Rob Krams

Atherosclerotic Lesion Size and Vulnerability Are Determined by Patterns of Fluid Shear Stress

Background— Atherosclerotic lesions are predominantly observed in curved arteries and near side branches, where low or oscillatory shear stress patterns occur, suggesting a causal connection. However, the effect of shear stress on plaque vulnerability is unknown because the lack of an appropriate in vivo model precludes cause-effect studies. Methods and Results— We developed a perivascular shear stress modifier that induces regions of lowered, increased, and lowered/oscillatory (ie, with vortices) shear stresses in mouse carotid arteries and studied plaque formation and composition. Atherosclerotic lesions developed invariably in the regions with lowered shear stress or vortices, whereas the regions of increased shear stress were protected. Lowered shear stress lesions were larger (intima/media, 1.38±0.68 versus 0.22±0.04); contained fewer smooth muscle cells (1.9±1.6% versus 26.3±9.7%), less collagen (15.3±1.0% versus 22.2±1.0%), and more lipids (15.8±0.9% versus 10.2±0.5%); and showed more outward vascular remodeling (214±19% versus 117±9%) than did oscillatory shear stress lesions. Expression of proatherogenic inflammatory mediators and matrix metalloproteinase activity was higher in the lowered shear stress regions. Spontaneous and angiotensin II–induced intraplaque hemorrhages occurred in the lowered shear stress regions only. Conclusions— Lowered shear stress and oscillatory shear stress are both essential conditions in plaque formation. Lowered shear stress induces larger lesions with a vulnerable plaque phenotype, whereas vortices with oscillatory shear stress induce stable lesions.

Relationship Between Neointimal Thickness and Shear Stress After Wallstent Implantation in Human Coronary Arteries

BackgroundIn-stent restenosis by excessive intimal hyperplasia reduces the long-term clinical efficacy of coronary stents. Because shear stress (SS) is related to plaque growth in atherosclerosis, we investigated whether variations in SS distribution are related to variations in neointima formation. Methods and ResultsIn 14 patients, at 6-month follow-up after coronary Wallstent implantation, 3D stent and vessel reconstruction was performed with a combined angiographic and intravascular ultrasound technique (ANGUS). The bare stent reconstruction was used to calculate in-stent SS at implantation, applying computational fluid dynamics. The flow was selected to deliver an average SS of 1.5 N/m2. SS and neointimal thickness (Th) values were obtained with a resolution of 90° in the circumferential and 2.5 mm in the longitudinal direction. For each vessel, the relationship between Th and SS was obtained by linear regression analysis. Averaging the individual slopes and intercepts of the regression lines summarized the overall relationship. Average Th was 0.44±0.20 mm. Th was inversely related to SS: Th=(0.59±0.24)−(0.08±0.10)×SS (mm) (P <0.05). ConclusionsThese data show for the first time in vivo that the Th variations in Wallstents at 6-month follow-up are inversely related to the relative SS distribution. These findings support a hemodynamic mechanism underlying in-stent neointimal hyperplasia formation.

Evaluation of Endothelial Shear Stress and 3D Geometry as Factors Determining the Development of Atherosclerosis and Remodeling in Human Coronary Arteries in Vivo Combining 3D Reconstruction from Angiography and IVUS (ANGUS) with Computational Fluid Dynamics

The predilection sites of atherosclerotic plaques implicate rheologic factors like shear stress underlying the genesis of atherosclerosis. Presently no technique is available that enables one to provide 3D shear stress data in human coronary arteries in vivo. In this study, we describe a novel technique that uses a recently developed 3D reconstruction technique to calculate shear stress on the endothelium with computational fluid dynamics. In addition, we calculated local wall thickness, the principal plane of curvature, and the location of plaque with reference to this plane, relating these results to shear stress in a human right coronary artery in vivo. Wall thickness and shear stress values for the entire vessel for three inflow-velocity values (10 cm/second, 20 cm/second, and 30 cm/second equivalents with the Reynolds numbers 114,229, and 457) were as follows: 0.65 +/- 0.37 mm (n = 1600) and 19.6 +/- 1.7 dyne/cm2; 46.1 +/- 8.1 dyne/cm2 and 80.1 +/- 16.8 dyne/cm2 (n = 1600). Curvature was 25 +/- 9 (m-1), resulting in Dean numbers 20 +/- 8; 46 +/- 16, and 93 +/- 33. Selection of data at the inner curvature of the right coronary artery provided wall thickness values of 0.90 +/- 0.41 mm (n = 100), and shear stress was 17 +/- 17, 38 +/- 44, and 77 +/- 54 dyne/cm2 (n = 100), whereas wall thickness values at the outer curve were 0.37 +/- 0.17 mm (n = 100) and shear stress values were 22 +/- 17, 60 +/- 44, and 107 +/- 79 dyne/cm2 (n = 100). These findings could be reconciled by an inverse relationship between wall thickness and shear stress for each velocity level under study. For the first time for human vessels in vivo, evidence is presented that low shear stress promotes atherosclerosis. As the method is nondestructive, it allows repeated measurements in the same patient and will provide new insights in the progress of atherosclerosis.

