Low molecular weight heparins reduce the number of heart attacks and cause fewer complications after an acute coronary syndrome compared to unfractionated heparin

Blood clots in the arteries leading to the heart can cause acute coronary syndrome: unstable angina (a feeling of tightness in the chest) or a type of heart attack. Drugs that dissolve clots (such as aspirin) or thin the blood (such as heparin) can relieve the problem. Unfractionated heparin (UFH) thins the blood, but can cause a serious but rare adverse effect. Low molecular weight heparin (LMWH) is a new type of heparin. The review of trials found that UFH and LMWH were equally effective in preventing death, but LMWH prevented more heart attacks and caused fewer complications.

Authors' conclusions: 

LMWH and UFH had similar risk of mortality, recurrent angina, and major or minor bleeding but LMWH had decreased risk of MI, revascularization and thrombocytopenia. New trials with longer follow up are required.

Read the full abstract...

Acute coronary syndromes (ACS) are an important source of morbidity and mortality. Despite weak evidence for the use of unfractionated heparin (UFH) for acute coronary syndromes it is considered an accepted treatment for unstable angina and non-ST segment elevation myocardial infarction (MI). However, evidence suggests low molecular weight heparins (LMWH) are safer and more effective than UFH in the treatment and prevention of other thrombotic disorders.


To assess the effects of LMWH compared to UFH for acute coronary syndromes.

Search strategy: 

We searched the Cochrane Controlled Trials Register (The Cochrane Library Issue 4, 2000), MEDLINE (January 1966 to December 2000), EMBASE (1980 to December 2000) and CINAHL (1982 to December 2000) and reference lists of articles. Authors of all include studies, and pharmaceutical industry representatives, were contacted to determine if unpublished studies, which met the inclusion criteria, were available.

Selection criteria: 

Randomized controlled trials of subcutaneous LMWH versus intravenous UFH in people with acute coronary syndromes (unstable angina or non-ST segment elevation MI).

Data collection and analysis: 

Two reviewers independently assessed quality of studies. Data were extracted independently by two reviewers. Study authors were contacted to verify and clarify missing data.

Main results: 

We identified 27 potentially relevant studies, 7 studies (11,092 participants) were included in this review.
We found no evidence for difference in overall mortality between the groups treated with LMWH and UFH (RR = 1.0; 95% CI: 0.69, 1.44).
Some pooled outcomes showed some evidence of heterogeneity, few of the pooled outcomes were statistically heterogeneous most were homogeneous.
LMWH reduced the occurrence of MI (RR = 0.83; 95% CI: 0.70, 0.99) and the need for revascularization procedures (RR = 0.88; 95% CI: 0.82, 0.95). We found no evidence for difference in occurrence of recurrent angina (RR = 0.83; 95% CI: 0.68, 1.02), major bleeds (RR = 1.00; 95% CI: 0.80, 1.24) or minor bleeds (RR = 1.40; 95% CI: 0.66, 2.90). A decrease in the incidence of thrombocytopenia (RR = 0.64; 95% CI: 0.44, 0.94) was observed for patients given LMWH. From these results, 125 patients need to be treated with LMWH to prevent 1 additional MI and 50 patients need to be treated to prevent 1 revascularization procedure. Insufficient data exist to compare different types of LMWH.