A pacemaker is a device that delivers battery-supplied electrical stimuli over leads to electrodes that are in contact with the heart. It primarily treats bradycardia. An implantable cardioverter-defibrillator (ICD) is a multiprogrammable antiarrhythmic device for treating ventricular tachycardia and ventricular fibrillation. ICDs also possess pacemaking capability to treat bradycardia. The electrical leads are placed either directly over the epicardium or inserted transvenously into the cardiac chambers. Electronic circuitry regulates the timing and characteristics of the stimuli. The power source is usually a lithium–iodine battery.
Physicians encounter an increasing number of children with either temporary or permanent pacemakers. Basic knowledge about the pacemaker and the pacemaker rhythm strip is essential in taking care of these children. This chapter presents examples of electrocardiography (ECG) rhythm strips from children with various types of pacemakers and elementary information regarding pacemaker and ICD therapy in children.
Electrocardiograms of Artificial Cardiac Pacemakers
The need to recognize the rhythm strips of artificial pacemakers has increased in recent years, especially in intensive care and emergency room settings. The position and number of the pacemaker spikes on the ECG rhythm strip are used to recognize different types of pacemakers. Thus, a pacemaker may be classified as a ventricular pacemaker, atrial pacemaker, or P wave–triggered ventricular pacemaker.
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When the pacemaker stimulates the atrium, the resulting P wave demonstrates an abnormal P axis.
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When the pacemaker stimulates the ventricle, wide QRS complexes result.
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The ventricle that is stimulated (or the ventricle on which the pacemaker electrode is placed) can be identified by the morphology of the QRS complexes. With the pacing electrode on the right ventricle, the QRS complex resembles a left bundle branch block (LBBB) pattern; with the pacemaker placed on the left ventricle, a right bundle branch block (RBBB) pattern results.
Ventricular Pacemaker (Ventricular Sensing and Pacing)
This mode of pacing is recognized by vertical pacemaker spikes that initiate ventricular depolarization with wide QRS complexes ( Fig. 26-1 , A ). The electronic spike has no fixed relationship with atrial activity (P wave). The pacemaker rate may be fixed (as in Fig. 26-1 , A ), or it may be on a demand (or standby) mode in which the pacemaker fires only after a long pause between the patient’s own ventricular beats.
Atrial Pacemaker (Atrial Sensing and Pacing)
The atrial pacemaker is recognized by a pacemaker spike followed by an atrial complex; when atrioventricular (AV) conduction is normal, a QRS complex of normal duration follows (see Fig. 26-1 , B ). This type of pacemaker is indicated in patients with sinus node dysfunction with bradycardia. When the patient has high-degree or complete AV block in addition to sinus node dysfunction, an additional ventricular pacemaker may be required (AV sequential pacemaker, not illustrated in Fig. 26-1 ). The AV sequential pacemaker is recognized by two sets of electronic spikes—one before the P wave, and another before the wide QRS complex.
P Wave–Triggered Ventricular Pacemaker (Atrial Sensing, Ventricular Pacing)
This pacemaker may be recognized by pacemaker spikes that follow the patient’s own P waves at regular PR intervals and with wide QRS complexes (see Fig. 26-1 , C ). The patient’s own P waves are sensed and trigger a ventricular pacemaker after an electronically preset PR interval. This type of pacemaker is the most physiologic and is indicated when the patient has advanced AV block but a normal sinus mechanism. Advantages of this type of pacemaker are that the heart rate varies with physiologic need, and the atrial contraction contributes to ventricular filling and improves cardiac output.
Pacemaker Therapy in Children
Remarkable technologic advances have been made in pacemaker design and function. Surgical corrections of cardiac defects and their late sequelae have increased the need for pacemaker therapy in children. New permanent pacemakers (physiologic pacemakers) are capable of closely mimicking normal cardiac rhythm, and most of them are small enough to be implanted in an infant.
