Myointimal hyperplasia: pathogenesis and implications. 2. Animal injury models and mechanical factors

Balloon-based Injury to Induce Myointimal Hyperplasia in the Mouse Abdominal Aorta

The use of animal models is essential for a better understanding of MH, one major cause for arterial stenosis.In this article, we demonstrate a murine balloon denudation model, which is comparable with established vessel injury models in large animals. The aorta denudation model with balloon catheters mimics the clinical setting and leads to comparable pathobiological and physiological changes. Briefly, after performing a horizontal incision in the aorta abdominalis, a balloon catheter will be inserted into the vessel, inflated, and introduced retrogradely. Inflation of the balloon will lead to intima injury and overdistension of the vessel. After removing the catheter, the aortic incision will be closed with single stiches. The model shown in this article is reproducible, easy to perform, and can be established quickly and reliably. It is especially suitable for evaluating expensive experimental therapeutic agents, which can be applied in an economical fashion. By using different knockout-mouse strains, the impact of different genes on MH development can be assessed.

Improved Neo-Endothelialization of Small Diameter ePTFE Grafts with Titanium Coating

Background

Patency of small synthetic bypass grafts is inferior compared to autologous grafts for revascularization procedures. Titanium coating of foreign surfaces has shown to decrease thrombogenicity, enhance biocompatibility and promote adhesion of endothelial cells. The aim of this study was to test the effect of titanium coating of small diameter ePTFE grafts on short term patency, neo-endothelialization and neointimal proliferation.

Methods

Bilateral carotid graft interposition was performed in 5 pigs with uncoated (n=5) and titanium-coated (n=5) ePTFE grafts (internal diameter=4 mm, length=5 cm), thus each pig served as its own control. At the end of the study (30 ± 3 days), patency and stenosis severity was assessed by carotid angiography. Animals were sacrificed and grafts were excised for histology and scanning electron microscopy. Morphometry of histologic sections was carried out to determine neointimal proliferation and percentage of neo-endothelial coverage.

Results

Patency rate was 80% for uncoated and titanium-coated grafts. Quantitative angiography did not show any significant difference in lumen size between two groups. Morphometry revealed a significantly higher cellular coverage with CD 31 positive endothelial cells for titanium-coated (84 ± 19%) than uncoated grafts (48 ± 26%, p<0.001). There was a non significant trend (p=0.112) towards increased neointimal proliferation in titanium-coated (94 ± 61 μm2/μm) compared to uncoated grafts (60 ± 57 μm2/μm).

Conclusions

Patency rate in uncoated and titanium-coated ePTFE grafts is similar at one month. However, titanium coated grafts show a significant improvement in neo-endothelialization compared to uncoated grafts.

Carotid Bifurcation Geometry and Atherosclerosis

Hemodynamic changes occurring at the initial segments of the arterial bifurcations appear to play an important role in the development of atherosclerotic plaque. Therefore, arterial geometry might be a potential marker for atherosclerosis. Considerable evidence suggests that geometry can influence local hemodynamics at the carotid bifurcation contributing to the development of atheroma. Bifurcation angle, differences in the area ratios including the flare, proximal curvature, sinus bulb width, and tortuosity of the internal or external carotid artery have been listed as potential contributory elements. These morphometric details have been studied not only in postmortem examination but also with the help of imaging modalities such as ultrasound, digital subtraction angiography, computed tomography angiography, and the assistance of computational models and magnetic resonance angiography. The establishment of certain anatomical and geometrical details in addition to traditional risk factors may help in the identification of patients at high risk of developing carotid artery disease. We reviewed the literature to highlight the evidence on the importance of various geometrical details in the development of carotid atheroma and to suggest areas of future research.

Inducing myointimal hyperplasia versus atherosclerosis in mice: an introduction of two valid models

Various in vivo laboratory rodent models for the induction of artery stenosis have been established to mimic diseases that include arterial plaque formation and stenosis, as observed for example in ischemic heart disease. Two highly reproducible mouse models - both resulting in artery stenosis but each underlying a different pathway of development - are introduced here. The models represent the two most common causes of artery stenosis; namely one mouse model for each myointimal hyperplasia, and atherosclerosis are shown. To induce myointimal hyperplasia, a balloon catheter injury of the abdominal aorta is performed. For the development of atherosclerotic plaque, the ApoE -/- mouse model in combination with western fatty diet is used. Different model-adapted options for the measurement and evaluation of the results are named and described in this manuscript. The introduction and comparison of these two models provides information for scientists to choose the appropriate artery stenosis model in accordance to the scientific question asked.

