The term trifascicular block refers to a combination of conduction blocks in all three fascicles, either permanent or transient: (1) RBB and LASF with first-degree AV block, (2) RBB and LPIF with first-degree AV block, (3) LBB with first-degree AV block, or (4) alternating RBB and LBB block.
While bi- and trifascicular conduction blocks indicate advanced organic heart disease, long-term follow-up studies of ambulatory patients indicate that the risk of sudden progression to complete heart block and sudden death due to ventricular asystole is not high. Placement of a ventricular demand pacemaker is indicated only for symptoms due to documented bradyarrhythmias.
In the face of an acute myocardial infarction, the risks of complete heart block are much greater when new or preexistent bi- or trifascicular conduction blocks are present. In this setting, prophylactic placement of a ventricular demand pacemaker is indicated.
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