Second-degree blocks localized to the AV node are frequently benign and may be caused by β-blocker, calcium-channel−blocker, or digitalis toxic effects; therefore, they are often associated with sinus bradycardia. Unless unprovoked and symptomatic, AV nodal blocks rarely require pacemaker implantation.2 However, a distal block localized to the His-Purkinje system is usually malignant because if it proceeds to complete heart block, the patient is at risk for asystolic cardiac arrest.
The ECG is a powerful tool for the localization and prognostication of AV block; the most important determinant is QRS duration. In patients with bundle-branch block and syncope, any type of second-degree block is worrisome for possible distal location.3 On the other hand, second-degree blocks with narrow QRS complexes are frequently benign;2 the exception being a block localized to the His bundle in the His-Purkinje system above the level of the branching bundles.1 His bundle blocks are frequently overlooked. In general, 2:1 His bundle block should be suspected from the ECG when the PR intervals are normal and the QRS complexes are narrow, as was seen in the present case.4 Also, AV block occurring during treadmill testing is always abnormal.