six patches were harvested after 2 weeks for measurement of cell cycle entry by proliferating cell nuclear antigen (PCNA) immunohistochemistry. The remaining 26 patches were harvested after 4 weeks, perfusion fixed, and excised for morphometric analysis. RESULTS: Mean blood flow and tau at implantation ranged from 0.5 to 41 mL/min and 0.07 to 15 dyne/cm(2), respectively. At the time of harvest, 30 of 32 patches remained patent, and the artificially created aberrations in blood flow were maintained (range, 0.7-41 mL/min). After 2 weeks PCNA immunohistochemistry showed a significantly higher level of cell cycling in patches exposed to low tau (40 +/- 5 vs 1.6 +/- 0.3 PCNA-positive cells per high-power field; P <.001), which is equivalent to approximately 20% of the total cells present. In patches harvested after 4 weeks, NIT ranged from 42 to 328 microm and significantly correlated with mean tau at implantation. Patches with very low tau exhibited histologic characteristics similar to those of failing human bypass grafts, including laminar thrombus and flow-limiting luminal stenosis. The relationship between tau and NIT was nonlinear in that extremely low tau (< 2 dyne/cm(2)) resulted in NIT beyond that predicted by a simple linear correlation (P =.003). CONCLUSION: Extremely low tau (< 2 dyne/cm(2)) stimulates high rates of smooth muscle cellular proliferation in arterialized vein patches. NIT is accelerated in these regions of low tau far beyond that predicted by a simple linear model. The nonlinear nature of the cellular proliferative response and NIT at tau less than 2 dyne/cm(2) may explain the rapid progression of neointimal lesions in failing bypass grafts.

Atherosclerotic enlargement of the human abdominal aorta.

Aortic aneurysms usually develop in the atherosclerosis prone infrarenal abdominal aorta. To assess the role of atherosclerosis in aortic enlargement, we studied the relation between plaque formation and aortic size in 30 pressure-fixed male cadaver aortas (age 40-95 years, mean age 67 years). Morphometric analysis of transverse sections of the mid-thoracic and the mid-abdominal aortas included measurement of intimal plaque area, lumen area, plaque and media thicknesses. The area encompassed by the internal elastic lamina area (IEL area) was taken to be an index of aortic size. IEL area increased with age at both the thoracic (r=0.77, P<0.01) and abdominal (r=0.54, P<0.01) aortic levels. The aorta also enlarged with increasing plaque area at the thoracic (r=0.73, P<0.01) and abdominal (r=0.79, P<0.01) levels. Regression analysis of IEL area on age, body weight, height and plaque area revealed that the primary predictor of thoracic aortic size was age, whereas the primary predictor of abdominal aortic size was plaque area. Plaque thickness in the abdominal aorta was greater than in the thoracic aorta (P<0.01). Increased plaque area was associated with a significant decrease in media thickness in the abdominal aorta (r=-0.75, P<0.01) but not in the thoracic aorta. Aortas with relatively enlarged abdominal segments, i.e. those with a thoracic to abdominal ratio of <1.2 (n=13), were compared to those with a normal ratio (> or =1.2, n=17). Relatively large abdominal aortas had twofold greater plaque area (P<0.001), reduced medial thickness (P<0.05), fewer medial elastic lamellae (P<0.01) and greater mural tensile stress (P<0.05) than relatively normal abdominal aortas. We conclude that plaque formation in the infrarenal abdominal aorta in humans is associated with aortic enlargement and decreased media thickness. These changes may be predisposing factors for the preferential development of subsequent aneurysmal dilation in the abdominal aorta.

Aneurysmal and occlusive atherosclerosis of the human abdominal aorta.

