Paced ECG Morphology – Reveals More than What It Conceals

Ventricular paced rhythms can mask ECG changes of several conditions. This is because the paced ventricular rhythm does not follow the normal pattern of depolarization through the bundle of His and the bundle branches. Instead, depolarization initially occurs at the tip of the pacemaker lead which is usually at the apex or the septum. The depolarization then proceeds through the myocardium from one cardiomyocyte to another rather than through the conduction system of the heart. This results in abnormal depolarization and repolarization patterns. Thus the QRS complex is usually widened and the T wave is usually of opposite polarity to the QRS complex. Reaching a diagnosis from a single ECG already rendered abnormal by the paced rhythm is not easy. It is thus useful to have a baseline ECG available for comparison. Changes in the axis, QRS width and QRS complex morphology can give important clues to the diagnosis. ECG changes of acute myocardial infarction in patients with a ventricular paced rhythm are well described. There is very limited data on the usefulness of paced ECG in the diagnosis of other conditions.