Prevention of coronary in-stent restenosis and vein graft failure: does vascular gene therapy have a role?

Coronary artery bypass grafting (CABG) and percutaneous coronary intervention (PCI), including stent insertion, are established therapies in both acute coronary syndromes (ACS) and symptomatic chronic coronary artery disease refractory to pharmacological therapy. These continually advancing treatments remain limited by failure of conduit grafts in CABG and by restenosis or thrombosis of stented vessel segments in PCI caused by neointimal hyperplasia, impaired endothelialisation and accelerated atherosclerosis. While pharmacological and technological advancements have improved patient outcomes following both procedures, when grafts or stents fail these result in significant health burdens. In this review we discuss the pathophysiology of vein graft disease and in-stent restenosis, gene therapy vector development and design, and translation from pre-clinical animal models through human clinical trials. We identify the key issues that are currently preventing vascular gene therapy from interfacing with clinical use and introduce the areas of research attempting to overcome these.

Endothelial injury induces vascular smooth muscle cell proliferation in highly localized regions of a direct contact co-culture system

Though previous studies have indicated a relationship between the proliferation of endothelial cells and vascular smooth muscle cells (VSMCs) in co-culture, the results have been contradictory and the signaling mechanism poorly understood. In this transmembrane co-culture study, VSMCs and endothelial cells were grown to confluence on opposite sides of a microporous membrane to mimic the intima/media border of vessels. The endothelial layer was injured, and then cultured for 3 days, resulting in partial re-endothelialization. VSMC proliferation across from the injured/partially recovered endothelial region was significantly higher than across from the de-endothelialized region (a sevenfold increase) and the uninjured region (a threefold increase). ELISA indicated that PDGF, which was undetectable in uninjured co-culture and homotypic controls, increased after injury and the addition of a piperazinyl-quinazoline carboxamide PDGF receptor inhibitor blocked VSMC proliferation across from the injured/partially recovered region. We conclude that co-culture signaling initiated by endothelial cell injury locally stimulates VSMC proliferation and that this signaling could be mediated by PDGF-BB.

Matching the diameter of ePTFE bypass prosthesis with a native artery improves neoendothelialization

BACKGROUND AND AIM

The undersizing of the bypass graft diameter compared to native artery changes blood flow characteristics and velocity which may affect conduit neo-endothelialization, intimal hyperplasia reaction and patency. The aim of this study was to evaluate conduit neoendothelialization, intimal hyperplasia reaction and patency results between undersized and matched ePTFE grafts.

MATERIAL AND METHODS

In 16 male Sprague-Dawley rats, undersized (1-mm internal diameter) and matched (2-mm internal diameter) ePTFE grafts were anastomosed end-to-end in the infrarenal abdominal aorta. Blood flow volume per minute was measured and wall shear stress was calculated for each group. After 3 weeks of follow-up, angiography was performed via the left carotid artery just before sacrifice. Conduit neoendothelialization and intimal hyperplasia reaction were measured by computer-assisted morphometry.

RESULTS

Wall shear stress was 8 times higher for the undersized group (840.56 vs. 105.07 mPa). Three weeks after implantation, conduit neoendothelialization was better in matched grafts compared to undersized grafts (441 vs. 574 microm, p = 0.008). Intimal hyperplasia reaction was similar for both groups (8.7 vs. 6.7 microm(2)/microm for undersized and matched grafts, respectively). Patency rate was 7/8 for undersized and 8/8 for matched ePTFE grafts.

CONCLUSION

Although the graft patency and the intimal hyperplasia reaction were not different between the two groups after 3 weeks, matched grafts had a significantly better endothelialization compared to undersized grafts. This short-term beneficial effect may influence long-term patency results.