PURPOSE: The purpose of this study was to assess atherosclerotic plaque deposition and aortic wall responses in the abdominal aorta in relation to the development of aneurysmal and occlusive disease in the infrarenal aorta. METHODS: Morphologic differences at five standardized locations in the infrarenal aorta in 67 pressure perfusion-fixed male cadaver aortas (aged, 41-98 years; mean, 66 years) were studied and compared with the supraceliac segment. Quantitative computer-assisted morphometry of histologic sections included measurement of plaque area, lumen area, lumen diameter, media thickness, number of medial elastic lamellae, and the area encompassed by the internal elastic lamina that best represents the artery size of each segment. The ratio of the supraceliac segment to the midabdominal segment (normally greater than 1.3) was used to define three groups: Group I (normal): ratio greater than or equal to 1.30 (n = 31); Group II (intermediate): ratio greater than or equal to 1.20 but less than 1.30 (n = 20); and Group III: ratio less than 1.20 (n = 16), which represented dilated midabdominal aortas. There was no significant difference in age among the groups. RESULTS: Group I had minimal intimal plaque and little gross evidence of atherosclerosis. Group II had increased intimal plaque compared with Group I (P <.01) and gross evidence of atherosclerosis, which was maximally localized in the distal aorta; there was no aortic enlargement or thinning of the media underneath the plaque. Group III had more intimal plaque than Group I (P <.01) and Group II (P <.01) and was associated with localized aortic enlargement and media thinning compared with Group I (P <.05) and Group II (P <.01). Increasing intimal plaque in Group III correlated with an increase in lumen diameter (r = 0.61, P <.05), but this relationship was not significant in Group I and Group II. The aortic media in Group III had a reduced number of medial elastic lamellae, was reduced in thickness, and was more exposed to increased wall stress than the aortas in Groups I and II. CONCLUSION: These results suggest that there may be different local responses to atherosclerosis in the abdominal aorta in human beings. Plaque deposition associated with localized dilation, thinning of the media, and loss of medial elastic lamellae may predispose that segment of aorta to subsequent aneurysm formation. Plaque deposits without media thinning, without loss of elastic lamellae, and without artery wall dilation may predispose the aorta, in the event of continuing plaque accumulation, to the development of lumen stenosis.

Prevention of restenosis by a herpes simplex virus mutant capable of controlled long-term expression in vascular tissue in vivo.

Neointimal hyperplasia resulting from vascular smooth muscle cell (SMC) proliferation and luminal migration is the major cause of autologous vein graft failure following vascular coronary or peripheral bypass surgery. Strategies to attenuate SMC proliferation by the delivery of oligonucleotides or genes controlling cell division rely on the use of high concentrations of vectors, and require pre-emptive disruption of the endothelial cell layer. We report a genetically engineered herpes simplex virus (HSV-1) mutant that, in an in vivo rabbit model system, infects all vascular layers without prior injury to the endothelium; expresses a reporter gene driven by a viral promoter with high efficiency for at least 4 weeks; exhibits no systemic toxicity; can be eliminated at will by administration of the antiviral drug acyclovir; and significantly reduces SMC proliferation and restenosis in vein grafts in immunocompetent hosts.

Hypercholesterolemia superimposed by experimental hypertension induces differential distribution of collagen and elastin.

We studied the mural distribution of collagen types I and III and tropoelastin in enhanced experimental atherogenesis induced in rabbits by hyperlipidemia superimposed by hypertension. Animals were fed a high-cholesterol diet for 5 weeks and also subjected to midthoracic aortic coarctation for 4 weeks. Serum cholesterol levels were increased and blood pressure was elevated proximal to the coarctation. Foam cell lesions developed in the aorta proximal to the coarctation. In situ hybridization and immunohistochemistry showed that gene expression of collagen types I and III and tropoelastin was upregulated, with a differential distribution across the arterial wall. New collagen type I was mainly distributed in the intima, the outer media, and the adventitia. New collagen type III was spread more uniformly across the wall, including the adventitia, whereas tropoelastin was mainly localized in intimal foam cell lesions. Morphometric data showed an increase in wall thickness. These results suggest that collagen types I and III play a role in remodeling of the aortic wall in response to hypertension. The remarkable involvement of the adventitia in this response indicates that the adventitia is an important component of the arterial wall. Tropoelastin is closely associated with foam cell lesion formation, suggesting a role for this component in atherogenesis as well.

