ECG interpretation part 2: determination of bundle branch and fascicular blocks

Recognition of the ECG characteristics associated with bundle branch and fascicular blocks is important for identification of new or worsening cardiac disease. These characteristics often herald an increased risk of mortality for patients with acute coronary syndrome (ACS). Clinicians need to have an understanding of the ECG changes associated with bundle branch blocks in order to distinguish between these changes and those associated with ST-segment elevation myocardial infarction (STEMI). Intraventricular conduction defects result from a delay or obstruction in an area of the conduction system below the bundle of His.1 Conduction defects may be due to a complete obstruction of either the right or left bundle branches or may be located further down the conduction system in either the left anterior or left posterior fascicle. These regions of the conduction system are responsible for activation of the ventricles. Therefore, ECG changes associated with intraventricular conduction defects are mostly seen in the QRS complex. The ability of clinicians to identify newly acquired intraventricular conduction defects such as bundle branch blocks is important because their presence in ACS has implications for patient prognosis and urgency of treatment aimed at revascularization.2,3 This article is the second in a 2-part series on ECG interpretation. In Part 1, concepts required for an understanding of mean electrical axis were explored. In Part 2, the mechanisms responsible for the ECG characteristics of bundle branch and fascicular blocks are examined.