In vivo quantitative assessment of catheter patency in rats

Formation of fibrin sleeves around catheter tips is a central factor in catheter failure during chronic implantation, and such tissue growth can occur despite administration of anticoagulants. We developed a novel method for monitoring catheter patency. This method recognizes the progressive nature of catheter occlusion, and tracks this process over time through measurement of changes in catheter resistance to a standardized 1 mL bolus infusion from a pressurized reservoir. Two indirect measures of catheter patency were used: (a) reservoir residual pressure and (b) reservoir discharge time. This method was applied to the study of catheter patency in rats comparing the effect of catheter material (silastic, polyurethane, Microrenathanetrade mark), lock solution (heparin, heparin/dexamethasone) and two different cannulation sites (superior vena cava via the external jugular vein, inferior vena cava via the femoral vein). Our findings reveal that application of flexible smaller-size silastic catheters and a dexamethasone lock solution resulted in prolonged catheter patency. Patency could be maintained over nine weeks with the femoral vein catheters, compared with five weeks with the external jugular vein catheters. The current method for measuring catheter patency provides a useful index for the assessment of tissue growth around the catheter tip. The method also provides an objective and quantitative way of comparing changes in catheter patency for different surgical methods and catheter types. Our method improves on the conventional method of assessing catheter occlusion by judging the ability to aspirate from the catheter.

Comparison of a new recirculation thrombectomy catheter with other devices of the same type: in vitro and in vivo evaluations

RATIONALE AND OBJECTIVES

To compare a new 7 Fr. Helix thrombectomy catheter with Amplatz thrombectomy devices (ATD) with respect to clot fragmentation efficiency, hemolytic potential, and risk for vascular trauma.

MATERIALS AND METHODS

Particle size was evaluated following the maceration of 8-to-10-day-old clots, each weighing 6 +/- 0.01 g. The clots were macerated using devices of various sizes, including the 7-Fr. Helix thrombectomy catheter, the 7-Fr. over-the-wire (OTW), 8-Fr. ATD and the 6-Fr. ATD. The number of particles by weight was quantified. The 7-Fr. Helix and the 8-Fr. ATD were tested in the native iliac vein of six dogs without presence of clots. Blood samples were obtained before, during, and at 1, 3, 6, 24, and 48 hours after the procedure, to monitor the hemolytic effects. The treated iliac veins were examined histologically.

RESULTS

Most of the clot was fragmented into particles <10 microm. The mean percentage by weight of the original clot that remained as particles larger than 10 microm was 1.59% in the 7-Fr. Helix group. This was significantly less than 3.10% with the 6-Fr. ATD, 2.57% with 7-Fr. OTW and 2.44% in the 8-Fr. ATD group (<0.01). In vivo results showed a higher plasma free hemoglobin (PFH) level starting 5 minutes after initiating the ATD activation, reaching its peak after completion of the activation, and declining afterward, with return to the baseline at 24 hours. The haptoglobin level tended to decline slightly at 10 minutes, reaching its lowest level at 24 hours, and starting to recover at 48 hours. A similar pattern of PFH and haptoglobin changes was found in both groups. There were no significant differences regarding hemolytic effect of the two tested devices. No significant changes of creatinine were observed for up to 48 hours after procedures. Focal microthrombosis was seen in two sites of two vessels treated with the 7-Fr. Helix and a focal injury of a venous valve was noted in the 8-Fr. ATD group. The media and internal elastic lamina (IEL) were intact in all cases.

CONCLUSION

The newly designed 7-Fr. Helix is more effective in fragmentation in vitro clots than the 6 Fr. ATD, 7-Fr. OTW and the 8-Fr. ATD. The 7-Fr. Helix and the 8-Fr. ATD produced hemolytic effects, but they appeared to be tolerated by the animals. The in vivo results indicated that the 7-Fr. Helix appears to be as safe as the 8-Fr. ATD.

The effect of proximal artery flow on the hemodynamics at the distal anastomosis of a vascular bypass graft: computational study