Apolipoprotein (a) fragments in relation to human carotid plaque instability.

PURPOSE: An elevated plasma level of lipoprotein (a) is an independent risk factor for atherothrombotic cardiovascular disease by yet undefined mechanisms. We have previously reported that matrix metalloproteinases cleave apolipoprotein (a) into 2 main fragments, F1 and F2, the latter (the C-terminal domain) exhibiting in vitro a high-affinity binding to extracellular matrix components, including fibrin(ogen). We therefore tested the hypothesis that the lipoprotein (a) matrix metalloproteinase-derived F2 is localized in potentially or morphologically unstable human carotid plaque at regions of increased matrix metalloproteinase activity. METHODS: Carotid plaques removed after endarterectomy (n = 18) were evaluated for structural features indicative of instability (thin fibrous cap, inflammation, and proximity of the necrotic core to the lumen); each plaque was classified as unstable (n = 10) or stable (n = 8). Western blot analysis was performed to quantitate apolipoprotein (a) and its fragments F1 and F2 in plaque extracts. Immunohistochemical staining was used to localize apolipoprotein (a) and its fragments within the atherosclerotic plaque. In situ zymography was used to determine regions of gelatinase (matrix metalloproteinase 2 and matrix metalloproteinase 9) activity. RESULTS: Western blot analyses demonstrated a 2.5-fold higher density of F2 in unstable plaques than in stable plaques (3.07 +/- 1.9 vs 1.18 +/- 0.8; P <.05). In morphologically unstable plaques, there was preferential distribution of F2 within regions of fibrous cap inflammation and/or foam cell accumulation and within abluminal necrotic cores. In morphologically stable plaques, however, localization was predominantly found in the medial smooth muscle cells. Regions of enhanced matrix metalloproteinase 2 and matrix metalloproteinase 9 activity co-localized with the transmural distribution of F2 within the plaque. CONCLUSIONS: These findings suggest that F2 in regions of increased matrix metalloproteinase activity is a potential mechanism for superimposed thrombotic events in morphologically unstable human carotid plaques. The relationship between plasma lipoprotein (a) levels and accumulation of F2 and the potential correlation of F2 to human plaque disruption and thrombosis warrant further study.

Differential transmural distribution of gene expression for collagen types I and III proximal to aortic coarctation in the rabbit.

To assess the effects on the biosynthesis of collagen types I and III associated with an acute increase in blood pressure, we established a mid-thoracic aortic coarctation in the rabbit and studied gene expression and protein accumulation of these collagen types proximal to the stenosis 1, 3 and 7 days and 2, 4 and 8 weeks after coarctation. The mRNA level of type I collagen pro-alpha2(I) was maximal at 3 days and returned to normal at 4 weeks. mRNA of pro-alpha2(I) was localized mainly in the outer media, adventitia and intima. Accumulation of type I collagen and its precursors was increased by 3 days, peaked at 4 weeks, and decreased toward normal by 8 weeks, corresponding to the distribution of pro-alpha2(I) mRNA. Gene expression for pro-alpha1(III) was similar to that of pro-alpha2(I) but was distributed throughout the media. We conclude that the mechanical stresses associated with an acutely induced alteration in pressure initiate rapid gene expression for collagen types I and III in the aortic wall. The response for collagen type I, predominantly in the outer media and adventitia, suggests that these regions play an immediate role in the resistance to excessive dilatation of the aorta. The diffuse response for collagen type III in the media suggests participation in a more extensive remodeling response associated with the reinforcement and reorganization of the musculo-elastic fascicles.

Increased flow and shear stress enhance in vivo transforming growth factor-beta1 after experimental arterial injury.

