Effects of aspirin, dipyridamole, and cod liver oil on accelerated myointimal proliferation in canine veno-arterial allografts

Consensus on the Rational Use of Antithrombotics in Veterinary Critical Care (CURATIVE): Domain 3-Defining antithrombotic protocols

OBJECTIVES

To systematically examine the evidence for use of a specific protocol (dose, frequency, route) of selected antithrombotic drugs, in comparisons to no therapy or to other antithrombotic therapies, to reduce the risk of complications or improve outcomes in dogs and cats at risk for thrombosis.

DESIGN

Standardized, systematic evaluation of the literature, categorization of relevant articles according to level of evidence (LOE) and quality (Good, Fair, or Poor), and development of consensus on conclusions via a Delphi-style survey for application of the concepts to clinical practice.

SETTINGS

Academic and referral veterinary medical centers.

RESULTS

Databases searched included Medline via PubMed and CAB abstracts. Eight different antithrombotic drugs were investigated using a standardized Patient, Intervention, Comparison, Outcome (PICO) question format both for dogs and cats, including aspirin, clopidogrel, warfarin, unfractionated heparin (UFH), dalteparin, enoxaparin, fondaparinux, and rivaroxaban, generating a total of 16 worksheets. Most studies identified were experimental controlled laboratory studies in companion animals (LOE 3) with only four randomized controlled clinical trials in companion animals (LOE 1).

CONCLUSIONS

Overall, evidence-based recommendations concerning specific protocols could not be formulated for most antithrombotic drugs evaluated, either because of the wide range of dosage reported (eg, aspirin in dogs) or the lack of evidence in the current literature. However, clopidogrel administration in dogs and cats at risk of arterial thrombosis, notably in cats at risk of cardiogenic thromboembolism, is supported by the literature, and specific protocols were recommended. Comparably, aspirin should not be used as a sole antithrombotic in cats with cardiomyopathy. Using the available safety profile information contained in the literature, the panel reached consensus on suggested dosage schemes for most antithrombotics. Significant knowledge gaps were highlighted, which will hopefully drive novel research.

Antiproliferative effects of low molecular weight heparin

AIM

To investigate the effect of low molecular weight heparins (LMWH) on the inhibition of intimal hyperplasia (IH) developing in prosthetic vascular patch graft implanted into sheep carotid artery.

METHODS

A gelatin sealed Dacron patch graft was implanted into the common carotid artery of sheep, which were then allocated to a control group (n = 10) or to one of four treatment groups (each group n = 10) receiving either a low dose (LD) or high dose (HD) of one of two LMWH (enoxaparin 1 or 2 mg/kg/day, dalteparin 100 or 200 units/kg/day) administered subcutaneously for 4 weeks. Anti-activated factor X and activated partial thromboplastin time were assayed from blood collected prior to and at 1 and 2 h after LMWH administration on days 3, 7, 14, 21 and 28. Animals were killed on day 28 after taking blood samples prior to, then at 0.5, 1, 2, 3, 4, 6, 8, 12 and 24 h following the last injection. Grafts were collected for analysis and measurements of intimal thickness obtained under light microscopy from eight transverse sections of each grafted artery aided by computer image analysis. An IH index was calculated by dividing the area of IH (mm2) by the width of the graft (mm).

RESULTS

Intimal hyperplasia index measurements (mean +/- SD) were: controls 0.574 +/- 0.077, LD enoxaparin 0.471 +/- 0.056, LD dalteparin 0.404 +/- 0.025, HD enoxaparin 0.398 +/- 0.068, HD dalteparin 0.332 +/- 0.048. The reductions in IH index compared to controls were significant (P < 0.05) for both LD and HD dalteparin and for HD enoxaparin.

CONCLUSION

Both LMWH dalteparin and enoxaparin reduced the amount of IH formation with dalteparin showing a greater effect in the present animal study. The possibility that different LMWH might exert differing antiproliferative effects requires further investigation.

Verapamil reduces intimal hyperplasia in a sheep carotid artery patch graft model

BACKGROUND

The current study investigated the effect of verapamil on the development of intimal hyperplasia (IH) using a sheep model.

METHODS

A gelatin-sealed Dacron patch graft was implanted into the left common carotid artery in 40 adult Merino sheep. Sheep were then randomly allocated to four groups. Group 1 were controls given no treatment (n = 10). Groups 2-4 were treated with intravenous verapamil 0.5 mg/kg per day in two divided doses for different lengths of time. Group 2 (n = 10) received treatment for 1 week; group 3 (n = 10) received treatment for 2 weeks and group 4 (n = 10) received treatment for 4 weeks. All sheep were killed at 4 weeks and the grafted segments of artery were harvested for IH assessment by an image analysis system.

