Spontaneous Mahaim Fiber Automaticity as a Rare Cause of PVC-Like Complexes.

backgroundMahaim fibers are rare types of accessory pathways with atrioventricular node-like decremental conduction properties. They present a significant challenge in terms of diagnosis and treatment. CASE SUMMARY: We present the case of a patient with bouts of left bundle branch block-morphology wide complex tachycardia, ultimately diagnosed as atrioventricular reciprocating tachycardia with antegrade conduction over an atriofascicular accessory pathway. We demonstrate a systematic approach to diagnosis of left bundle branch block-morphology tachycardia along with several classic features of Mahaim fibers. These include emergence of an "M potential" with decremental conduction during incremental atrial pacing, early right ventricular apical activation during tachycardia, and radiofrequency-induced automaticity. We also demonstrate the rare finding of spontaneous Mahaim automaticity, captured as premature ventricular contraction-like beats on electrocardiogram. DISCUSSION: Prior accounts of spontaneous automaticity in atriofascicular pathways are exceptionally rare. In the literature, automaticity is most often described after radiofrequency ablation. TAKE-HOME MESSAGE: The case exemplifies the unusual electrophysiological property of this rare entity and uniquely illustrates Mahaim spontaneous automaticity masquerading as right ventricular apical ectopy.