Cont. SICILIAN GAMBIT

10. The excitable gap

In anatomical reentry, the head of an activating wave front and its relative refractory tail are usually separated by tissue that is completely excitable: the excitable gap. The excitable gap may be short or long. This may have consequences for the type of antiarrhythmic agents chosen to terminate a tachycardia.

Reentrant circuit with a long excitable gap. Within the ring, there is a segment of impaired conduction and excitability. Further depression of conduction and excitability will result in conduction block in that segment.

Prolongation of refractoriness will cause block of the reentrant wave front in its own refractory tail, when the conduction enroaches on the relative refractory period.

Atrial fibrillation can be maintained by the presence of many independent wave fronts.

Prolongation of the wavelength of refractoriness reduces the number of wave fronts in a given chamber below a critical number; block and collision terminate the arrhythmia.

We know that sometimes, although we try to suppress the arrhythmia, we create a new one. An explanation of this proarrhythmia phenomenon can be the following:

a) excitability and conduction in a segment are depressed so that unidirectional block sets the stage for induction of reentry by a premature impulse entering the circuit;
b) excitability and conduction are further impaired, creating a zone of bidirectional block so that reentry can no longer be initiated;
c) the segment is only mildly depressed. Bidirectional conduction is responsible for collision of wave fronts (causing bidirectional block);
d) further depression of block (through drugs) will convert the zone of bidirectional conduction into a zone of unidirectional block, which will permit the induction of reentry.

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