RESULTS

The IH index from the three groups treated with verapamil was significantly less than that of the control (group 1,0.287+/-0.077: group 2,0.205+/-0.064, P<0.05; group 3, 0.193+/-0.059, P<0.01; group 4,0.171+/-0.046, P<0.01).

CONCLUSION

The present study suggests that verapamil inhibits the development of IH even when treatment is given for only 1 week.

Both dietary fish-oil supplementation and aspirin fail to inhibit atherosclerosis in long-term vein bypass grafts in moderately hypercholesterolemic nonhuman primates

BACKGROUND

Aortocoronary vein bypass grafts are vulnerable to late atherosclerotic occlusion. Conventional platelet inhibitor therapy provides early but not persistent protection against graft failure. Evidence suggests that consumption of marine foods may reduce cardiovascular disease, possibly because of the unique long-chain unsaturated omega-3 fatty acids present in these foods. We hypothesized that dietary fish-oil supplementation would protect against atherosclerosis in vein bypass grafts.

METHODS AND RESULTS

Thirty-three moderately hypercholesterolemic cynomolgus macaques were divided into four groups: control, control+aspirin, fish oil, and fish oil+aspirin. Each control group received olive oil as placebo to equalize calorie and fat consumption with that of the fish-oil groups. Both oils were in ethyl ester form, with the fish oil providing 0.88 g/d eicosapentaenoic acid. The aspirin dose was 40 mg/d. Cephalic vein grafts were interposed bilaterally in the carotid arteries and excised for analysis at 4 years. Bleeding time was significantly prolonged in all groups receiving fish oil or aspirin (P<.05). Plasma cholesterol levels were similar among groups, averaging 6.9+/-2.4 mmol/L (267+/-94 mg/dL). The extent of atherosclerosis in vein grafts did not differ among groups as evaluated both by Sudan IV staining of intimal lipid lesions (27+/-21% of total surface area, P=.89) and analysis of cholesterol content (236+/-203 nmol/mg, 9.1+/-7.8 microg/mg, P=.85). Vein graft connective tissue composition was also unaffected by treatment.

CONCLUSIONS

Our findings do not support the use of concentrated dietary fish-oil supplements or aspirin for the prevention of atherosclerosis in long-term vein bypass grafts. Consumption of fish flesh or less refined oil preparations could have effects different from those of the purified fish-oil ethyl esters we used.

Fish oil and the prevention and regression of atherosclerosis

Epidemiological studies in the seventies have put forward that dietary rather than genetic factors are responsible for the lower incidence of ischemic heart disease in Greenland Inuit and have generated a large body of both in vitro and in vivo experimental studies, exploring the putative favorable effects of fish (oil) on atherogenesis and its risk factors. The first part of this report reviews the in vivo animal studies, concentrating on the hypercholesterolemic models and the arterialized vein graft model. In the hypercholesterolemic animal studies, the results are inconclusive as the studies reporting a protective effect are matched by the number of studies showing no effect or an adverse effect. The diversity in species, dose of fish oil, duration of study, type of vessel studied and type of fish oil preparation (content of n-3 fatty acids, unesterified n-3 fatty acids, ethylesters or triglycerides) could all contribute. Furthermore, the definitions and criteria used in the literature to evaluate atherogenesis are diverse and it appears that while one parameter is affected, another is not necessarily modified in the same direction, stressing the importance of extending the analysis of the effects on atherogenesis to more than one parameter. We also believe that it is time to reach a consensus as to which animal model mimics most closely a particular human situation. Only in appropriate models, investigating more than one atherosclerosis variable, can the effects of a putative anti-atherogenic drug or diet be verified. In the veno-arterial autograft model, mimicking the patient after coronary bypass grafting, dietary fish oil has been consistently effective in preventing accelerated graft intima proliferation. It could therefore be of interest to evaluate the effects of fish oil on graft patency in patients after coronary bypass surgery after a period of years. The results from studies on restenosis after percutaneous transluminal angioplasty are also reviewed and it is concluded that the two large scale trials, that are currently underway, might reliably answer the question whether fish oil is effective as a non-pharmacological adjuvants in the prevention of restenosis. Lastly, the studies on the effects of fish oil on the regression of experimental atherosclerosis are reviewed. In view of the small number of studies (i.e., four) investigating the effects of fish oil on the regression of atherosclerosis, it is premature to draw any conclusion, and therefore further experimental work is required.

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.