We have previously demonstrated that high-flow (HF) conditions inhibit experimental intimal hyperplasia. We hypothesized that such flow conditions may alter transforming growth factor-beta1 (TGF-beta1) after mural injury. The right common carotid artery (CCA) was balloon-injured in 54 New Zealand White male rabbits. Flow was thereafter preserved (normal flow [NF]), reduced by partial outflow occlusion (low flow [LF]), or increased by ligation of the left CCA (HF). Four sham-operated animals served as uninjured controls. Mean blood flow and pressure in the right CCA were measured before and after flow modulation and before euthanasia (3, 7, and 14 days). TGF-beta1 mRNA and protein levels in the right CCA were determined by Northern and ELISA analyses at each time point. At 7 and 14 days, intimal hyperplasia was quantified, and the transmural localization of TGF-beta1 was determined by immunohistochemical analysis. Mean flow was reduced from 22+/-1 to 10+/-3 mL/min in the LF group and increased to 34+/-2 mL/min in the HF group (P<0.001). Blood pressure was not different among the flow groups for all time points. Wall shear stress was markedly decreased in the LF group to 14+/-4 dyne/cm(2) and increased in the HF group to 63+/-6 dyne/cm(2) at 7 days compared with values in uninjured controls (39+/-2 dyne/cm(2), P<0.001) and the NF group (44+/-7 dyne/cm(2), P<0.001). At 14 days, wall shear stress was similar among the flow groups. The intima-to-media ratio was 5- and 2-fold greater in the LF group than in the HF and NF groups at 14 days. mRNA levels for TGF-beta1 and its active ligand were increased in the HF group by at least 2- and 3-fold, respectively, at 3 and 7 days compared with levels in uninjured controls and the LF group (P<0.05) but were not different among the flow groups at 14 days. TGF-beta1 preferentially localized in the abluminal vascular smooth muscle cells of the HF arterial segments. Flow- and shear-mediated release of TGF-beta1 may therefore play a role in abrogating the proliferative and migratory response of vascular smooth muscle cells in the early stages after mural injury.

Gene therapy enhances the antiproliferative effect of radiation in intimal hyperplasia.

BACKGROUND: Although ionizing radiation (IR) has been demonstrated to attenuate vessel wall restenosis and intimal hyperplasia (IH), dose-related mural injury and atrophy are possible deleterious side effects. We tested the hypothesis that a radiosensitizing strategy may improve IR-induced inhibition of in vivo vascular smooth muscle cells (VSMCs) without influencing apoptotic cell death. METHODS: In 28 New Zealand White rabbits, the right common carotid artery (CCA) was injured and subjected to low-flow conditions to promote IH. The CCA was transfected with an adenoviral vector incorporating the cytosine deaminase (CD) gene (1 x 10(9) PFU/ml). 5-Fluorocytosine (5-FC), a prodrug that is converted to the radiosensitizing agent 5-fluorouracil (5-FU) by CD, was thereafter administered intravenously. The CCA was exposed to 5 Gy IR at 24 h. Intimal/medial (I/M) area and thickness ratios were determined in the harvested CCAs at 14 days. VSMC proliferative and apoptotic indices were assessed with immunohistochemistry. RESULTS: A 50% reduction in I/M area was found in rabbits treated with IR and IR + CD/5-FC (0.19 +/- 0.03 and 0.18 +/- 0.02) when compared with untreated controls (UC) (0.37 +/- 0.06) (P = 0.005). This finding was substantiated by attenuation of I/M thickness in the IR groups [0.47 +/- 0.13 (IR), 0.41 +/- 0.11 (IR + CD/5-FC), 0.61 +/- 0.17 (UC)] (P = 0.007). The number of proliferating VSMCs was notably smaller when IR was combined with CD/5-FC (4.17 +/- 1.16 vs 2.97 +/- 1.09 log transformed cells/mm(2), P < 0.07). Apoptosis was similar in all groups. CONCLUSIONS: Both IR alone and IR combined with a radiosensitizing agent are effective in attenuating experimental IH. However, combination therapy is synergistic and achieves greater inhibition of VSMC proliferation and may involve selective killing of radioresistant S-phase VSMCs. IR + CD/5-FC represents a novel therapeutic strategy that offers potential for long-term control of IH.

