Perioperative suppression of platelet adherence to small-diameter polytetrafluoroethylene (PTFE) grafts

Antiplatelet agents for preventing thrombosis after peripheral arterial bypass surgery

BACKGROUND

Peripheral arterial disease (PAD) may cause occlusions (blockages) in the main arteries of lower limbs. One treatment option is bypass surgery using autologous (the patient's own tissue) vein graft or prosthetic (artificial) graft. A number of factors influence occlusion rates in these patients, including the material used. To prevent graft occlusion patients are usually treated with antiplatelet, antithrombotic drugs, or a combination of both.

OBJECTIVES

To determine the effects of antiplatelet agents for the prevention of thrombosis in people with lower limb atherosclerosis who were undergoing femoropopliteal or femorodistal bypass grafting. Outcomes included the overall success of therapy (graft patency and limb salvage rates) and complications of treatment.

SEARCH METHODS

For this update the Cochrane Peripheral Vascular Diseases Group Trials Search Co-ordinator searched the Specialised Register (last searched June 2014) and the Cochrane Central Register of Controlled Trials (CENTRAL) (2014, Issue 5). We sought additional trials through screening the reference lists of relevant papers.

SELECTION CRITERIA

Two review authors, RB and AL, independently reviewed studies found in the search and evaluated them based on the inclusion and exclusion criteria, resolving disagreements through discussion.

DATA COLLECTION AND ANALYSIS

RB and AL independently extracted details of the selected studies for the update. We compared the treatment and control groups for important prognostic factors and differences described. If any data were unavailable, we sought further information from study authors. We synthesised data by comparing group results. We addressed unit of analysis issues by subgroup analysis.

MAIN RESULTS

We include 16 studies with 5683 randomised participants. Nine different treatment groups were evaluated: aspirin (ASA) or aspirin and dipyridamole (ASA/DIP) versus placebo or nothing (six studies); ASA or ASA/DIP versus pentoxifylline (two studies); ASA/DIP versus indobufen (one study); ASA or ASA/DIP versus vitamin K antagonists (two studies); ASA/DIP versus low molecular weight heparin (one study); ticlopidine versus placebo (one study); ASA versus prostaglandin E1 (one study); ASA versus naftidrofuryl (one study); and clopidogrel and ASA versus ASA alone (one study). The treatment comparisons were evaluated separately, and, where possible, we performed subgroup analysis for venous grafts and prosthetic grafts and at different follow-up time points. The quality of evidence was low to moderate as many of the treatment comparisons had very few studies to contribute data, several of the included studies had unit of analysis issues, the treatment dosages varied between studies, and data for many outcomes important to this review were not given in any of the studies, or differed greatly between studies. Overall study quality was moderate, with the largest problem being that the majority of studies did not describe their methods of randomisation, allocation concealment or blinding of outcome assessors, leading to risk ratings of 'unclear'. The other main issue with study quality was studies not blinding participants or personnel.The treatment comparison with the most number of included studies, which allowed for robust conclusions, was that of aspirin (ASA) or ASA and dipyridamole (ASA/DIP) versus placebo or nothing, covered by six studies. For this treatment group, there was improved graft patency in the ASA or ASA/DIP treatment group, odds ratio (OR) 0.42 (95% confidence interval (CI) 0.22 to 0.83; P = 0.01; 952 participants). This effect was not seen for venous grafts alone at any of the time points, but was observed for all time points in prosthetic grafts, including the final time point of 12 months (OR 0.19, 95% CI 0.10 to 0.36; P < 0.00001; 222 participants). Only a single study evaluated secondary patency, for which there was no difference between treatment groups. For the comparison ASA or ASA/DIP versus placebo or nothing there was no difference for any of the side effects, including general, gastrointestinal, bleeding and wound/graft infection. Amputations, cardiovascular events and mortality were also similar between the treatment groups. The comparison of ASA or ASA/DIP versus vitamin K antagonists included two studies, one of which was very large, with over 2000 participants. There were no differences between treatment for primary graft patency at three, six, 12 or 24 months, and there was also no evidence of a difference for limb amputation, cardiovascular events or mortality. One large study (851 participants) evaluated clopidogrel and ASA versus ASA alone, and for all grafts there was no evidence of a difference of primary patency at 24 months. There was evidence of increased total bleeding in the clopidogrel and ASA group (OR 2.65, 95% CI 1.69 to 4.15) from an increase in mild (OR 2.34, 95% CI 1.37 to 4.00), and moderate bleeding (OR 4.13, 95% CI 1.37 to 12.45), but no difference in severe or fatal bleeding. There was no difference between the treatment groups for limb amputation or mortality. For the remaining treatment comparisons there is not currently enough evidence to draw any robust conclusions about the efficacy or safety of the treatment on graft patency after peripheral bypass.

