John: Hello, I'm John Hilton, editor of the Cochrane Editorial unit. Venous thromboembolism is a common and potentially life-threatening condition in which a clot forms in the veins of the leg or pelvis or moves up to the lungs where it blocks a blood vessel. Heparin is one of the treatments and an updated Cochrane Review from February 2017 examines the evidence of its effects. Lindsay Robertson from the Freeman Hospital in Newcastle upon Tyne in the UK tells us about these latest findings.

Lindsay: The standard treatment for someone with a venous thromboembolism is an anticoagulant, to prevent further clots from forming, and heparin is the commonest of these. It comes in two forms: low molecular weight heparin (LMWH) or unfractionated heparin (UFH). UFH is an older drug and is given either intravenously or by injection. When administering it, doctors have to monitor clotting factors carefully and adjust the dose as necessary. On the other hand, LMWH is given by an injection under the skin once or twice a day and doesn’t need to be monitored as closely as UFH.
Although LMWH has been used in the prevention of recurrent thromboembolism, there is accumulating evidence that it is also safe and effective for the initial treatment of venous thromboembolic events and we wanted to bring all the evidence together in this, the third update of the Cochrane Review that was first published in 1999.
We examined the efficacy and safety of fixed dose subcutaneous low molecular weight heparin compared to adjusted dose unfractionated intravenous or unfractionated heparin for the initial treatment of people with venous thromboembolism. We assessed the number of times that patients developed another blood clot, the number of patients whose original clot got smaller and safety outcomes, such as death and major bleeding episodes during the initial treatment or within 48 hours after it had finished.
We found 29 randomised trials involving a total of over 10,000 patients with venous thromboembolism. Pooling the results showed that fewer patients treated with LMWH formed further blood clots and that fewer cases of bleeding occurred than with the other heparin treatments, but we need to be cautious because of the moderate quality of this evidence. There was no difference in the number of deaths between the LMWH and UFH groups, and LMWH reduced the size of the original blood clot when compared to UFH but the quality of this evidence was low as results were not similar across the studies.
In summary, it appears that LMWH reduces the incidence of further blood clots and major bleeding during initial treatment; and that it might reduce clot size more than UFH. However, the moderate to low quality of this evidence and the relatively short follow-up of the patients in the existing trials, means that further large scale studies are required, in particular to determine the relative effects of LMWH and UFH on the longer-term outcomes of venous thromboembolism.

John: If you would like to explore the existing evidence in more detail and watch for future updates of Lindsay’s review, you can find the full version online. Go to Cochrane Library dot com and search ‘venous thromboembolism and fixed dose heparin’.