Atypical Wenckebach AV Block in the Infra-Hisian Region: Clinical Implications and Management

BACKGROUND Second-degree atrioventricular (AV) block is a frequently encountered conduction abnormality on surface electrocardiogram (ECG). However, it does not always imply a block at the AV nodal level. In rare cases, this block can occur below the bundle of His, within the infra-Hisian region of the His-Purkinje system. While the incidence of infra-Hisian block is generally low in the general population, it becomes more common in specific high-risk groups, such as older adults and individuals with structural heart disease. Infra-Hisian block carries a significant risk of progressing to complete heart block, particularly if the patient shows a markedly prolonged His-ventricular (HV) interval or evidence of a bi-fascicular block. CASE REPORT We present the case of a 65-year-old woman who experienced recurrent episodes of syncope. Her surface ECG revealed a bi-fascicular block along with Wenckebach AV block, and 24-h Holter monitoring showed no other significant abnormalities. A baseline short PR interval with second-degree AV block indicated a possible infra-nodal block. An electrophysiology study confirmed an atypical Wenckebach AV block with second-degree infra-Hisian AV block. The patient subsequently underwent permanent pacemaker implantation. CONCLUSIONS Given its potential to develop into more severe forms of heart block, infra-Hisian block is a critical condition that requires accurate identification and management. Several ECG indicators can help diagnose second-degree AV block. For example, RP-dependent PR interval or RP-PR reciprocity is characteristic of Wenckebach AV block. A short PR interval at baseline and minimal amount of PR interval lengthening before block in type 1 second-degree AV block suggest involvement of the His-Purkinje system.