[Linking in a phase-3 branch block: the electrocardiographic manifestations]

Two cases of phase 3 intraventricular block and linking phenomenon of type 1 and type 2 (or bigeminal rhythm linking) are presented. In the first case, complexes that follow the extrasystolic blocked beat are wide, although they are associated with relative long R-R cycle:type 1 linking. In the second case, bundle branch block and normal conduction alternate in bigeminal extrasystolic beats:type 2 linking. The linking phenomenon is due to concealed retrograde penetration of the bundle branch that is blocked in anterograde direction. It delays the cycle and the refractory period of the bundle branch. Therefore, in type 1 linking, the subsequent stimulus, although occurring with a long R-R cycle, is again blocked in the same bundle branch. In type 2 linking, the R-R cycle and refractory period, occurring after bundle branch block, are shorted because comprised between retrograde activation by extrastimulus and anterograde activation by subsequent sinus stimulus. The subsequent ectopic impulse, although premature, is normally conducted.