Intermittent Intraventricular Block
Speeding heart rate shortens 1 the refractory period of atrial and ventricular muscle, as well as the refractory period of the specialized conduction system. Refractoriness in some fibers may abbreviate more than in others as the rate increases, and this fact explains why functional right bundle-branch block (RBBB) is more easily elicited during slow rates or following long, preceding cycle lengths.1 Faster rates disproportionately shorten the refractory period of the right bundle branch (RBB)—which normally exceeds that of the left bundle branch (LBB) during slow rates—so that the refractory period of the RBB approaches that of LBB. However, refractoriness eventually reaches a critical value beyond which an increase in heart rate no longer abbreviates it. At this point, conduction block may occur. If, during rapid rates, the refractory period of the AV node exceeds that of the bundle branches, the AV node becomes the limiting factor
Zipes DP, Fisch C. Supraventricular Arrhythmias With Abnormal QRS Complex. Arch Intern Med. 1972;130(5):781–784. doi:10.1001/archinte.1972.03650050099021
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