Gap phenomenon in AV conduction

Acknowledgement

Laura Vitali Serdoz

Electrophysiologists

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The Gap phenomenon is observed during atrial extrastimulation in which AV block at longer coupling intervals is followed by restored AV conduction at shorter coupling intervals. Meaning that paradoxically a faster pace improves conduction.
It appears only when the initial site of block is in the distal conduction system (infra-his) and as the coupling interval is further decreased, a further (decremental) delay occurs in the proximal conducting site (supra-his). This "extra" time caused by the supra-his delay gives the distal site time to recover and conduct again
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