The effect of supplemental oxygen on the transarterial wall oxygen gradients at a prosthetic vascular graft to artery anastomosis in the rabbit

Application of laser scanning cytometry in vascular smooth muscle remodeling

Pulmonary artery hyperplasia is the result of proliferation of the pulmonary arterial smooth muscles (PASM). Hypoxia-induced PASM proliferation in the fetus and the newborn is the primary cause of persistent pulmonary hypertension of the newborn (PPHN). This study was performed to characterize the utility of the Laser Scanning Cytometry (LSC) method in elucidating arterial cytoskeletal remodeling in an in vitro model of PPHN. The aim was to demonstrate the following: (a) LSC is a valid method for the analysis of nuclear and cytosolic fluorescence and (b) the cumulative effects of mechanical stretch together with hypoxia promote reactive oxygen species (ROS) formation. The molecular events in response to hypoxia and the mechanical overload of the pulmonary circuit were demonstrated in vitro by subjecting hypoxic cultured primary PASM or human airway smooth muscles (hASM) to repetitive stretch-relaxation cycles at rates comparable to dynamic stretch in vivo. The altered cytoskeleton in the form of filamentous to globular actin (F:G actin) ratio was imaged and quantified at the cellular level by LSC as an endpoint. LSC can remove the nuclear G-actin fluorescence from the total G-actin fluorescence. Pulsatile stretch was found to significantly increase the total endogenous ROS and superoxide anion release in normoxic and hypoxic conditions in primary PASM fibers. The effect of stretch was predominant in increasing superoxide anion release, only under hypoxic conditions. These findings, obtained by LSC in vitro are amenable to validation in any in vivo model of interest. The in vitro model is clinically relevant to human pulmonary vascular remodeling.

Hyperbaric oxygen inhibits venous neointimal hyperplasia following arteriovenous fistulization

Hypoxia following arteriovenous fistulization results in venous neointimal hyperplasia (VNH), potentially causing early arteriovenous fistula (AVF) dysfunction. In this study, we used hyperbaric oxygen (HBO) in a rabbit model of AVF to determine whether it could ameliorate early AVF failure. Chronic renal failure was induced by adenine in 96 adult rabbits randomly divided into 3 groups (n=32 in each group). The sham + HBO group underwent sham operation and received HBO. The AVF alone group underwent fistulization, but did not receive HBO. The AVF + HBO group underwent fistulization and received HBO. Each group was further divided into 4 subgroups of 8 rabbits each that were euthanized at 1, 7, 14 or 28 days post-operatively. At each time point, blood flow changes in the AVF venous segment were detected using a high-frequency duplex ultrasonography system. Immunohistochemical staining for proliferating cell nuclear antigen (PCNA), and hematoxylin and eosin staining were performed to evaluate VNH. Western blot analysis was performed to confirm the expression of hypoxia-inducible factor (HIF)-1α. At 14 and 28 days following HBO treatment, blood flow in the AVF + HBO group was greater than that at day 0. The AVF + HBO group had a smaller ratio of intima to media area, a lower HIF-1α protein expression, and a smaller percentage of PCNA-positive cells in the proximal vein than did the AVF alone group. Our results thus suggest that continuous HBO treatment following AVF significantly inhibits VNH and promotes blood flow. Therefore, early AVF failure may be prevented by the use of HBO therapy.

The role of short-term oxygen administration in the prevention of intimal hyperplasia

OBJECTIVE

Intimal hyperplasia (IH) is the cause of most failed arteriovenous fistulas (AVFs), resulting in repeat procedures and leading to increased utilization of scarce health care resources. Our laboratory has previously demonstrated the role of supplemental oxygen in preventing IH and smooth muscle cell proliferation (SMCp) at an artery-to-graft anastomosis and at the deployment site of an intra-arterial stent. This study examines the effect of supplemental oxygen in preventing IH and SMCp in an AVF in a rabbit model.

METHODS

Ninety-six rabbits were randomized into four groups: group 1, control; group 2, no surgery with supplemental oxygen; group 3, AVF without supplemental oxygen; and group 4, AVF with supplemental oxygen. Rabbits receiving supplemental oxygen received 30% oxygen for up to 42 days. Specimens were collected in all groups at days 1, 3, 7, 21, 42, and 90. IH and SMCp were measured at the AVF site as well as in the artery and vein proximal and distal to the AVF.

RESULTS

IH was first noted at day 7 and significantly increased through day 90 at all locations in the nonoxygen-supplemented groups. No significant IH was noted in the oxygen-supplemented group at any location or any time point. SMCp was noted at day 3 through day 21 in the nonoxygen-supplemented group, whereas almost no SMCp was noted in the oxygen-supplemented group at any location or time point.