The formation of distal anastomotic intimal hyperplasia (IH), one common mode of bypass graft failure, has been shown to occur in the areas of disturbed flow particular to this site. The nature of theflow in the segment of artery proximal to the distal anastomosis varies from case to case depending on the clinical situation presented. A partial stenosis of a bypassed arterial segment may allow residual prograde flow through the proximal artery entering the distal anastomosis of the graft. A complete stenosis may allow for zero flow in the proximal artery segment or retrograde flow due to the presence of small collateral vessels upstream. Although a number of investigations on the hemodynamics at the distal anastomosis of an end-to-side bypass graft have been conducted, there has not been a uniform treatment of the proximal artery flow condition. As a result, direct comparison of results from study to study may not be appropriate. The purpose of this work was to perform a three-dimensional computational investigation to study the effect of the proximal artery flow condition (i.e., prograde, zero, and retrograde flow) on the hemodynamics at the distal end-to-side anastomosis. We used the finite volume method to solve the full Navier-Stokes equations for steady flow through an idealized geometry of the distal anastomosis. We calculated the flow field and local wall shear stress (WSS) and WSS gradient (WSSG) everywhere in the domain. We also calculated the severity parameter (SP), a quantification of hemodynamic variation, at the anastomosis. Our model showed a marked difference in both the magnitude and spatial distribution of WSS and WSSG. For example, the maximum WSS magnitude on the floor of the artery proximal to the anastomosis for the prograde and zero flow cases is 1.8 and 3.9 dynes/cm2, respectively, while it is increased to 10.3 dynes/cm2 in the retrograde flow case. Similarly, the maximum value of WSSG magnitude on thefloor of the artery proximal to the anastomosis for the prograde flow case is 4.9 dynes/cm3, while it is increased to 13.6 and 24.2 dynes/cm3, respectively, in the zero and retrograde flow cases. The value of SP is highest for the retrograde flow case (13.7 dynes/cm3) and 8.1 and 12.1 percent lower than this for the prograde (12.6 dynes/cm3) and zero (12.0 dynes/cm3) flow cases, respectively. Our model results suggest that the flow condition in the proximal artery is an important determinant of the hemodynamics at the distal anastomosis of end-to-side vascular bypass grafts. Because hemodynamic forces affect the response of vascular endothelial cells, the flow situation in the proximal artery may affect IH formation and, therefore, long-term graft patency. Since surgeons have some control over the flow condition in the proximal artery, results from this study could help determine which flow condition is clinically optimal.

Pulsatile flow in an end-to-side vascular graft model: comparison of computations with experimental data

Various hemodynamic factors have been implicated in vascular graft intimal hyperplasia, the major mechanism contributing to chronic failure of small-diameter grafts. However, a thorough knowledge of the graft flow field is needed in order to determine the role of hemodynamics and how these factors affect the underlying biological processes. Computational fluid dynamics offers much more versatility and resolution than in vitro or in vivo methods, yet computations must be validated by careful comparison with experimental data. Whereas numerous numerical and in vitro simulations of arterial geometries have been reported, direct point-by-point comparisons of the two techniques are rare in the literature. We have conducted finite element computational analyses for a model of an end-to-side vascular graft and compared the results with experimental data obtained using laser-Doppler velocimetry. Agreement for velocity profiles is found to be good, with some clear differences near the recirculation zones during the deceleration and reverse-flow segments of the flow waveform. Wall shear stresses are determined from velocity gradients, whether by computational or experimental methods, and hence the agreement for this quantity, while still good, is less consistent than for velocity itself from the wall shear stress numerical results, we computed four variables that have been cited in the development of intiimal hyperplasia-the time-averaged wall shear stress, an oscillating shear index, and spatial and temporal wall shear stress gradients in order to illustrate the versatility of numerical methods. We conclude that the computational approach is a valid alternative to the experimental approach for quantitative hemodynamic studies. Where differences in velocity were found by the two methods, it was generally attributed to the inability of the numerical method to model the fluid dynamics when flow conditions are destabilizing. Differences in wall shear, in the absence of destabilizing phenomena, were more likely to be caused by difficulties in calculating wall shear from relatively low resolution in vitro data.

Porous polyurethane tubes as vascular graft

A vascular graft with the inner diameter of about 3 mm was prepared from segmented poly (ether urethane) with an extrusion technique. To make the wall of the vascular grafts porous, NaCl salts were added to the polyurethane solution to be extruded and removed with water extraction after evaporating the solvent in the extruded tube. The wall was reinforced with elastic fiber to prevent dilation. The compliance of the vascular graft measured with the method of Hayashi et al. ranged from 0.2 to 0.3% mmHg -1. The initial Young's modulus was close to that of canine carotic artery, to which the porous polyurethane graft 4-cm long was anastomosed. Vascular grafts were occluded within 2 weeks after implantation, when their pore size was 0, 1.7, or 4.4 mum, whereas those with the pore size of 5.5, 7.4, and 30 mum were patent for longer than 4 weeks. When the vascular graft with the pore size of 30 mum was implanted for 6 months, the luminal surface was covered with neointima, but the endothelium-like cells appearing in the middle of the intima of the vascular graft were immature and sometimes had a very big nucleus. In addition, spindle-shaped, modified smooth muscle cells were noticed in the deep layer of the neointima, especially in the tissue where anastomotic intimal hyperplasia occurred.

