If the pacemaker rate is higher than the spontaneous heart rate of the patient, then every heartbeat is paced. Distinct morphologies appear on the ECG tracing. The pattern depends on the impulses generated by the pacemaker and the spontaneous impulses produced in the heart. 

On a one-chamber pacemaker, the artificial impulse is stimulated over the right ventricle. The right ventricle is activated first, and then the left ventricle is activated over the septum. Conduction is delayed over the left septum, and this produces a left bundle-branch block pattern on ECG. If the electrode is placed on the left ventricle, then a right bundle-branch pattern is seen on ECG. 

The electrical stimulus from the pacemaker produces small and short spikes on the ECG tracing. These are seen immediately before the QRS complex. Unipolar pacemakers generate higher spikes than bipolar pacemakers. Depending on the location of the ECG leads, the spikes vary in morphology. They can be positive, negative, or invisible.

One-Chamber Ventricular-Paced Rhythm With Two Spontaneous Beats

The ECG above represents a one-chamber pacemaker with ventricular pacing spikes that precede each wide QRS complex. Only the limb leads are presented. The ECG represents a regular-paced rhythm with two spontaneous beats. The pacemaker function is normal.

When the spontaneous rate of the patient is higher than the pacemaker rate, the pacer function is inhibited. Likewise, if the pacemaker sensor detects one or several premature beats, then pacing is inhibited shortly afterward. In these instances, there are no spikes on the ECG to give the clinician a clue that a patient has an implanted pacemaker. However, an on-demand pacing switch can be used to manually cause the pacemaker to discharge, allowing the clinician to capture an ECG with the pacemaker spike.

A fusion beat may appear on the ECG if the pacemaker rate is the same as the spontaneous rate (see figure 2). In this instance, the ventricles are partially activated by both the pacemaker and the patient’s spontaneous rhythm. 

A “pseudo-fusion” beat may also be observed. This occurs when a stimulus produced from the pacemaker occurs too late in the absolute refractory period and is rendered useless, and the heart is exclusively activated by the spontaneous impulse. These are normal features of a pacemaker ECG.

Fusion Beats ⁶⁰

Patients may have pacemakers with rate-responsive devices that adjust according to the physiological demands of a patient. These devices provide heart rate support in response to patient activity.