Electrophysiology Unit (EPU) Practice Exam 2026 - Free Electrophysiology Questions and Study Guide

The earliest atrial signal during normal sinus rhythm comes from the high right atrium because the SA node, the natural pacemaker, is located there near the superior vena cava. An electrode placed in the high right atrium is closest to the site of initial atrial depolarization, so it detects the onset of atrial activation first. The other channels reflect activation that occurs later: the coronary sinus channel largely tracks left atrial activation, which follows right atrial activation; the right atrial channel elsewhere in the RA may be a bit later depending on exact placement; and the His channel captures the conduction through the His bundle after atrial activation, not the very start of atrial depolarization. Therefore the high right atrium channel shows the earliest atrial electrogram in NSR.