Significance of porosity and compliance of microporous, polyurethane-based microarterial vessel on neoarterial wall regeneration

The microporous structure of artificial vascular grafts, which increases compliance and porosity simultaneously, may enhance neoarterial regeneration. In order to differentiate these effects, three models of segmented polyurethane grafts (inner diameter, 1.5 mm; wall thickness, 100 microns) with or without micropores fabricated using an excimer laser ablation technique, were prepared, and their neoarterial regenerative potentials were studied upon implantation: Model I (microporous, permeable, compliant); Model II (smooth-surfaced, impermeable, compliant); and Model III (smooth-surfaced, impermeable, noncompliant). In Models I and II, the pore or groove size (diameter, 100 microns) and pore or groove arrangement were fixed, and consequently their compliances were almost identical. Irrespective of model, the luminal surfaces were coated with benzophenone-derivatized gelatin and subsequently photocured. Twenty grafts (length, 20 mm) of each model were implanted in the aortas of rats. Predetermined implantation periods were 4, 12, and 24 weeks. Total patency rate decreased in the order Model I (100%), II (87%), and III (59%) grafts. All patent grafts were completely endothelialized after 12 weeks of implantation, irrespective of model. After 12- and 24-week implantations, in Model I grafts, the neoarterial wall was thin, and smooth muscle cells (SMCs) were of the contractile phenotype. In Model II grafts, the neoarterial wall exhibited considerable thickening. In Model III grafts, the neoarterial wall exhibited marked thickening, and SMCs were of the synthetic phenotype. The neoarterial wall thickness at the midportion of the grafts after 24 weeks of implantation increased in the order Model I (48 +/- 8 microns), II (146 +/- 87 microns), and III (385 +/- 21 microns) grafts. These results strongly suggest that compliance matching and porosity synergistically resulted in neoarterial wall restoration without appreciable thickening.

Dipyridamole inhibits PDGF- and bFGF-induced vascular smooth muscle cell proliferation

Dipyridamole is the only pharmacologic agent demonstrated to reduce polytetrafluoroethylene (PTFE) graft occlusion in hemodialysis patients. However, the mechanism of action of dipyridamole in preventing graft occlusion is unknown. The purpose of this study was to examine the direct effects of dipyridamole on both platelet-derived growth factor (PDGF) and basic fibroblast growth factor (bFGF)-induced vascular smooth muscle cell (VSMC) proliferation. Human aortic smooth muscle cells were grown to confluence in 96 well plates. A total of 5 x 10(-6) molar dipyridamole, PDGF 10 ng/ml, or bFGF 10 ng/ml were added to appropriate wells at the start of each experiment. Cell proliferation at 48 hours was determined using tritiated thymidine uptake. Intracellular cyclic AMP (cAMP) was measured using a competitive enzyme immunoassay. Treatment of VSMC with 5 microM dipyridamole dramatically reduced basal proliferation rates compared to controls [5229 +/- 1131 counts per minute (CPM) versus 387 +/- 68 CPM, P < 0.001]. Treatment with dipyridamole also reduced PDGF-stimulated VSMC proliferation (7311 +/- 1655 CPM vs. 593 +/- 110 CPM, P < 0.001) as well as the response to bFGF (5632 +/- 1270 CPM vs. 310 +/- 31 CPM, P < 0.001). Treatment of VSMC with either 5 or 20 microM dipyridamole did not change intracellular cAMP levels. Furthermore, the addition of dibutyryl cAMP to VSMC demonstrated only a modest inhibitory effect on proliferation. We conclude that dipyridamole inhibits both PDGF- and bFGF-stimulated VSMC proliferation. The effects of dipyridamole on VSMC proliferation do not appear to be entirely mediated by changes in intracellular cAMP concentrations. The direct effect of dipyridamole on VSMC proliferation may account for its efficacy in reducing PTFE graft thrombosis in hemodialysis patients.

Computational design of a bypass graft that minimizes wall shear stress gradients in the region of the distal anastomosis

PURPOSE

Recent experimental and theoretic studies show that large wall shear stress gradients characterize disturbed flow patterns associated with the location of myointimal hyperplasia, atheroma, or both. Graft-to-artery anastomoses that minimize wall shear stress gradients may reduce the degree of myointimal development and the propensity for thrombosis. This study analyzes the distribution of distal anastomotic wall shear stress gradients for conventional geometries and for the optimized geometry assuming idealized merging of the graft with the artery.

