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Pacemakers and Implantable Cardioverter–Defibrillators: Essentials for Clinicians


This chapter provides a brief introduction to an important aspect of everyday electrocardiogram (ECG) analysis related to the two major types of cardiac implantable electronic devices (CIEDs): pacemakers and implantable cardioverter–defibrillators (ICDs). The topic of implantable loop recorders (ILRs) is briefly described in 4, 13 . Additional material on CIEDs is provided in the online supplement and Bibliography.

Pacemakers: Definitions and Types

Key Point

Pacemakers are electronic devices designed to correct or compensate for symptomatic abnormalities of cardiac impulse formation (sinus node dysfunction) and/or conduction (atrioventricular [AV] heart block or left ventricular conduction delays).

An electronic pacemaker consists of two primary components: (1) a pulse generator (battery and microcomputer) and (2) one or more electrodes (also called leads ). The electrodes can be attached to the skin (in the case of emergency transcutaneous pacing), but more often they are attached directly to the inside of the heart ( Fig. 22.1 ).

Fig. 22.1, Schematic of an implanted pacemaker consisting of a generator (battery with microcomputer) connected to a single wire electrode (lead) inserted through the left subclavian vein into the right ventricle. This is the simplest type of pacemaker. Dual-chamber pacemakers have electrodes in both the right atrium and right ventricle. Biventricular pacemakers can pace both ventricles and the right atrium.

Pacemaker therapy can be temporary or permanent. Temporary pacing is used when the electrical abnormality is expected to resolve within a relatively short time. Temporary pacing electrodes are inserted transvenously and connected to a generator located outside the body. Less commonly, these leads are placed via a transcutaneous approach. For example, temporary pacing is used in severe, symptomatic bradycardias associated with cardiac surgery, inferior wall myocardial infarction (MI), Lyme disease, or drug toxicity. When normal cardiac electrical function returns, the temporary pacing electrode can be easily removed.

Permanent pacemakers have both the generator and electrode(s), implanted inside the body (see Fig. 22.1 ). Electronic pacemakers are used for three major purposes:

  • To restore properly timed atrial impulse formation in severe sinus node dysfunction

  • To restore properly timed ventricular contractions during atrioventricular block (AV synchrony a

    a The terms synchrony and synchronization refer to a harmonization of chamber activation and contraction. Specifically, the terms are used in cardiology to describe events occurring either (1) at a fixed interval (lag or delay) or (2) simultaneously. The first type is exemplified by AV synchrony in which the ventricles are stimulated to contract by the initiation of atrial depolarization after a physiologic delay (native PR interval) or the electronic (pacemaker) interval. The second type is exemplified by intraventricular synchrony in which ventricular pacemaking (biventricular, His bundle or left bundle branch) is used stimulate the right ventricle and left ventricle to contract in a coordinated way, simulating the normal activation process.

    )

  • To compensate for left bundle branch block (LBBB) conduction abnormalities, especially with heart failure, by synchronizing right and left ventricular contraction. Cardiac resynchronization therapy (CRT) was initially accomplished with biventricular pacing. Ongoing work is leading to the increased use of His bundle or left bundle branch area (“conduction system” or “physiologic”) pacing designed to restore synchrony.

Depending on the indication, pacemakers have from one to three leads.

Most often the pacemaker leads are implanted transvenously (through cephalic or subclavian veins) with the generator unit (consisting of the power supply and a microcomputer) positioned subcutaneously in the anterior shoulder area. In some instances the leads are implanted on the epicardial (outer) surface of the heart, using a surgical approach (e.g., to avoid intravascular exposure in patients with a high risk of endocarditis).

All contemporary pacemakers are capable of sensing intrinsic electrical activity of the heart and are externally programmable (adjustable) using special computer devices provided by the manufacturers. Pacemakers are usually set to operate in an on-demand mode, providing electronic pacing support only when the patient’s own electrical system fails to generate impulses in a timely fashion. Modern pacemaker batteries last on average between about 12 and 15 years, depending on usage.

More recently, leadless pacemakers have been developed. These devices (currently about 2 g in weight and about the length of AAA batteries) combine the pulse generator and lead system in a compact cylinder implanted into the right ventricle via the femoral vein. Because these pacemakers do not require an incision to implant a subcutaneous generator and leads, the risk of infection is substantially reduced. Whereas the first-generation leadless pacemakers could only sense/pace the right ventricle, the newest generation devices are capable of atrial activity sensing and, therefore, can function effectively in the VDD mode ( Table 22.1 ).

Table 22.1
Standard Four-Letter Pacemaker Code
I: Chamber Paced II: Chamber Sensed III: Response to Sensing IV: Rate Modulation
A Atrium A Atrium I Inhibit R Rate-responsive
V Ventricle V Ventricle T Trigger
D Both (A and V) D Both (A and V) D Both (I and T)
O None O None O None

Single- and Dual-Chamber Pacemakers

Single-lead (or single-chamber) pacemakers (see Fig. 22.1 ), as their name indicates, are used to stimulate only the 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. 22.2 ). In the United States, single-lead atrial pacemakers are rarely implanted. Even patients with isolated sinus node dysfunction usually receive dual-chamber devices because AV conduction abnormalities can develop as the patient ages, thus requiring the additional ventricular lead.

Fig. 22.2, With the pacemaker electrode placed in the right atrium, a pacemaker stimulus ( A ) is seen before each P wave. The QRS complex is normal because the ventricle is depolarized by the atrioventricular conduction system.

