Atypical Wenckebach block and concealed conduction.

Following exercise testing, a 71-year-old man developed atrial tachycardia with a unusual form of conduction disturbance: 4:2 atrioventricular (A-V) block. This pattern of A-V block is explainable by two different mechanisms of concealed conduction: two different levels of block in the A-V nodal region and dual A-V nodal pathways with concealed retrograde conduction of P waves. These concepts can also explain a variety of other unusual conduction disturbances.