Atrial and ventricular pacing can be seen on the electrocardiogram (ECG) as a pacing stimulus (spike) followed by a P wave or QRS complex, respectively. The ECG has the ability to show normal and abnormal pacemaker function.
ATRIAL PACING ONLY
Atrial pacing appears on the ECG as a single pacemaker stimulus followed by a P wave (waveform 1). (See "Modes of cardiac pacing: Nomenclature and selection".) The morphology of the P wave depends upon the location of the atrial lead; it may be normal, diminutive, biphasic, or negative. The PR interval and configuration of the QRS complex are similar to those seen in sinus rhythm. They are independent of the atrial pacemaker; thus, the duration and configuration are determined by the intrinsic characteristics of the patient's conduction system.
There is 100 percent capture if the rhythm is entirely paced. However, there may be intermittent capture when the atrial pacemaker is in a demand mode and is activated only when the intrinsic atrial rate falls below a preset level. In these cases, a paced beat will be seen after a pause that is equal to this lower predetermined heart rate. For example, if the pacemaker is set at 60 beats/min, the pacemaker will only pace if the rate falls below 60 beats/min or there is a pause of one second (60 beats/min ÷ 60 sec/min).
VENTRICULAR PACING ONLY
Ventricular demand pacing appears on the electrocardiogram as a single pacemaker spike followed by a QRS complex that is wide, bizarre, and resembles a ventricular beat (waveform 2). (See "Modes of cardiac pacing: Nomenclature and selection".) The pacemaker lead is usually in the right ventricular apex; thus, the paced QRS complex has a left bundle branch block (LBBB) configuration since right ventricular activation occurs before activation of the left ventricle, and is negative in the inferior leads. Sometimes, the lead may be placed higher up in the right ventricular septum or outflow tract, and while there is still a LBBB pattern, the inferior leads may have variable axis. There may or may not be atrial activity noted, depending upon the nature of the patient's underlying rhythm, the atrial rate, and the occurrence of ventriculoatrial conduction via the atrioventricular (AV) node. If intrinsic or native atrial activity is present with a single chamber ventricular-only pacemaker, it occurs at a rate that differs from the ventricular rate since it is dissociated from the QRS complex. Frequently, ventricular demand pacing is seen in association with atrial fibrillation.
There may be episodic pacing in patients who have a ventricular demand pacemaker. The pacemaker is activated and delivers a stimulus only when the intrinsic ventricular rate falls below a predetermined lower limit; pacemaker activity is suppressed when the intrinsic heart rate is faster (ventricular inhibited). The escape interval (the time between the last intrinsic beat and the paced beat) is equivalent to the rate at which the pacemaker is set to activate. Similar to atrial pacing, if the pacemaker is set at 60 beats/min, the pacemaker will only pace the ventricle if the rate falls below 60 beats/min or there is a pause of one second (60 beats/min ÷ 60 sec/min). If the native rate is slow, there will be 100 percent ventricular pacing (figure 1). The electrocardiogram may have evidence of fusion or pseudofusion beats if the pacemaker rate and intrinsic heart rate are nearly identical, and the native and paced QRS complex occur simultaneously.