Enoxaparin and ST elevation myocardial infarction

The ExTRACT-TIMI 25 investigators conclude that enoxaparin is superior to intravenous unfractionated heparin in patients with ST elevation myocardial infarction undergoing fibrinolysis, with either a fibrin-specific agent or streptokinase.1 Since intravenous unfractionated heparin has not been demonstrated to be superior to placebo in this context,2 the relevance of this finding to clinical practice is not clear. The investigators cite a review3 as evidence that unfractionated heparin is efficacious for patients with ST elevation myocardial infarction treated with fibrinolysis and aspirin. In fact, this review3 concludes with the statement ‘Nor is there good evidence that, among patients who are given aspirin and fibrinolytic therapy, the routine addition of either intravenous or subcutaneous heparin produces any worthwhile improvement in outcome’. The pertinent clinical question is: does the addition of enoxaparin to standard therapy (fibrinolysis plus aspirin) for ST elevation myocardial infarction improve outcome? Unfortunately, this question remains unanswered. We cannot assume that intravenous unfractionated heparin is a surrogate for placebo. The small number of studies that have compared unfractionated heparin with placebo have been underpowered to detect any likely benefit either individually or as part of a meta-analysis.2 Therefore, whether unfractionated heparin is slightly better or worse than placebo remains unknown. The finding that enoxaparin is marginally superior to unfractionated heparin does not allow us to conclude that it is superior to placebo.