Lingering misconceptions about type I second-degree atrioventricular block

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Statements from the literature

“Type I block is AV nodal”14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26 with no mention of QRS duration; type I block is “almost always in the AV node” without mention of QRS duration27, 28; type I block “is usually accompanied by a QRS of normal duration”29; “Type I block is localized in the AV node or the His bundle”30 without mention of QRS duration or bundle branches except that “type II block often occurs in the bundle branches”; “Type I block occurs in the AV junction”31 without

Statements from the literature

“Type I AV block with a wide QRS is infranodal in about 25% to 30% of cases”8, 9, 10, 11, 33, 34; “Type I block and bundle branch block (BBB) is more likely (or usually) to be within the AV node than the His-Purkinje system”11 or type I with BBB “is frequently at the AV node”; “a remote chance exists of Mobitz I block occurring in an infranodal structure,” if the QRS is wide.35

These statements probably stem from misinterpretation of data from the Puech group5, 6, 7 and that shown in Table 1,

Statement from the literature

“Wenckebach type of block in the His bundle is uncommon and may be seen in 9% of patients and may occur in the presence of a narrow or wide QRS complex.”42 This statement would be more precise by explaining that it refers to 9% (similar to the 8% in Table 1) of all patients (narrow and wide QRS complexes) with type I block and chronic conduction system disease. According to the data outlined in Table 2, 16% of narrow QRS type I blocks are intra-Hisian. This figure may be true for symptomatic

Relative frequency of type I and type II in the His-Purkinje system

In the His-Purkinje system, type II block is about twice as common as type I block (Table 1).

Incidence of narrow QRS type I block

“In patients with a narrow QRS, type I second-degree AV block is nodal in approximately 70% of cases.”10 Similarly, it has been stated (without mention of QRS duration) that “type I second-degree AV block is localized in the AV node in 75% of cases and in the His-Purkinje system in the remainder.”48 These general statements are misleading because they do not specifically refer to chronic conduction system disease. Considering all the causes of narrow QRS type I block, the vast majority of

Conclusion

Type I second-degree AV block should not be automatically labeled as AV nodal in the setting of chronic conduction system disease. Although uncommon, type I second-degree AV block with BBB as a manifestation of chronic conduction system disease requires special consideration because the second-degree AV block is in the His-Purkinje system in 60% to 70% of cases. A His bundle recording is required to determine the site of second-degree block.12 The prognosis of type I infranodal block is poor

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