Adaptive remodeling of internal elastic lamina and endothelial lining during flow-induced arterial enlargement.

Gaps in the internal elastic lamina (IEL) have been observed in arteries exposed to high blood flow. To characterize the nature and consequences of this change, blood flow was increased in the carotid arteries of 56 adult, male, Japanese white rabbits by creating an arteriovenous fistula between the common carotid artery and the external jugular vein. The common carotid artery proximal to the arteriovenous fistula was studied at intervals from 1 hour to 8 weeks after exposure to high flow. In the controls, the IEL showed only the usual, small, physiological holes, 2 to 10 microm in diameter. At 3 days, some of the holes in the IEL had become enlarged, but they could not be detected by scanning electron microscopy, despite manifest endothelial cell proliferation. At 4 days, gaps in the IEL appeared as small, luminal surface depressions, 15 to 50 microm wide. At 7 days, the gaps in the IEL had enlarged and formed circumferential, luminal depressions occupying 15+/-5% of the lumen surface. Endothelial cell proliferation persisted in the gaps while proliferative activity decreased where the IEL remained intact. At 4 weeks, as the artery became elongated and dilated, the gaps in the IEL widened as intercommunicating circumferential and longitudinal luminal depressions occupying 64+/-5% of the lumen surface. At 8 weeks, the rate of elongation and dilatation of the artery slowed and the widening of the gaps in the IEL diminished. Endothelial cells covered the gaps throughout. We conclude that flow-induced arterial dilatation is accompanied by an adaptive remodeling of the intima. The gaps in the IEL permit an increase in lumen surface area while endothelial cell proliferation assures a continuous cell lining throughout.

Biomechanical factors as regulators of biological responses to vascular grafts.

Biomechanical forces have been implicated in the induction and progression of intimal hyperplastic thickening in vein, prosthetic, and endovascular bypass grafts. Graft implantation imposes significant alterations is shear and tensile forces. Such physical forces play an important role in modulating those cellular and molecular events that underlie regulation of vascular healing and adaptation. Characterization of such hemodynamic variables that induce perpetual medial vascular smooth muscle cell proliferation and migration will help in identification of those grafts at risk for occlusion and limited long-term patency and in design of therapeutic strategies that attenuate progressive intimal hyperplasia.

Irradiation for the treatment of intimal hyperplasia.

Intimal hyperplasia represents a serious complication limiting the long-term benefits of vascular interventions such as balloon angioplasty and stent placement. Although pharmacological interventions have attempted to curtail restenosis, they have not been shown to be effective to date. Radiotherapy is one alternative that has shown promise as an inhibitor of intimal hyperplasia in several animal models. Irradiation causes cell death by producing irreparable damage to DNA. This is believed to be the mechanism of inhibition of VSMC proliferation. Delivery of irradiation can be either intraluminal via an angiographically directed catheter or extraluminal using an external radiation source such as an x-ray device. Intraluminal irradiation has generally utilized either gamma or beta-emitting sources. Both have been effective in producing a dose response, although some studies advocate the use of beta-type irradiation as a safer, more efficient means of delivery. Extraluminal irradiation also has been an effective inhibitor of intimal hyperplasia. Studies suggest that this form of irradiation provides a more even-dose distribution to vessel walls than an intravascular delivery system. The use of radiotherapy has more recently been extended to clinical trials, and initial studies have shown promising results. The success of irradiation must be balanced with its potential complications including radiation-induced arteritis, coronary artery stenosis, and secondary development of malignancy. Although these have been associated with irradiation, the dose used in these cases was often considerably higher than those used in the treatment of intimal hyperplasia. Finally, with the advent of gene therapy, irradiation may provide an additional means of supplementing this new type of therapy through radiation-inducible gene therapy.

