Rare accessory pathways, such as Mahaim fibres, have been postulated to result in cardiac pre-excitation. However, most lack the histopathological correlation that has been demonstrated for the Wolff–Parkinson–White pattern. This chapter will discuss the Mahaim fibre tachycardias with left bundle branch block pattern. Mahaim fibres are characterized by the presence of different accessory pathways with decremental conduction (Mahaim physiology, i.e. conduction slows at faster heart rates), which only conducts in an antegrade fashion. The Mahaim fibres have a different anatomical location and distribution that characteristically terminate in the ventricles into or near the conducting system and are responsible for the constellation of electrophysiological features that define Mahaim tachycardias. Similar to other types of pre-excitation syndromes, there are two treatment options available for the treatment of Mahaim fibre tachycardias: pharmacological therapy and/or curative ablation. Catheter ablation offers the greatest chance at definitive therapy for symptomatic patients experiencing frequent Mahaim tachycardias.
Decremental conduction properties in overt and concealed atrioventricular accessory pathways