METHODS

A validated computational fluid dynamics program was used to solve the transient three-dimensional partial differential equations and auxiliary equations that describe laminar incompressible blood flow. Time-averaged wall shear stresses and wall shear stress gradients were calculated for three distal graft-artery anastomoses: a standard end-to-side, a Taylor patch, and an optimized geometry. The latter was obtained iteratively by minimizing the local wall shear stress gradients and was analyzed under resting and exercise inflow waveforms.

RESULTS

Both the standard and Taylor patch anastomoses have relatively high wall shear stress gradients in the regions of the toe and heel. For all flow inputs studied nonuniform hemodynamics in the optimized graft design are largely eliminated, and the time-averaged wall shear stress gradients are greatly reduced throughout the anastomotic zone. At resting flow the Taylor patch produces slightly lower wall shear stress gradients in the anastomotic region than the standard end-to-side anastomosis. The optimized design reduces wall shear stress gradients to almost one half of that of the standard and Taylor patch geometries. At exercise flow wall shear stress gradients almost triple in the standard anastomosis and increase approximately 30% in the Taylor patch. In contrast, the geometrically optimized design is basically independent of the type of flow input waveform in terms of time-averaged wall shear stress gradients and disturbed flow patterns.

CONCLUSION

This study demonstrates that it is possible to design a terminal graft geometry for an end-to-side anastomosis that significantly reduces wall shear stress gradients. If the wall shear stress gradient is confirmed to be a major hemodynamic determinant of intimal hyperplasia and restenosis, these results may point to the design of optimal bypass graft geometries.

Effect of carotid artery geometry on the magnitude and distribution of wall shear stress gradients

PURPOSE

Recent information indicates that large, sustained wall shear stress gradients are a dominant hemodynamic parameter associated with the location and severity of atherosclerosis and myointimal hyperplasia. This study computes the spatial values of wall shear stresses and their gradients for three carotid artery bifurcation geometries.

METHODS

A computational fluid dynamics program was used to solve the transient two-dimensional partial differential equations that describe fluid flow. Blood was treated as both a Newtonian and a non-Newtonian incompressible fluid. Solutions for the velocities, wall shear stresses, and wall shear-stress gradients were obtained for three carotid bifurcation geometries: a normal carotid bifurcation (similar to a primarily reconstructed carotid endarterectomy), a patch-reconstructed carotid endarterectomy, and a gradually tapered, low-angle carotid bifurcation (no carotid bulb).

RESULTS

Computed velocity profiles closely match published experimental ones. Disturbed flow velocities are largest in the bulb segment of the normal carotid bifurcation. Peak and minimum wall shear stresses and peak shear stress gradients occurred in the lateral internal carotid artery wall. These were binodal in the normal or primarily reconstructed carotid artery, localized at the distal end of the patch-reconstructed carotid bifurcation, and minimal in the smooth, tapered carotid bifurcation. Wall shear stresses and their gradients were slightly higher for non-Newtonian than Newtonian fluids in the normal carotid artery but were similar in the other two geometric configurations.

CONCLUSION

These results indicate that flow disturbances in general and wall shear stress gradients in particular are markedly reduced in carotid artery bifurcations that are smooth and gradually tapered and do not have a bulb. Abrupt geometric wall changes such as those occurring in the normal carotid bulb and at the distal end of a patch-reconstruction after carotid endarterectomy are harbingers of disturbed flow and high wall shear stress gradients. These results suggest that carotid endarterectomy reconstruction geometry characterized by a gradually tapered internal carotid artery may minimize the hemodynamically induced component of early myointimal hyperplasia and thrombosis and late atherosclerotic restenosis.

