Second-degree atrioventricular block: A reappraisal

In this review, we discuss the various forms and causes of second-degree at rioventricular (AV) block and the reasons they remain poorly understood. Bo th type I and type II block characterize block of a single sinus P wave. Ty pe I block describes visible, differing, and generally decremental AV condu ction. Type II block describes what appears to be an all-or-none conduction without visible changes in the AV conduction time before and after the blo cked impulse. Although the diagnosis of type II block is possible with an i ncreasing sinus rate, absence of sinus slowing is an important criterion of type II block because a vagal surge (generally a benign condition) can cau se simultaneous sinus slowing and AV nodal block, which can superficially r esemble type II block. The diagnosis of type II block cannot be established if the first postblock P wave is followed by a shortened PR interval or is not discernible. A pattern resembling a narrow QRS type II block in associ ation with an obvious type I structure in the same recording leg, Holter) e ffectively rules out type II block because the coexistence of both types of narrow QRS block is exceedingly rare. Concealed His bundle or ventricular extrasystoles confined to the specialized conduction system without myocard ial penetration and depolarization can produce electrocardiographic pattern s that mimic type I and/or type II block (pseudo-AV block). All correctly d efined type LI blocks are infranodal, A narrow QRS type I block is almost a lways AV nodal, whereas a type I block with bundle branch block barring acu te myocardial infarction is infranodal in 60% to 70% of cases. A 2:1 AV blo ck cannot be classified;in terms of type I or type II block, but it can be nodal or infranodal, Infranodal blocks require pacing regardless of form or symptoms. The widespread use of numerous disparate definitions of type II block appears primarily responsible for many of the problems surrounding se cond-degree AV block. Adherence to the correct definitions provides a logic al and simple framework for clinical evaluation.