Flow regulation of 72-kD collagenase IV (MMP-2) after experimental arterial injury.

BACKGROUND: MMP-2 plays a key role in basement membrane degradation and in the migration of proliferating smooth muscle cells after vascular injury. Because low flow and shear stress have been related to the localization and progression of intimal hyperplasia, we hypothesized that flow conditions modulate in vivo MMP-2 transcription and activity in a model of injury-induced intimal thickening. METHODS AND RESULTS: The right common carotid artery (CCA) was balloon-injured in 21 New Zealand White male rabbits. Flow was thereafter preserved (normal flow, n=7), reduced by partial outflow occlusion (low flow, n=7), or increased by ligation of the left CCA (high flow, n=7). In 15 other animals (controls without injury), flow was reduced (n=5), increased (n=5), or preserved (n=5). Mean blood flow and pressure in the right CCA were measured before and after flow modulation (day 0) and before the rabbits were killed (day 7). Northern analysis, gelatin-gel zymography, and fluorometric assays were performed on day 7 to determine MMP-2 mRNA levels and activity in relation to flow and intimal thickening. Mean flow was reduced from 21+/-1 to 7+/-1 mL/min (P<0.05) by outflow occlusion and increased to 31+/-2 mL/min (P<0.05) by ligation of the contralateral CCA. Blood pressure was not different between the flow groups. Hemodynamic parameters were similar for days 0 and 7 after flow modulation. In the injured right CCA, there was a 186% increase in MMP-2 mRNA with normal flow (P<0.05), a 366% increase with low flow (P<0.005), and only a 38% increase with high flow (P>0.05) compared with the uninjured CCA with normal flow. In the uninjured CCA, MMP-2 mRNA levels were increased by only 39% and 26% in the low- and high-flow groups, respectively, compared with normal-flow controls. The zymographic signal and quantitative fluorescent activity of gelatinase were markedly increased in both injured and uninjured CCAs subjected to low flow. Intimal thickening was observed after 1 week only in CCA segments with low flow and injury. CONCLUSIONS: Hemodynamic forces such as low flow upregulate injury-induced MMP-2 mRNA and appear to be more important in regulating MMP-2 activity than injury alone. This may facilitate migration of the smooth muscle cells and subsequent development of intimal thickening.

Low flow enhances platelet activation after acute experimental arterial injury.

PURPOSE: Vascular smooth muscle cell (VSMC) proliferation and migration to the subintima or intimal hyperplasia (IH) occur after arterial injury and are thought to be induced by mitogenic factors released from activated platelets. Because low flow (LF) and shear have been attributed to the localization and progression of IH, we postulated that hemodynamic factors may regulate the degree of platelet activation, as measured by plasma thromboxane B2 (TXB2) and platelet-derived growth factor-AB (PDGF-AB) release at regions of experimental arterial injury. METHODS: The right common carotid artery (CCA) was subjected to balloon injury in 18 New Zealand White male rabbits. Flow in the injured CCA was reduced by out-flow ligation (LF group, n = 6) or increased by ligation of the left CCA (high flow [HF] group, n = 6). In six other animals, flow was preserved (normal flow [NF] group). Mean blood flow and pressure in the right CCA were measured thereafter at 10 and 30 minutes. Plasma TXB2 and PDGF-AB levels were determined with the enzyme-linked immunosorbent assay method in each animal with blood samples taken systematically before injury (baseline) and in the distal CCA at similar time points. RESULTS: At 10 minutes, mean blood flow was reduced from 20 +/- 2 ml/min in the NF group to 7 +/- 1 ml/min in the LF group animals (p < 0.01) and increased to 32 +/- 2 ml/min in the HF group animals (p < 0.05). Mean arterial blood pressure did not differ among the groups. Hemodynamic parameters were similar at 10 and 30 minutes. TXB2 levels were more than fourfold greater in the LF group than in the HF and NF groups at both time points (p < 0.05). In addition, there was a twofold increase in plasma PDGF-AB level at 10 minutes in the LF group compared with baseline levels (p < 0.05). CONCLUSION: Platelet activation at regions of acute vascular injury was determined to be flow dependent. Upregulated platelet activity in low flow conditions may be due to increased platelet exposure time to subendothelial collagen and is greatly attenuated if normal or increased flow is present.

