Background--Patients with an acute anterior ST-segment elevation myocardial infarction and right bundle-branch block (RBBB) have a high mortality risk, which may be stratified by early ECG changes.

Methods and Results--In the Hirulog Early Reperfusion Occlusion (HERO-2) trial, 17 073 patients with acute myocardial infarction (AMI) within 6 hours of symptom onset were treated with streptokinase and randomized to receive bivalirudin or heparin. There was no difference in the primary end point of 30-day mortality. ECGs were recorded at randomization and 60 minutes after fibrinolytic therapy was begun. The 30-day mortality rate was 31.6% in the 415 patients with RBBB and anterior AMI at randomization and 33% in the 100 patients who developed new RBBB at 60 minutes from normal baseline conduction accompanying an anterior AMI. An increase in QRS duration by 20-ms increments was associated with increasing 30-day mortality rate in both RBBB groups on multivariable analyses with covariates of age, Killip class, systolic blood pressure, pulse, and prior infarction. Patients with QRS duration 160 ms had higher 30-day mortality rate than those with QRS duration <160 ms (37.2% versus 27.2%, P=0.03, and 46.2% versus 24.5%, P=0.025, in the 2 groups, respectively). For the patients with RBBB and anterior MI at randomization, RBBB resolved at 60 minutes in 40 patients, but 30-day mortality rate was unchanged. For those with persisting RBBB at 60 minutes, 30-day mortality rate was lower if ST-segment elevation had resolved by 50% (20.4% versus 35.3%, P=0.006).

Conclusions--In patients with anterior AMI and RBBB, increasing QRS duration is associated with increasing 30-day mortality. Early ST-segment resolution after fibrinolytic therapy despite persisting RBBB is associated with lower mortality rate.