Transitory R wave growth in the midst of ST-segment elevation myocardial infarction: A case of left septal fascicular block with atypical electrocardiographic presentation.

We report the case of a patient who presented with angina and ST-segment elevation in the precordial leads owing to a proximal occlusion of the left anterior descending coronary artery. Serial electrocardiography (ECG) showed signs consistent with the left septal fascicular block (LSFB). The latter was observed in conjunction with a pre-existing left anterior fascicular block and presented atypical ECG features, such as intermittent prominent anterior QRS forces (prominent R wave) in V2 only and preserved septal q waves in I and aVL. In the discussion, we present an overview of the electrocardiographic criteria for the diagnosis of the LSFB together with reasons for which LSFB may present with an atypical ECG picture.