First-degree trifascicular block unmasked by sinus arrhythmias. Report of a case studied with intracardiac electrocardiography.

A case is presented in which first-degree block in all three fascicles of the intraventricular conduction system results in a QRS complex with no specific features of fascicular block. During spontaneous sinus arrhythmia the typical features of RBBB and LAH appear at the longest sinus cycles. This is associated with shortened P-R and H-Q intervals. During shorter sinus cycles, near equalization of delay in each fascicle is associated with delay in His-Purkinje conduction but loss of the typical features of bifascicular block as the ventricular myocardium becomes activated more synchronously. Documentation of the changing patterns of ventricular activation is provided by His bundle electrocardiography.