Dosage and timing of Photofrin for photodynamic therapy of intimal hyperplasia

Photodynamic therapy has been recommended as a method of preventing intimal hyperplasia. The purpose of this study was to determine the dose and timing of Photofrin porfimer sodium needed to achieve a 3:1 or higher ratio between injured and control arteries after balloon endothelial injury. New Zealand White rabbits were anesthetized and their right femoral artery surgically exposed. A 4Fr Fogarty balloon catheter was passed retrograde into the lower abdominal aorta, inflated and pulled distally into the external iliac artery six times. All rabbits received heparin 100 IU/kg. Arteriotomies were closed and the animals recovered. Rabbits (n = 5 per group) were given intravenous Photofrin at a dose and time according to the following scheme: group I, 5.0 mg/kg immediately after balloon injury; group II, 2.5 mg/kg immediately after injury; group III, 5.0 mg/kg after 1 week; group IV, 5.0 mg/kg after 2 weeks; or group V, 2.5 mg/kg after 2 weeks. Animals were killed 24h after drug administration and the aortoiliac segments removed for spectrophotofluorometric determination of Photofrin levels from injured and control segments. Mean(s.d.) ratios of injured: control arteries for groups I to V were 4.8 (2.6), 2.8 (1.2), 3.0 (1.0), 1.4 (0.3) and 1.0 (0.0) respectively. This ratio was significantly higher for group I rabbits compared with groups IV and V (P < 0.01, ANOVA). Fluorescence and light microscopy showed that Photofrin was localized primarily in the tunica media, and that the drug must be administered before significant intimal hyperplasia occurs.(ABSTRACT TRUNCATED AT 250 WORDS)

Preventing intimal hyperplasia with photodynamic therapy using an intravascular probe

The purpose of this study was to determine the efficacy of intravascular photodynamic therapy (PDT) to prevent the development of intimal hyperplasia. Anesthetized New Zealand white rabbits underwent placement of Fogarty balloon catheters introduced via femoral artery cutdowns. Catheters were passed retrograde 10 cm into the lower abdominal aorta, inflated six times, and withdrawn toward the inguinal ligament. Rabbits were then randomly assigned to one of the following groups: group 1, drug with no light; group 2, no drug with 240 joules of light; group 3, drug plus 120 joules of light; or group 4, drug plus 240 joules of light. Uninjured carotid arteries served as negative control vessels (N) and injured but non-PDT-treated iliac artery segments served as positive controls (P). Porfimer sodium (photofrin) was administered in a dose of 5.0 mg/kg. Light was provided by a fiberoptic probe with a 1 cm cylindric diffuser attached to an argon pumped dye laser tuned to 630 nm to provide 1 W of laser light for 120 or 240 seconds. One month after PDT, rabbits were killed, perfusion fixed with glutaraldehyde, and vessels removed and examined microscopically. Intimal thickness (mean +/- SD) was calculated and expressed as ratios of the intima/media at four equal positions. Results for N, P, and groups 1, 2, 3, and 4 were 0.02 +/- 0.00, 1.18 +/- 0.71, 0.76 +/- 0.33, 0.96 +/- 0.43, 0.14 +/- 0.22, and 0.36 +/- 0.16, respectively. Intimal thickness was significantly reduced in groups 3 and 4 when compared with P, group 1, and group 2 (p < 0.001, ANOVA). These results showed that intravascular PDT was effective in reducing intimal hyperplasia following arterial injury. This may be a practical method of delivering light for PDT.

Flow instabilities induced by coronary artery stents: assessment with an in vitro pulse duplicator

An in vitro pulse duplicator system was used to investigate whether coronary artery stents induce downstream flow instabilities. Hot film or electrochemical probes were used to measure wall shear stress before and after deployment of both single and multiple (overlapping) stents in normal and diseased coronary geometries. Left main coronary diameters ranged from 4 to 5 mm, whereas left anterior descending (LAD) and left circumflex (LCX) diameters ranged from 2 to 4 mm. Under resting conditions, all coronary flow waveforms remained laminar, even after stent placement. However, disturbances were found downstream from a stent placed in the proximal LAD under mild exercise conditions. These disturbances were found 5 mm distal to the stent, in both the LAD and the proximal LCX. Turbulence intensities of order 5% were induced by a single slotted stent in a normal LAD geometry. In cases of distal disease, the turbulence intensity was 9% with one stent and 11% with tandem stents. In cases of proximal disease, these values were 19 and 25%, respectively. The shear stress from these disturbances (20-200 dynes cm-2) is sufficient to delay re-endothelialization and promote restenosis. Therefore, the disturbances could contribute to the increased incidence of restenosis reported with multiple stents, and with stents used in cases of diffuse coronary disease.