Ventricular single-lead pacemakers (with the lead positioned in the right ventricle) are primarily used to generate a reliable heartbeat in patients with chronic atrial fibrillation with an excessively slow ventricular response. The atrial fibrillation precludes effective atrial stimulation such that there is no reason to insert an atrial lead ( Fig. 22.3 ).

Fig. 22.3, 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 V 1 . 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 overlook the underlying atrial fibrillation because of the regular rate.

In dual-chamber pacemakers , electrodes are inserted into both the right atrium and right ventricle ( Figs. 22.4 and 22.5 ). 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 stimuli) is analogous to the PR interval under physiologic conditions.

Fig. 22.4, Dual-chamber 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.

Fig. 22.5, Paced beat morphology in dual-chamber pacemaking. Both atrial and ventricular pacing stimuli are present. The atrial ( A ) pacing stimulus is followed by a P wave with very low amplitude. The ventricular ( V ) pacing stimulus is followed by a wide QRS complex with a T wave pointing in the opposite direction (discordant). The QRS after the pacing stimulus resembles a left bundle branch block, with a leftward axis, consistent with pacing from the right ventricular apex.

ECG Morphology of Paced Beats

Paced beats are characterized by the pacing stimulus (often called a “pacing spike”), which is seen as a sharp vertical deflection. If the pacing threshold is low, the amplitude of pacing stimuli can be very small and easily overlooked on the standard ECG.

A paced P wave demonstrates a pacing stimulus followed by a P wave (see Fig. 22.2 ).

A paced QRS beat also starts with a pacing stimulus, followed by a wide QRS complex (see Figs. 22.3 and 22.6 ). The wide QRS reflects the slow activation of the ventricles starting at the tip of the lead and spreading to the other ventricle through slowly conducting myocardium, similar to what occurs with bundle branch blocks, premature ventricular complexes (PVCs), 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 the LBBB pattern; see Chapter 8 ) with a leftward axis (QRS deflections are typically negative in leads II, III, and aVF and positive in leads I and aVL).

Fig. 22.6, Note the negative P wave ( arrows ) after the ventricular demand (VVI) paced beats because of retrograde activation of the atria from bottom to top after the paced ventricular beats.

As with PVCs, the T waves in paced beats normally are discordant —directed opposite to the main QRS direction (see Figs. 22.3 and 22.5 ). Concordant T waves (i.e., pointing in the same direction as the QRS complexes during ventricular pacing) may indicate acute myocardial ischemia.

Ventricular paced beats, similar to PVCs, can also sometimes conduct in a retrograde manner to the atria, producing near simultaneous atrial and ventricular depolarization and contraction (see Fig. 22.6 ). 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 intermittent, large (“cannon”) A waves in the neck examination. These abrupt pressure changes, in turn, may activate autonomic reflexes, causing very bothersome symptoms (palpitations, pulsation in the neck, dizziness, and a 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 stimulation will be timed to occur after atrial activation, maintaining physiologic AV synchrony.

Electronic Pacemaker Programming: Shorthand Code

Historically, pacemaker programming has been described by a standard three- or four-letter code, usually followed by a number indicating the lower rate limit. Although many new pacing enhancements have been introduced since the inception of this code, it is still widely used ( Table 22.1 ). Depending on the atrial rate and the status of intrinsic AV conduction, dual-chamber pacemaker function can produce four different combinations of pacing/sensing ECG patterns ( Figs. 22.4, 22.7 , and 22.8 ):

  • A sense, V sense

  • A sense, V pace

  • A pace, V sense

  • A pace, V pace

Fig. 22.7, VVI pacing cycle (10 beats). Sensing of intrinsic activity in the ventricles inhibits pacemaker output. Once the pause after the last QRS complex reaches 1 sec, the pacemaker produces a pacing stimulus resulting in a paced beat (wide QRS). Beats 2 and 8 are fusion beats (coinciding conducted and paced beats). Note also that the normally conducted beats (narrow QRS complexes) have inverted T waves, probably due to “cardiac memory” associated with intermittent pacing, not to ischemia.

Fig. 22.8, DDD pacing. Four different pacing/sensing combinations can be present depending on the sinus rate and atrioventricular (AV) conduction. See also Fig. 22.4 .

This programming corresponds to the code in Table 22.1 .

Single-Chamber Pacemaker Programming

As noted, modern pacemakers are programmed in the on-demand mode providing pacing support only when needed. In the case of a single-chamber pacemaker, usually VVI, this function is accomplished by specifying the lower rate limit (for example 60 beats/min). The pacemaker constantly monitors the patient’s heart rate in the implanted chamber on a beat-to-beat basis. Any time the rate drops below the lower rate limit (in the case of 60 beats/min, the critical pause after a spontaneous QRS complex will be >1 sec), the pacemaker will deliver a pacing stimulus ( Fig. 22.7 ). This corresponds to the code: VVI 60.

To simulate the heart rate increase that normally occurs with exertion, pacemakers can be programmed in a rate-responsive or adaptive mode. The purpose of this mode is to increase the lower rate limit dynamically, depending on the level of physical activity as detected by a sensor incorporated in the generator unit. For example, one of your patients may have rate-responsive ventricular single-chamber pacemaker programming, referred to as VVIR 60 to 110, in which the R indicates “rate-responsive” and the second number (110 in this case) represents the upper pacing limit, which is the maximum rate that the device will pace the ventricles in response to its activity sensor.

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