AUTHORS' CONCLUSIONS

Antiplatelet therapy with aspirin or with aspirin plus dipyridamole had a beneficial effect on primary patency of peripheral bypass grafts compared to placebo or no treatment. This effect was not evident when evaluating venous grafts alone, but antiplatelet therapy did have a beneficial effect on patency in those who had prosthetic grafts. There was no evidence of differences in side effects (including general, gastrointestinal, bleeding or infection), amputation, cardiovascular events or mortality between the treatment groups. However, the number of participants included in this analysis might be too small to detect a statistically significant effect for side effects, amputation, cardiovascular morbidity or mortality. We found no difference in primary graft patency when aspirin or aspirin with dipyridamole was compared to a vitamin K antagonist or when clopidogrel with aspirin was compared to aspirin alone. However, there was evidence of increase bleeding in the clopidogrel with aspirin group for the latter comparison. The remaining six treatment comparisons did not include enough data to draw any robust conclusions about their efficacy or safety at this time.

Antiplatelet agents for preventing thrombosis after peripheral arterial bypass surgery

BACKGROUND

Peripheral arterial disease (PAD) may cause occlusions (blockages) in the main arteries of lower limbs. One treatment option is bypass surgery using autologous (the patient's own tissue) vein graft or artificial graft. A number of factors influence occlusion rates, including the material used. To prevent graft occlusion patients are usually treated with antiplatelet, antithrombotic drugs, or a combination of both.

OBJECTIVES

To evaluate whether antiplatelet treatment in patients with symptomatic PAD undergoing infrainguinal bypass surgery improves graft patency, limb salvage and survival.

SEARCH STRATEGY

The authors searched the Cochrane Peripheral Vascular Diseases Group Specialised Register (January 2008) and the Cochrane Central Register of Controlled Trials (CENTRAL) (The Cochrane Library 2007, Issue 4). Additional trials were sought through reference lists of papers and proceedings from the vascular surgical society meetings.

SELECTION CRITERIA

For this update the methodological quality of each original trial was assessed independently by review authors (JB, HM, AW) with emphasis on concealment of allocation.

DATA COLLECTION AND ANALYSIS

Details of the selected studies were extracted independently by JB and HM for the update. The treatment and control groups were compared for important prognostic factors and differences described. If any data were unavailable, further information was sought from authors. Data were synthesised by comparing group results. Unit of analysis issues were addressed by subgroup analysis.

MAIN RESULTS

The administration of a variety of platelet inhibitors resulted in improved venous and artificial graft patency when compared to no treatment. However, analysing patients for graft-type indicated that those patients receiving a prosthetic graft were more likely to benefit from administration of platelet inhibitors than patients treated with venous grafts.

AUTHORS' CONCLUSIONS

Antiplatelet therapy with aspirin had a slight beneficial effect on the patency of peripheral bypass grafts but seemed to have an inferior effect on venous graft patency compared with artificial grafts. The effect of aspirin on cardiovascular outcomes and survival was small and not statistically significant. This might be due to the fact that the majority of patients receiving a peripheral graft have an advanced stage of PAD with critical ischaemia. They are usually seriously ill as a result of cardiovascular disease and have high mortality rates, of 20% per year. Additionally, the number of patients included in this analysis might be too small to reach a statistically significant effect for mortality and cardiovascular morbidity.

Antiplatelet agents for preventing thrombosis after peripheral arterial bypass surgery

BACKGROUND

Peripheral arterial disease (PAD) may cause occlusions (blockages) in the main arteries of the lower limbs. It is frequently treated by implantation of either an infrainguinal autologous (using the patient's own tissue) venous or artificial graft. A number of factors influence occlusion rates, including the material used. To prevent graft occlusion, patients are usually treated with either an antiplatelet or antithrombotic drug, or a combination of both.

