Single-lead (or single-chamber) pacemakers (see Fig. 21-1), as their name indicates, are used to stimulate only one chamber (right atrium or right ventricle). Atrial single-lead pacemakers (with the lead positioned in the right atrium) can be used to treat isolated sinus node dysfunction with normal AV conduction (Fig. 21-2). In the United States, single-lead atrial pacemakers are rarely used. Even patients with isolated sinus node dysfunction usually receive dual-chamber devices because AV conduction abnormalities often develop later as the patient ages.

Figure 21-2 With the pacemaker electrode placed in the right atrium, a pacemaker spike (A) is seen before each P wave. The QRS complex is normal because the ventricle is not being electronically paced and the atrioventricular (AV) conduction system is functional.

Ventricular single-lead pacemakers (with the lead positioned in the right ventricle) are primarily used to generate the heartbeat in patients with chronic atrial fibrillation with an excessively slow ventricular response. The atrial fibrillation precludes effective atrial stimulation so that there is no reason to insert an atrial lead (Fig. 21-3).

Figure 21-3 The ventricular (QRS) rhythm is completely regular because of ventricular pacing. However, the underlying rhythm is atrial fibrillation. Fibrillatory waves are small in amplitude in most leads and best seen in lead V₁. Most computer ECG interpretations will read this as “ventricular pacing” without noting atrial fibrillation. Unless the reader specifies “atrial fibrillation” in the report, this important diagnosis, which carries risk of stroke, will go unnoticed. Furthermore, on physical examination, the clinician may be “tricked” into thinking the regular rhythm is sinus.

In dual-chamber pacemakers, electrodes are inserted into both the right atrium and right ventricle (Figs. 21-4 and 21-5). The circuitry is designed to allow for a physiologic delay (normal synchrony) between atrial and ventricular stimulation. This AV delay (interval between the atrial and ventricular pacemaker spikes) is analogous to the PR interval under physiologic conditions.

Figure 21-4 Dual-chamber (DDD) pacemakers sense and pace in both atria and ventricles. The pacemaker emits a stimulus (spike) whenever a native P wave or QRS complex is not sensed within some programmed time interval.

Figure 21-5 Paced beat morphology in dual-chamber pacemaking. Both atrial and ventricular pacing spikes are present. The atrial (a) pacing spike is followed by a P wave with very low amplitude. The ventricular (V) pacing spike is followed by a wide QRS complex with T wave pointing in the opposite direction (discordant). The QRS after the pacing spike resembles a left bundle branch block (LBBB), with a leftward axis, consistent with pacing from the right ventricular apex.

ECG Morphology of Paced Beats

Paced beats start with a pacing spike—a sharp vertical deflection from the pacemaker stimulus. If the pacing threshold is low, the amplitude of pacing spikes can be very small and they can be easily missed.

A paced P wave demonstrates a pacing spike followed by a P wave (see Fig. 21-2).

A paced QRS beat also starts with a pacing spike, followed by a wide QRS complex (see Figs. 21-3 and 21-6). The wide QRS is due to the fact that activation of the ventricles starts at the tip of the lead and spreads to the other ventricle through slowly conducting myocardium, similar to what occurs with ventricular premature beats (VPBs) or ventricular escape beats. The QRS morphology depends on the lead (electrode) position. The most commonly used ventricular electrode site is the right ventricular apex. Pacing at this location produces a wide QRS (usually resembling an LBBB pattern; see Chapter 7) with a leftward axis (QRS deflections are predominantly negative in leads II, III, and aVF and positive in leads I and aVL).

As with VPBs, the T waves in paced beats normally are discordant—directed opposite to the main QRS direction (see Figs. 21-3 and 21-5). Concordant T wave (i.e., pointing in the same direction as the paced QRS) during ventricular pacing may indicate acute myocardial ischemia (see following discussion).

Ventricular paced beats, similar to VPBs, can also sometimes conduct in a retrograde manner to the atria, producing near simultaneous atrial and ventricular depolarization and contraction (Fig. 21-6). When this occurs repeatedly, atrial contraction against the closed AV valves produces recurrent, sudden increases in jugular (and pulmonary) vein pressures, which may be seen as “cannon” A waves in the neck examination. Of note for clinicians, these abrupt pressure changes, in turn, may activate a vagal reflex and cause severe symptoms (palpitations, pulsation in the neck, dizziness, and blood pressure drop), often referred to as the pacemaker syndrome. Therefore, patients in sinus rhythm with AV block are usually implanted with dual-chamber pacemakers so that ventricular pacing will occur after atrial pacing, maintaining physiologic AV timing (synchrony).