Misleading ECG appearance of AV block due to concealed AV nodal conduction caused by interpolated ventricular ectopic beats.

Concealed conduction commonly occurs when a retrogradely conducted interpolated ectopic impulse enters the atrioventricular (AV) node; thus, the next sinus beat is not conducted to the ventricle or conducted with a prolonged PR interval because of increased refractoriness of AV conduction system. A 67-year-old man had complaints of exertional fatigue and palpitations at rest. His blood pressure was 110/70 mmHg and heart rate was 78 beats/min. Auscultation revealed a mild systolic murmur at the apex and an irregular rhythm. His electrocardiogram was normal, except for the presence of frequent premature ventricular complexes (PVC) of right bundle branch block morphology. Echocardiographic examination showed only grade-1 mitral regurgitation. Further evaluation with 24-h Holter monitoring showed frequent interpolated PVCs in bigeminal rhythm. Progressive prolongation of the PR interval was observed after each PVC, which ended with Mobitz type I AV block. The patient was treated with metoprolol which resulted in immediate and marked improvement in the symptoms. Control Holter recording showed very rare PVCs, without PR prolongation or AV block.