A presumed abnormal electrocardiogram (ECG) was obtained from an asymptomatic patient with a pacemaker. Systematic evaluation of the ECG revealed that the artifact was due to a physiological sensor in the pacemaker which was displayed when the enhanced pacemaker detection features on the ECG machine was activated. The article discusses the possible causes and an approach to similar artifacts. K E Y W O R D Sartifact, closed loop stimulation, electrocardiogram, electromagnetic interference, pacemaker