The most common sites for accessory pathways are connections between muscle tissue in the
atria and the ventricles (atrio-ventricular pathways), bypassing the
atrioventricular node. Rarer sites include connections between atrial muscle and the
conducting tissue within the ventricles (atrio-fascicular pathways), between the atrioventricular node and the muscle tissue of the ventricle (nodo-ventricular pathways), and between the conducting tissue of the ventricle and the ventricular muscle (fasciculo-ventricular pathways). These rarer accessory pathways are sometimes collectively referred to as Mahaim pathways or Mahaim fibres. from a 9-year-old girl with
Ebstein's anomaly and a Mahaim accessory pathway. Mahaim pathways are typically seen on the right side of the heart, with their ventricular connection lying within or close to the right bundle branch. The fibres often conduct slowly and in one direction only - from the atria to the ventricles (antegrade conduction); not from the ventricles to the atria (retrograde conduction). Unlike most atrio-ventricular accessory pathways which conduct electrical impulses at a relatively fixed speed, conduction through a Mahaim pathway varies according to how rapidly it is stimulated. More frequent stimulation leads to slower conduction, known as decremental conduction. If conduction to the ventricles occurs solely through the pathway (maximal pre-excitation), as occurs during arrhythmias like antidromic
atrioventricular re-entrant tachycardia, the ECG appearance is of
QRS complexes with a
left bundle branch block morphology which can be mistaken for
ventricular tachycardia. However, due to their slow decremental conduction, during sinus rhythm the 12-lead ECG will often show little pre-excitation. ==References==