Fundamentals of pacemakers ECG interpretation - Part 1

Background Permanent pacemakers and cardiac implantable electronic devices have become integral to contemporary cardiology practice. Despite advances in remote monitoring and intracardiac electrograms, the surface electrocardiogram remains the critical bedside tool for assessing device function, particularly during symptomatic presentations or emergencies. However, modern pacing modalities—including cardiac resynchronization therapy and conduction system pacing—have substantially increased ECG complexity, creating interpretive challenges for clinicians and trainees. Objective This review, the first in a two-part series, aims to provide a systematic framework for pacemaker ECG interpretation that remains applicable across device platforms, vendors, and evolving pacing practices. Summary We propose a three-step approach integrating timing assessment, morphological pattern recognition, and malfunction detection. First, we explain the fundamental timing logic of pacemakers—escape, inhibition, and triggering—and how these principles manifest on the surface ECG across single-chamber and dual-chamber modes. Second, we describe the characteristic QRS morphologies associated with different pacing sites, including conventional right ventricular pacing, biventricular pacing, His-bundle pacing, and left bundle branch area pacing. Third, we outline the ECG recognition of basic malfunctions: failure to pace, failure to capture, undersensing, and oversensing. Conclusions Modern paced ECG interpretation requires understanding timing logic as the foundation for recognizing normal device behavior and detecting malfunction. This approach helps clinicians distinguish appropriate device function from true malfunction across single-chamber, dual-chamber, CRT, and conduction system pacing.