OBJECTIVES

To evaluate whether antiplatelet treatment in patients with symptomatic PAD undergoing infrainguinal bypass surgery improves graft patency, limb salvage and survival.

SEARCH STRATEGY

The reviewers searched the Cochrane Peripheral Vascular Diseases Group Specialised Register, (last searched April 2003), and the Cochrane Central Register of Controlled Trials (CENTRAL) (last searched Issue 1, 2003). Additional trials were sought through reference lists of papers and by reviewing proceedings from the vascular surgical society meetings.

SELECTION CRITERIA

The methodological quality of each trial was assessed independently by two reviewers (JD, MMK), with emphasis on concealment of randomisation.

DATA COLLECTION AND ANALYSIS

Details of the studies selected were extracted independently by two reviewers (JD, MMK), and an 'intention-to-treat' analysis performed. The treatment and control groups were compared for important prognostic factors and differences described. If any data were not available, further information was sought from the author. Data were synthesized by comparing group results.

MAIN RESULTS

The administration of a variety of platelet-inhibitors resulted in improved venous and artificial graft patency compared to no treatment. However, analysing patients for graft-type indicated that patients receiving a prosthetic graft were more likely to profit from administration of platelet-inhibitors than those treated with a venous graft.

REVIEWER'S CONCLUSIONS

Antiplatelet therapy with aspirin had a slight beneficial effect on the patency of peripheral bypasses, but seemed to have an inferior effect on venous graft patency compared with artificial grafts. The effect of aspirin on cardiovascular outcomes and survival was mild and not statistically significant; this might be due to the fact that the majority of patients receiving a peripheral graft have an advanced stage of PAD with critical ischemia. These patients are usually seriously ill with respect to cardiovascular diseases with high mortality rates of 20% per year. Additionally, the number of patients included in this analysis might still be too small to reach a statistically significant effect for mortality and cardiovascular morbidity.

Original Article: Changes in Platelet Function, von Willebrand Factor and Factor VIII in Patients Undergoing Aorto-bifemoral By-pass with Dacron Grafts

Platelets play a major role in the development of patency complications in vascular grafts. The aim of this study was to evaluate changes in platelet count and function, and also in factor VIII:C (FVIII:C) and von Willebrand factor (vWF) plasma levels, induced by aorto-bifemoral by-pass with Dacron grafts in seven patients. Platelet count, platelet aggregate ratio (PAR), and platelet aggregability induced by several stimuli, as well as FVIII:C and vWF plasma levels were evaluated before and on days 1,4,9 and 11 after surgery. We observed a mild thrombocytopenia on day 1, followed by a progressive increase in platelet count, which attained a relative thrombocytosis on the 11th day. PAR did not vary significantly during the whole observation period. Platelet aggregation, assayed by the optical method using ADP, epinephrine, arachidonic acid, collagen and ristocetin, (decreased on days 1 and 4). Thereafter, an increase in aggregation was observed until day 11 when hyperaggregability was verified. FVIII:C and vWF peaked on the 4th day, decreasing progressively to pre-surgery values on day 11.

When do vascular surgeons prescribe antiplatelet therapy? Current attitudes

A large number of studies have addressed the efficacy of antiplatelet agents in the prevention of primary and secondary atherosclerotic events. We have undertaken to review the literature and conclude that there is good evidence for the routine prescription of antiplatelet therapy in the prevention of secondary atherosclerotic events in patients with unstable angina, myocardial infarction, transient ischaemic attacks and post-arterial reconstruction. The evidence for any benefit in the prevention of vascular graft occlusion is less clear cut. We therefore conducted a postal survey of Vascular Surgeons in Britain and Ireland, receiving 112 responses to 134 questionnaires. Forty-seven percent of surgeons used antiplatelet therapy following any vascular procedure that they undertook. The rest were more selective in their use of these drugs, reserving them for specific vascular reconstruction, e.g. with synthetic grafts. Five percent of surgeons used anti-platelet aggregating prophylaxis only in patients following transluminal balloon angioplasty. In view of extensive evidence of reduction in long-term vascular mortality and non-fatal vascular events by the use of these drugs in patients who have had a primary vascular event, we would suggest that there is a strong argument for the routine use of anti-platelet drugs in patients presenting with arterial disease to a Vascular Surgeon, regardless of vascular reconstruction, angioplasty or type of graft used.