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Objective Among other adjunctive medication, heparin is widely used in the therapy of acute myocardial infarction (AMI) today. Large randomized trials, however, have shown inconclusive data on the benefit of adjunctive heparin therapy for patients with AMI. The aim of this study was to describe the use of heparin and complication rates in routine clinical practice today. Methods MITRA and MIR were multicenter registries of AMI patients in Germany. During the years 1994 to 1998, 22697 patients were registered with MITRA and MIR. Of these patients 49.9% received reperfusion therapy. Results 21004 patients (92%) received heparin during acute therapy of AMI. The following factors were associated with withholding heparin: Bleeding at admission (OR 4.7; CI 3.2-6.8), cardiogenic shock (OR 1.8; CI 1.4-2.3) and fibrinolytic therapy with streptokinase (OR 2.1; CI 1.8-2.3). Complication rates of patients with heparin were only slightly higher than among those without heparin: 1.7% strokes and 1.9% bleedings were reported among the patients with fibrinolysis and heparin compared to 1.3% strokes and 1.4% bleedings among patients without heparin (p=ns). Mortality rates were 14.1% for patients with and 27.3% for patients without heparin (p<0.001). Conclusions Of the patients in MITRA and MIR 92% received heparin during AMI. Patients with active bleeding or in critical condition received heparin significantly less often. The selection of critically ill patients may have contributed to the high mortality of patients without heparin for AMI. Bleeding complication rates of patients with adjunctive heparin were only sightly higher than reported in randomized trials.