Mechanical analysis of heterogeneous, atherosclerotic human aorta.

An experimental technique was developed to determine the finite strain field in heterogeneous, diseased human aortic cross sections at physiologic pressures in vitro. Also, the distributions within the cross sections of four histologic features (disease-free zones, lipid accumulations, fibrous intimal tissue, and regions of calcification) were quantified using light microscopic morphometry. A model incorporating heterogeneous, plane stress finite elements coupled the experimental and histologic data. Tissue constituent mechanical properties were determined through an optimization strategy, and the distributions of stress and strain energy in the diseased vascular wall were calculated. Results show that the constituents of atherosclerotic lesions exhibit large differences in their bilinear mechanical properties. The distributions of stress and strain energy in the diseased vascular wall are strongly influenced by both lesion structure and composition. These results suggest that accounting for heterogeneities in the mechanical analysis of atherosclerotic arterial tissue is critical to establishing links between lesion morphology and the susceptibility of plaque to mechanical disruption in vivo.

Juxtalumenal location of plaque necrosis and neoformation in symptomatic carotid stenosis.

PURPOSE: The structural features that underlie carotid plaque disruption and symptoms are largely unknown. We have previously shown that the chemical composition and structural complexity of critical carotid stenoses are related to plaque size regardless of symptoms. To further determine whether the spatial distribution of individual plaque components in relation to the lumen corresponds to symptomatic outcome, we evaluated 99 carotid endarterectomy plaques. METHODS: Indications for operation were symptomatic disease in 59 instances (including hemispheric transient ischemic attack in 29, stroke in 19, and amaurosis fugax in 11) and angiographic asymptomatic stenosis > 75% in 40. Plaques removed after remote symptoms beyond 6 months were excluded. Histologic sections from the most stenotic region of the plaque were examined using computer-assisted morphometric analysis. The percent area of plaque cross-section occupied by necrotic lipid core with or without associated plaque hematoma, by calcification, as well as the distance from the lumen or fibrous cap of each of these features, were determined. The presence of foam cells, macrophages, and inflammatory cell collections within, on, or just beneath the fibrous cap was taken as an additional indication of plaque neoformation. RESULTS: The mean percent angiographic stenosis was 82% +/- 11% and 79% +/- 13% for the asymptomatic and symptomatic groups, respectively (p > 0.05). The necrotic core was twice as close to the lumen in symptomatic plaques when compared with asymptomatic plaques (0.27 +/- 0.3 mm vs 0.5 +/- 0.5 mm; p < 0.01). The percent area of necrotic core or calcification was similar for both groups (22% vs 26% and 7% vs 6%, respectively). There was no significant relationship to symptom production of either the distance of calcification from the lumen or of the percent area occupied by the lipid necrotic core or calcification. The number of macrophages infiltrating the region of the fibrous cap was three times greater in the symptomatic plaques compared with the asymptomatic plaques (1114 +/- 1104 vs 385 +/- 622, respectively, p < 0.009). Regions of fibrous cap disruption or ulceration were more commonly observed in the symptomatic plaques than in the asymptomatic plaques (32% vs 20%). None of the demographic or clinical atherosclerosis risk factors distinguished between symptomatic and asymptomatic plaques. CONCLUSIONS: These findings indicate that proximity of plaque necrotic core to the lumen and cellular indicators of plaque neoformation or inflammatory reaction about the fibrous cap are associated with clinical ischemic events. The morphologic complexity of carotid stenoses does not appear to determine symptomatic outcome but rather the topography of individual plaque components in relation to the fibrous cap and the lumen. Imaging techniques that precisely resolve the position of the necrotic core and evidence of inflammatory reactions within carotid plaques should help identify high-risk stenoses before disruption and symptomatic carotid disease.

