On Bundle Branch Block and Acute Myocardial Infarction

Abstract

Studies from the pre-thrombolytic era showed that patients with bundle branch block suffering acute myocardial infarction had worse prognosis compared to patients without bundle branch block. It seems that this difference still exists in the thrombolytic era, at least for short-term mortality. Patients with bundle branch block have potentially much to gain from thrombolytic therapy, therefore diagnosing threatening or ongoing acute myocardial infarction and deciding on thrombolytic treatment is important, but this is difficult. Many ECG criteria for left bundle branch block have been suggested, but have shown limited diagnostic abilities. Studies on the diagnostic abilities in the presence of right bundle branch block are limited.In two populations, a prospective multi-centre study (257 patients) and a prospective register study (33 patients) from one centre, we studied the use thrombolytic treatment, demographics and short- and long-term outcome of patients admitted to coronary care units with bundle branch block and suspicion of acute myocardial infarction. The usefulness and diagnostic abilities of continuous vectorcardiographic monitoring and the diagnostic abilities of the 12-lead ECG were also studied.Patients with bundle branch block and suspicion of acute myocardial infarction received sub-optimal treatment with thrombolytics. Only 16% of those with left- and 36% of those with right bundle branch block and acute myocardial infarction received thrombolytic treatment. Patients with right- had lower mortality compared to those with left bundle branch block at 1-year follow-up (14 vs. 28%). After 1-year there was no difference in mortality between those with and without acute myocardial infarction and left bundle branch block emerged as an independent risk-factor while acute myocardial infarction did not.For left bundle branch block, previously suggested 12-lead ECG criteria showed limited diagnostic abilities (sensitivity 17% and specificity 94%). When monitoring with continuous vectorcardiography, distinct differences where seen in ST-Vector Magnitude and ST Change-vector magnitude. A cut-off value of >65 microV for ST Change-vector magnitude after 90 minutes of monitoring gave a sensitivity of 54% and specificity 72% for the diagnosis of acute myocardial infarction. 12-lead ECG criteria used for diagnosis of acute myocardial infarction for narrow QRS-complexes showed inferior diagnostic abilities when applied to right bundle branch block. Continuous vectorcardiographic monitoring showed distinct differences in QRS and ST parameters for patients with, compared to those without acute myocardial infarction. Using a cut-off value of > 125 microV for initial ST-vector magnitude resulted in a sensitivity of 55% and specificity of 87% for the diagnosis of acute myocardial infarction. Continuous vectorcardiography is thus applicable to patients with bundle branch block and suspicion of acute myocardial infarction and seems to be of use for early diagnosis and subsequent monitoring.