Gap phenomenon

The Gap phenomenon in AV conduction was first described as an observation during atrial extrastimulation in which AV block at longer coupling intervals is followed by restored AV conduction at shorter coupling intervals. It occurs when the initial site of block is in the distal conduction system and as the coupling interval is further decreased, a critical delay occurs in the proximal conducting site, which gives the distal site time to recover excitability. A key condition for occurrence of the gap phenomenon is that the effective refractory period of the distal site is longer than the functional refractory period of the proximal site.