Infrainguinal vein graft stenosis

Although knowledge of the biological processes involved in the development of intimal hyperplasia has increased markedly in recent years, the precise aetiology of infrainguinal vein graft stenosis remains undetermined. Current therapy is therefore directed at treatment of the established lesion rather than its prevention. There seems little doubt, however, that recent advances in understanding of the vascular biology of normal and pathological saphenous vein will eventually lead to specific targeted therapy that will allow the prevention of vein graft stenosis.

Permanent vascular access: a nephrologist's view

Vascular access complications are the greatest cause of morbidity in hemodialysis patients in the United States. Although arteriovenous fistulas have been recommended as the preferred mode of vascular access, recent data indicate that the majority of patients on hemodialysis in the United States have prosthetic graft fistulas. The most frequent complications of prosthetic graft fistulas are thrombosis and stenosis. Hospitalization rates for fistula complications are higher in patients with diabetes mellitus and of black race. Pathogenesis of intimal hyperplasia may include elaboration of platelet-derived growth factor and mechanical endothelial injury. Screening for stenosis and impaired blood flow in fistulas can be carried out with recirculation measurements, venous and intra-access pressure measurements, and Doppler ultrasound. A combination of the techniques is probably the best current strategy for fistula screening and further evaluation. Surgical thrombectomy and fistula revision remain the standard for comparison of newer approaches to management of complications. Percutaneous angioplasty with or without stent placement, thrombolysis, and use of atherectomy devices may play an increasing role in the treatment of complications, although comparative trials of these modalities need to be performed. No satisfactory long-term pharmacologic means of preventing thrombosis, stenosis, or restenosis have been found for graft arteriovenous fistulas. It is hoped that future directions in the field of vascular access placement and management will include better strategies for allowing primary arteriovenous fistula development, advances in graft materials, improved understanding of the pathogenesis of thrombosis and stenosis, and development strategies to prevent complications.

Effect of seeding time and density on endothelial cell attachment to damaged vascular surfaces

An in vitro model to facilitate the study of endothelial cell seeding of damaged vascular surfaces has been developed. This may have applications in the study of endothelial seeding of angioplasty and endarterectomy sites. Using this model, the optimum endothelial seeding time for attachment to damaged vascular surfaces should not exceed 30 min and, to achieve confluent cell attachment, a seeding density > 5 x 10(5) cells/cm2 should be used.

Doppler color flow images of iliofemoral graft end-to-side distal anastomotic models

Hemodynamics within the distal anastomoses of iliofemoral bypass grafts were simulated using Plexiglas models (2.5 cm ID) within a pulsatile flow loop system (Re(mean) = 92, Re(peak) = 459 and alpha = 3.56). End-to-side distal anastomoses were constructed with angles of 30 degrees, 45 degrees and 60 degrees to bypass proximal artery segments with stenoses of 60% and 100% diameter reduction. Velocities were obtained over a two-dimensional field within the artery using an ultrasonic Doppler color flow imager operating at 5 MHz at positions from 1.5 diameters upstream to 3 diameters downstream of the anastomosis. Flow patterns downstream of an occlusion demonstrated definite skewing effects toward the outer wall with resultant flow separation along the inner wall. Presence of a partial (60% diameter reduction) arterial stenosis upstream of the anastomosis produced flow separation along both artery walls and a more symmetric profile downstream. Measurement of a separation area index (SAI) along each arterial wall demonstrated a minimum area exposed to low velocities for the 30 degrees anastomotic angle compared to the 45 degrees and 60 degrees anastomotic angles and for the occluded proximal artery cases compared to corresponding stenotic artery cases. The SAI values were minimal at peak systole compared to successive quarter cycle intervals. The findings of this study provide further information regarding the relationship between local fluid mechanics and predominant sites for intimal hyperplasia formation.

Arterial response to mechanical injury: balloon catheter de-endothelialization

Coronary angioplasty has been used clinically for over a decade. Its initial promise as an alternative to coronary bypass surgery has only partially been fulfilled because of the high rate of post-operative restenosis. A number of animal models have been devised to study this phenomenon and although none is entirely satisfactory, they have, together with recent advances in molecular biology provided an insight into the cellular mechanisms that may contribute to this complication. This knowledge may ultimately lead to a means of therapeutic intervention. This review summarises our present understanding of the pathology of post-angioplasty re-stenosis as revealed by studies using the balloon catheter de-endothelialization model, and discusses some of the intervention strategies that have been attempted.