Measurements of velocity and wall shear stress inside a PTFE vascular graft model under steady flow conditions.

The flow field inside a model of a polytetrafluorethylene (PTFE) canine artery end-to-side bypass graft was studied under steady flow conditions using laser-Doppler anemometry. The anatomically realistic in vitro model was constructed to incorporate the major geometric features of the in vivo canine anastomosis geometry, most notably a larger graft than host artery diameter. The velocity measurements at Reynolds number 208, based on the host artery diameter, show the flow field to be three dimensional in nature. The wall shear stress distribution, computed from the near-wall velocity gradients, reveals a relatively low wall shear stress region on the wall opposite to the graft near the stagnation point approximately one artery diameter in axial length at the midplane. This low wall shear stress region extends to the sidewalls, suture lines, and along the PTFE graft where its axial length at the midplane is more than two artery diameters. The velocity distribution inside the graft model presented here provides a data set well suited for validation of numerical solutions on a model of this type.

Role of hypercholesterolemia in accelerated transplant coronary vasculopathy: results of surgical therapy with partial ileal bypass in rabbits undergoing heterotopic heart transplantation.

BACKGROUND: We tested the hypothesis that plasma cholesterol lowering action of partial ileal bypass (PIB) is beneficial in mitigating accelerated transplantation coronary vasculopathy. METHODS: Forty-one New Zealand white rabbits were randomized to receive a normal (n = 21) or 1% cholesterol diet (n = 20). They underwent heterotopic heart transplantation with sham-PIB (n = 19) or PIB (n = 22) and immunosuppression with cyclosporine A (CyA). RESULTS: CyA increased plasma cholesterol of rabbits receiving a normal diet. This effect was mitigated by PIB (101 +/- 50 mg/dl CyA vs baseline 24 +/- 8, p < 0.001; vs 54 +/- 25 mg/dl with PIB, p < 0.05). In cholesterol-fed rabbits, PIB decreased plasma cholesterol levels (520 +/- 236 mg/dl PIB vs baseline 720 +/- 359, p < 0.05; vs 1502 +/- 253 mg/dl with sham PIB, p < 0.00001). Coronary arteries (CA) of 21 5-week survivors were evaluated by light microscopy and digital morphometry. No rejection was noted. Histologic study revealed vasculopathy in 3% of 705 native and 18% of 654 transplant CA (p < 0.05). Graft vasculopathy (GV) was present in 25% of 365 CA of sham-PIB and 10% of 289 CA of PIB rabbits (p = 0.07). In cholesterol-fed rabbits, GV was characterized by fatty proliferative lesions in 75% of 91 pathologic CA of sham and 21% of 28 pathologic CA of PIB rabbits (p < 0.05). Graft intimal hyperplasia was not correlated with cholesterol intake or PIB and was present in 18 of 119 pathologic CA. CONCLUSIONS: GV was characterized by fatty intimal proliferation, fibrous intimal hyperplasia, and a "mixed type." Fibrous intimal hyperplasia developed in native and transplanted hearts, and CyA seemed to promote this state. Hypercholesterolemia promoted fatty proliferative lesions, worsening GV. PIB significantly decreased total cholesterol and retarded fatty proliferation of CA of native and transplanted hearts but did not prevent intimal hyperplastic vasculopathy. Therapy of hypercholesterolemia is recommended to at least mitigate the fatty intimal proliferation of GV.