A patient in their 60s presented with a several-day history of dizziness and light-headedness occurring during physical activity, including 2 episodes of loss of consciousness while running a short distance. Antihypertensive medications included daily lisinopril, 40 mg, and atenolol, 50 mg. On physical examination, the heart rate fluctuated from approximately 35 to 70 beats per minute (bpm), and after the patient moved from a prone to a sitting position several times, it decelerated rather than accelerating. Cardiovascular examination and routine laboratory findings were otherwise normal. Results of an electrocardiogram (ECG) on presentation revealed sinus rhythm of 66 bpm, PR interval of 180 milliseconds, and QRS duration of 118 milliseconds; QRS axis and morphologic findings were normal, with no ST-T abnormalities. The computerized interpretation software correctly indicated normal sinus rhythm. Because of symptomatic bradycardia, the β-blocker (atenolol) was withheld. A repeat ECG after 36 hours was interpreted by the software as sinus bradycardia at 52 bpm and otherwise normal (Figure, A).
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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.
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CME Disclosure Statement: Unless noted, all individuals in control of content reported no relevant financial relationships. If applicable, all relevant financial relationships have been mitigated.
Corresponding Author: Laszlo Littmann, MD, PhD, Department of Internal Medicine, Atrium Health Carolinas Medical Center, PO Box 32861, Charlotte, NC 28232 (firstname.lastname@example.org).
Published Online: May 9, 2022. doi:10.1001/jamainternmed.2022.1558
Conflict of Interest Disclosures: None reported.
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