Supernormality with cycle length-dependent intra-His alternans: a new cause of group beating

This article describes a patient with baseline infranodal conduction disease. Following the administration of intravenous procainamide, the patient developed group beating. Careful electrophysiological evaluation revealed that the group beating was due to alternating intra-His conduction times. Systematic analysis of paced and spontaneous events provides strong evidence that the alternans results from supernormality within the His bundle.