Mahaim fibres  are a type of accessory pathways connecting the atria to the fascicular conduction system in the ventricles. Nodofascicular pathways have also been described. They have decremental conduction properties like the AV (atrioventricular) node. No delta wave is usually seen in sinus rhythm with Mahaim fibres, though it is a form of pre-excitation. Mahaim fibres do not conduct retrogradely. So Mahaim fibre tachycardia is always with wide QRS complexes – antidromic with LBBB pattern. The fibres are usually located around the tricuspid annulus . The fibres are peculiar in that loss of conduction can occur even with trauma due to the electrophysiology (EP) catheter used for EP study. Specific Mahaim potentials (M potential) generated by the accessory pathway can be recorded in certain cases is useful in guiding successful radiofrequency (RF) ablation. Mahaim junctional acceleration occurs during RF ablation as in case of AV nodal re-entrant tachycardia (AVNRT). Mahaim acceleration during RF ablation has been considered as a predictor of successful ablation . Slow spontaneous automaticity of the distal Mahaim fibre during RF ablation has also been reported .
Left sided Mahaim fibres
Rare cases of left sided atriofascicular pathways have appeared in literature. In one case the Mahaim pathway (with decremental antegrade conduction) was located in the mitral annulus – aorta junction, between the left and right fibrous trigones . Thus it was situated in the supero-septal aspect of the mitral annulus rather than along the tricuspid annulus.
ECG in Mahaim fibre tachycardia
Typical Mahaim fibre tachycardia has a left bundle branch block (LBBB) morphology, with left axis deviation and a late precordial R/S transition . Baseline ECG may not show any significant change in case of Mahaim fibre, though an LBBB pattern with normal PR interval may be seen sometimes .
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