Characterization of atherosclerosis with a 1.5-T imaging system

It is shown that a conventional 1.5-T magnetic resonance (MR) imaging system can help characterize some of the key components of atherosclerotic plaque ex vivo. Fresh human aorta with atheromata was suspended in solutions of agarose and manganese chloride and heated to body temperature. The specimens were imaged with modified Dixon and projection-reconstruction imaging sequences. The specimens were then examined histologically to obtain direct correlation between images, spectra, and histologic characteristics. The results show that vessel wall and plaque components can be identified by means of their MR characteristics and correlated with their histologic appearance. The authors were able to identify normal vessel wall components, such as adventitial lipids and smooth muscle. They were also able to identify and localize plaque components such as fibrous tissue, calcification, lipids, and possible areas of hemorrhage and hemosiderin deposition.

Inhibition of intimal hyperplasia by photodynamic therapy using photofrin

Photodynamic therapy using photofrin and light energy inhibits human myofibroblast proliferation in cell culture. The purpose of this study is to evaluate its influence on intimal hyperplasia in vivo. Twenty New Zealand White rabbits underwent a standardized intimal injury to both common carotid arteries with a 2 Fr balloon catheter. One week later, half of the animals received photofrin (5 mg/kg) intravenously. The remaining 10 rabbits received no photofrin. Two days later, all neck incisions were reopened and a 1-cm segment of each of the 40 carotid arteries was exposed for 5 min to 80 mW of 630 nm light energy from a continuous wave tunable dye laser (fluence = 7.6 J/cm2). All vessels were harvested 5 weeks post-laser treatment following in vivo fixation with formalin. From each artery, separate cross-sections taken from both the lasered and non-lasered regions of each vessel were mounted and stained for histologic evaluation. Analyzed segments were then divided into four different treatment groups: group I segments consisted of arterial cross-sections which were taken from vessel regions that were injured but received neither photofrin nor laser treatment (group I, n = 20); group II segments also did not receive photofrin but were exposed to light energy (group II, n = 20); group III segments received photofrin but no light energy (group III, n = 20); and cross-sections in group IV were taken from those segments which received both photofrin and laser treatment. Using planimetry, the ratio of the area of intimal hyperplasia (IH) to the area enclosed by the internal elastic lamina (IEL) was measured for each specimen (IH/IEL).(ABSTRACT TRUNCATED AT 250 WORDS)

Dose response of fish oil versus safflower oil on graft arteriosclerosis in rabbit heterotopic cardiac allografts

With the advent of cyclosporin A, accelerated coronary arteriosclerosis has become the major impediment to the long-term survival of heart transplant recipients. Due to epidemiologic reports suggesting a salutary effect of fish oil, the dose response of fish oil on graft coronary arteriosclerosis in a rabbit heterotopic cardiac allograft model was assessed using safflower oil as a caloric control. Seven groups of New Zealand White rabbits (n = 10/group) received heterotropic heart transplants from Dutch-Belted donors and were immunosuppressed with low-dose cyclosporin A (7.5 mg/kg/day). Group 1 animals were fed a normal diet and served as control. Group 2, 3, and 4 animals received a daily supplement of low- (0.25 mL/kg/day), medium- (0.75 mL/kg/day), and high- (1.5 mL/kg/day) dose fish oil (116 mg n-3 polyunsaturated fatty acid/mL), respectively. Group 5, 6, and 7 animals were supplemented with equivalent dose of safflower oil (i.e., 0.25, 0.75, and 1.5 mL/kg/day). Oil-supplemented rabbits were pretreated for 3 weeks before transplantation and maintained on the same diet for 6 weeks after operation. The extent of graft coronary arteriosclerosis was quantified using computer-assisted, morphometric planimetry. When the animals were killed, cyclosporin A was associated with elevated plasma total cholesterol and triglyceride levels in the control group. While safflower oil prevented the increase in plasma lipids at all dosages, fish oil ameliorated the cyclosporin-induced increase in total cholesterol only with high doses. Compared to control animals, there was a trend for more graft vessel disease with increasing fish oil dose, as assessed by mean luminal occlusion and intimal thickness. A steeper trend was observed for increasing doses of safflower oil; compared to the high-dose safflower oil group, animals supplemented with low-dose safflower oil had less mean luminal occlusion (16.3% +/- 5.9% versus 41.4% +/- 7.6%, p less than 0.017) and intimal thickness (7.9 +/- 1.9 microns versus 34.0 +/- 13.0 microns, analysis of variance: p = 0.054). Low-dose safflower oil also had a slight, but nonsignificant, beneficial effect on graft vessel disease when compared to control rabbits. The same trends were observed in the degree of histologic rejection (0 = none to 3 = severe) in fish oil- and safflower oil-treated animals. Rejection score correlated weakly but significantly (p = 0.0001) with mean luminal occlusion (r = 0.52) and intimal thickness (r = 0.46). Therefore allograft coronary disease in this model appeared to exhibit an unfavorable, direct-dose response to fish oil and safflower oil, independent of effects on plasma lipids.(ABSTRACT TRUNCATED AT 400 WORDS)