True 3-Dimensional Reconstruction of Coronary Arteries in Patients by Fusion of Angiography and IVUS (ANGUS) and Its Quantitative Validation

BACKGROUND True 3D reconstruction of coronary arteries in patients based on intravascular ultrasound (IVUS) may be achieved by fusing angiographic and IVUS information (ANGUS). The clinical applicability of ANGUS was tested, and its accuracy was evaluated quantitatively. METHODS AND REUSLTS: In 16 patients who were investigated 6 months after stent implantation, a sheath-based catheter was used to acquire IVUS images during an R-wave-triggered, motorized stepped pullback. First, a single set of end-diastolic biplane angiographic images documented the 3D location of the catheter at the beginning of pullback. From this set, the 3D pullback trajectory was predicted. Second, contours of the lumen or stent obtained from IVUS were fused with the 3D trajectory. Third, the angular rotation of the reconstruction was optimized by quantitative matching of the silhouettes of the 3D reconstruction with the actual biplane images. Reconstructions were obtained in 12 patients. The number of pullback steps, which determines the pullback length, closely agreed with the reconstructed path length (r=0.99). Geometric measurements in silhouette images of the 3D reconstructions showed high correlation (0.84 to 0.97) with corresponding measurements in the actual biplane angiographic images. CONCLUSIONS With ANGUS, 3D reconstructions of coronary arteries can be successfully and accurately obtained in the majority of patients.

Coronary stent implantation changes 3-D vessel geometry and 3-D shear stress distribution.

Mechanisms of in-stent restenosis are not fully understood. Shear stress is known to play a role in plaque and thrombus formation and is sensitive to changes in regional vessel geometry. Hence, we evaluated the regional changes in 3-D geometry and shear stress induced by stent placement in coronary arteries of pigs.Methods. 3-D reconstruction was performed, applying a combined angiographic and IVUS technique (ANGUS), from seven Wallstents (diameter 3.5 (n=3) and 5mm (n=4)), which were implanted in seven coronary arteries of five pigs. This 3-D geometry was used to calculate locally the curvature, while the shear stress distribution was obtained by computational fluid dynamics. Local changes in shear stress were obtained at the entrance and exit of the stent for baseline (0. 65+/-0.22 ml/s) and hyperemic flow (2.60+/-0.86 ml/s) conditions. Results. After stent implantation, the curvature increased by 121% at the entrance and by 100% at the exit of the stent, resulting in local changes in shear stress. In general, at the entrance of the stent local maxima in shear stress were generated, while at the exit both local maxima and minima in shear stress were observed (p<0.05). Additionally, the shear stress at the entrance and exit of the stent were correlated with the local curvature (r: 0.30-0.84).Conclusion. Stent implantation changes 3-D vessel geometry in such a way that regions with decreased and increased shear stress occur close to the stent edges. These changes might be related to the asymmetric patterns of in-stent restenosis.

Shear stress biology of the endothelium

The relationships between blood flow, mechanotransduction, and the localization of arterial lesions can now be advanced by the incorporation of new technologies and the refinement of existing methods in imaging modalities, computational modeling, fluid dynamics, and high throughput genomics and proteomics. When combined with traditional cell and molecular technologies, a powerful palette of investigative approaches is available to address shear stress biology of the endothelium at levels extending from nanoscale subcellular detailed mechanistic responses through to higher organizational levels of regional endothelial phenotypes and heterogeneous vascular beds.

Decreased Coronary Flow Reserve in Hypertrophic Cardiomyopathy Is Related to Remodeling of the Coronary Microcirculation

BACKGROUND Ischemia occurs frequently in hypertrophic cardiomyopathy (HCM) without evidence of epicardial stenosis. This study evaluates the hypothesis that the occurrence of ischemia in HCM is related to remodeling of the coronary microcirculation. METHODS AND RESULTS End-diastolic septal wall thickness was significantly increased in patients with HCM (25.8+/-2.9 mm) in comparison with cardiac transplant recipients (control subjects: 11.4+/-3.0 mm; P<0.05). Although the diameter of the left anterior descending coronary artery was similar in both groups (3.0+/-0.8 versus 3.0+/-0.5 mm, P=NS), the coronary resistance reserve (CRR=CRRbasal/CRRhyperemic), corrected for extravascular compression (end-diastolic left ventricular pressure), was reduced to 1.5+/-0.6 in HCM (P<.05; control, 2.6+/-0.8). Arteriolar lumen (AL) divided by wall area was lower in HCM (21+/-5% versus 30+/-4%; P<.05), and capillary density tended to decrease (from 1824+/-424 to 1445+/-513 per mm2, P=.11) in HCM. CRR was linearly related to normalized AL according to the formula CRR=O.1 AL-0.45 (r=.57; P<.05). Further analysis revealed that CRR, AL, and capillary density were all linearly related to the degree of hypertrophy. CONCLUSIONS Decrements in CRR were related to changes of the coronary microcirculation. Both the decrease in CRR and these changes in the coronary microcirculation were related to the degree of hypertrophy. All these factors might contribute to the well-known occurrence of ischemia in this patient group.