CONCLUSIONS

Without oxygen supplementation, SMCp begins at day 3 and is no longer noted at day 21 after creation of an AVF, whereas IH begins by day 7 and increases at least through day 90 after creation of an AVF. Forty-two days of 30% supplemental oxygen inhibits IH and SCMp after creation of an AVF. These data suggest a role for the short-term administration of low-dose O2 to prevent both IH and SMCp after creation of an AVF that may prolong patency and function.

N-acetylcysteine prevents neointima formation in experimental venous bypass grafts

Background

Neointima formation, mainly characterized by smooth muscle cell proliferation, is an important cause of venous bypass graft failure. The therapeutic potential of the antioxidant N-acetylcysteine (NAC) to attenuate smooth muscle cell proliferation and neointima formation was examined in vivo. The effects of NAC on hyperoxia-induced venous smooth muscle cell (VSMC) cytokine production and proliferation were addressed in vitro.

Methods

Rats underwent autologous epigastric vein-to-femoral artery interposition grafting. Fourteen rats received oral NAC, and a similar control group received saline. Histomorphometric analysis was performed after 7 days or 3 weeks. Cytokine analysis and cell proliferation assay were performed in cultured human VSMCs after hyperoxic or normoxic exposure and NAC administration.

Results

NAC-treated rats displayed a threefold reduction in neointimal area, a sixfold reduction in stenosis rate, and a twofold reduction in VSMC proliferation after vein graft surgery. Incubation of VSMCs in 70 per cent oxygen stimulated the release of mitogenic inflammatory cytokines interleukin (IL) 6 and IL-8. Cytokine-rich medium from these VSMCs induced proliferation of normoxic VSMCs. NAC inhibited hyperoxia-induced cytokine release and VSMC proliferation.

Conclusion

NAC attenuated neointima formation and vein graft stenosis by reducing VSMC proliferation in vivo, and prevented hyperoxia-induced cytokine production and VSMC proliferation in vitro.

Long-term reduction of medial and intimal thickening in porcine saphenous vein grafts with a polyglactin biodegradable external sheath

OBJECTIVES

The development of neointimal hyperplasia with subsequent atherosclerotic deposition has been proposed to cause most late vein graft failures. Our unit has previously demonstrated that placement of a macroporous, loose-fitting polyester external stent prevents neointimal thickening in porcine vein grafts, and has been proposed as a therapeutic strategy to prevent late vein graft failure. To reduce any potential long-term complications of the permanent polyester stent, a study was undertaken to investigate the effect of a biodegradable external stent on porcine vein graft thickening at 1 month and to identify its longer term effects at 6 months.

METHODS

Bilateral saphenous vein to common carotid artery interposition grafting was performed in Large White pigs (25-32 kg; n = 6 per time course group) according to UK Home Office guidelines. A commercially constructed loose-fitting 8-mm-diameter polyglactin stent was placed externally around the vein graft on one side, and the contralateral side remained unstented to serve as control. The external stent was designed to biodegrade and hence disappear within 90 days. Grafts were left in situ for 1 month in 1 group of animals, and for up to 6 months in the other group, before explantation. Graft morphometric features were assessed with computer-aided planimetry.

RESULTS

At 1 month the vein grafts fitted with the polyglactin stent demonstrated a statistically significant decrease in neointimal thickening (0.038 mm; interquartile range [IQR], 0.035-0.039 mm) compared with the unstented control grafts (0.13 mm; IQR; 0.11-0.19; P = .0012), and also in medial thickening (0.09 mm; IQR, 0.086-0.093) compared with unsheathed control grafts (0.302 mm; IQR, 0.272-0.414; P = .0012). The 6-month polyglactin stented grafts also demonstrated a statistically significant reduction in neointimal thickening (0.049 mm; IQR, 0.047-0.07; P = .0012) compared with control grafts (0.178 mm; IQR, 0.164-0.19), and also in medial thickening (0.105 mm; IQR, 0.095-0.143) compared with unstented grafts (0.421 mm; IQR, 0.35-0.44; P = .0012, Mann-Whitney U test).

CONCLUSION

The loose-fitting biodegradable polyglactin external stent reduces porcine vein graft thickening at 1 month, which persists in the long term, even after degradation of the stent itself. This effective removal of the stent may therefore reduce the long-term risks for infection and mechanical complications associated with implanted prosthetic material while still eliciting the primary objective of preventing graft thickening over the long term. Biodegradable external stents therefore have potential advantages over permanent stent material in clinical application.

CLINICAL RELEVANCE

Arteriovenous bypass graft failure has a huge economic effect on health care resources, and a devastating effect o the patient. The attenuation of vein wall thickening, with subsequent luminal narrowing and occlusion, is a major goal in improving the longevity of the venous graft, to reduce secondary percutaneous and surgical interventions. The biodegradable external stent demonstrated in this study has possible clinical applications in bypass procedures with autogenous venous tissue, and represents a novel approach to ameliorating the problem of intimal hyperplasia that plagues these grafts.