Activation of Nrf2 in Endothelial Cells Protects Arteries From Exhibiting a Proinflammatory State

Objective—Proinflammatory mediators influence atherosclerosis by inducing adhesion molecules (eg, VCAM-1) on endothelial cells (ECs) via signaling intermediaries including p38 MAP kinase. Regions of arteries exposed to high shear stress are protected from inflammation and atherosclerosis, whereas low-shear regions are susceptible. Here we investigated whether the transcription factor Nrf2 regulates EC activation in arteries. Methods and Results—En face staining revealed that Nrf2 was activated in ECs at an atheroprotected region of the murine aorta where it negatively regulated p38–VCAM-1 signaling, but was expressed in an inactive form in ECs at an atherosusceptible site. Treatment with sulforaphane, a dietary antioxidant, activated Nrf2 and suppressed p38–VCAM-1 signaling at the susceptible site in wild-type but not Nrf2−/− animals, indicating that it suppresses EC activation via Nrf2. Studies of cultured ECs revealed that Nrf2 inactivates p38 by suppressing an upstream activator MKK3/6 and by enhancing the activity of the negative regulator MKP-1. Conclusions—Nrf2 prevents ECs at the atheroprotected site from exhibiting a proinflammatory state via the suppression of p38–VCAM-1 signaling. Pharmacological activation of Nrf2 reduces EC activation at atherosusceptible sites and may provide a novel therapeutic strategy to prevent or reduce atherosclerosis.

Extension of Increased Atherosclerotic Wall Thickness Into High Shear Stress Regions Is Associated With Loss of Compensatory Remodeling

Background Atherosclerosis preferentially develops at average low shear stress (SS) locations. SS‐related signaling maintains lumen dimensions by inducing outward arterial remodeling. Prolonged plaque accumulation at low SS predilection locations explains an inverse relation between wall thickness (WT) and SS. No data exist on WT‐SS relations when lumen narrowing and loss of compensatory remodeling commence. Methods and Results In 14 patients, an angiographically normal artery (stenosis <50%) was investigated with ANGiography and ivUS (ANGUS) to provide 3D lumen and wall geometry. Selection of segments >5 mm in length, in between side branches, yielded 25 segments in 12 patients. SS at the wall was calculated by computational fluid dynamics. WT smaller than 0.2*lumen diameter was defined as normal. Largest arc of normal WT defined reference cross sections. Lumen area relative to the reference cross sections defined area stenosis (AS). Average segmental AS smaller or greater than 10% defined preserved or narrowed lumen, respectively. Total vessel area relative to the reference defined vascular remodeling (VR). For the preserved lumens (n=11, AS=1.7±5.6%, P=NS), axially averaged WT and SS were inversely related (slope, ‐0.46±0.55 mm/Pa, P<0.05) and VR was positive (7±9%, P<0.05). Narrowed segments (n=13, 1 excluded, AS=18±6%, P<0.05) showed no relation between WT and SS or vascular remodeling. Conclusions In patient coronary arteries, the often‐reported inverse WT‐SS relationship appears restricted to lumen preservation and positive vascular remodeling. Its disappearance with lumen narrowing suggests a growing importance of non‐SS‐related plaque progression. (Circulation. 2003;108:17‐23.)

Shear stress–induced changes in atherosclerotic plaque composition are modulated by chemokines

We previously found that low shear stress (LSS) induces atherosclerotic plaques in mice with increased lipid and matrix metalloproteinase content and decreased vascular smooth muscle and collagen content. Here, we evaluated the role of chemokines in this process, using an extravascular device inducing regions of LSS, high shear stress, and oscillatory shear stress (OSS) in the carotid artery. One week of shear stress alterations induced expression of IFN-gamma-inducible protein-10 (IP-10) exclusively in the LSS region, whereas monocyte chemoattractant protein-1 (MCP-1) and the mouse homolog of growth-regulated oncogene alpha (GRO-alpha) were equally upregulated in both LSS and OSS regions. After 3 weeks, GRO-alpha and IP-10 were specifically upregulated in LSS regions. After 9 weeks, lesions with thinner fibrous caps and larger necrotic cores were found in the LSS region compared with the OSS region. Equal levels of MCP-1 expression were observed in both regions, while expression of fractalkine was found in the LSS region only. Blockage of fractalkine inhibited plaque growth and resulted in striking differences in plaque composition in the LSS region. We conclude that LSS or OSS triggers expression of chemokines involved in atherogenesis. Fractalkine upregulation is critically important for the composition of LSS-induced atherosclerotic lesions.