Indications
The indications for permanent pacemaker implantation in children are continually evolving as the reliability of pacing systems improves and clinical experience increases. Box 26-1 lists conditions for which pacemaker therapy is or is not indicated based on the 2008 joint recommendations of the American College of Cardiology (ACC), American Heart Association (AHA), and Heart Rhythm Society (HRS) ( Epstein et al, 2008 ). In the guidelines, class I conditions are those for which there is general agreement that the device will be beneficial, and class II conditions are those for which there is ambivalence regarding whether the device will be beneficial. Class IIa conditions are those for which the weight of evidence or opinion is in favor of usefulness or efficacy, and class IIb conditions are those for which usefulness or efficacy is less well established. Class III conditions are those for which there is agreement that the device will not be useful. Each recommendation is accompanied by the weight of evidence as follows. Level A if the data were derived from multiple randomized clinical trials, level B when data were derived either from a limited number of trials or nonrandomized studies, level C when the consensus of experts was the primary source of the recommendation.
Class I (is indicated)
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For advanced second- or third-degree associated with symptomatic bradycardia, ventricular dysfunction, or low cardiac output (level of evidence: C)
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For sinus node dysfunction with correlation of symptoms during age-inappropriate bradycardia; the definition of bradycardia varies with the patient’s age and expected heart rate (level of evidence: B)
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For postoperative advanced second- or third-degree AV block that is not expected to resolve or that persists at least 7 days after cardiac surgery (level of evidence: B)
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For congenital third-degree AV block with a wide QRS escape rhythm, complex ventricular ectopy, or ventricular dysfunction (level of evidence: B)
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For congenital third-degree AV block in an infant with a ventricular rate less than 55 beats/min or with congenital heart disease and a ventricular rate less than 70 beats/min (level of evidence: C)
Class IIa (is reasonable)
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For patients with congenital heart disease and sinus bradycardia for the prevention of recurrent episodes of intraatrial reentrant tachycardia; sinus node dysfunction may be intrinsic or secondary to antiarrhythmic treatment (level of evidence: C)
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For congenital third-degree AV block beyond the first year of life with an average heart rate less than 50 beats/min, abrupt pauses in ventricular rate that are two or three times the basic cycle length, or associated with symptoms caused by chronotropic incompetence (level of evidence: B)
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For sinus bradycardia with complex congenital heart disease with a resting heart rate less than 40 beats/minor pauses in ventricular rate longer than 3 seconds (level of evidence: C)
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For patients with congenital heart disease and impaired hemodynamics caused by sinus bradycardia or loss of AV synchrony (level of evidence: C)
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For unexplained syncope in the patient with prior congenital heart surgery complicated by transient complete heart block with residual fascicular block after a careful evaluation to exclude other causes of syncope (level of evidence: B)
Class IIb (may or might be reasonable)
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For transient postoperative third-degree AV block that reverts to sinus rhythm with residual bifascicular block (level of evidence: C)
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For congenital third-degree AV block in asymptomatic children or adolescents with an acceptable rate, a narrow QRS complex, and normal ventricular function (level of evidence: B)
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For asymptomatic sinus bradycardia after biventricular repair of congenital heart disease with a resting heart rate less than 40 beats/min or pauses in ventricular rate longer than 3 seconds (level of evidence: C)
Class III (is not indicated)
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For transient postoperative AV block with return of normal AV conduction in an otherwise asymptomatic patient (level of evidence: B)
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For asymptomatic bifascicular block with or without first-degree AV block after surgery for congenital heart disease in the absence of prior transient complete AV block (level of evidence: C)
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For asymptomatic type I second-degree AV block (level of evidence: C)
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For asymptomatic sinus bradycardia with the longest relative risk interval less than 3 seconds and a minimum heart rate more than 40 beats/min (level of evidence: C)
AV, Atrioventricular.
In general, the most common indications for permanent pacemaker implantation in children, adolescents, and patients with congenital heart disease fit into one of three categories: (1) symptomatic sinus bradycardia (with symptoms of syncope, dizziness, exercise intolerance, or congestive heart failure); (2) the bradycardia–tachycardia syndrome (caused by overdrive suppression after a period of tachycardia); and (3) advanced second- or third-degree AV block, either congenital or postsurgical.
Bradycardia is the most common and noncontroversial indication for permanent pacemaker therapy in both children and adults. The primary criterion for pacemaker implantation for bradycardia is the concurrent observation of a symptom (e.g., syncope) with bradycardia (e.g., heart rate less than 40 beats/min or asystole longer than 3 seconds). In children, significant bradycardia with syncope or near syncope results most commonly from extensive surgery involving the atria (e.g., the Senning operation, the Fontan operation, and surgery for atrial septal defect or total anomalous pulmonary venous return). Another noncontroversial indication is surgically acquired heart block that lasts more than 2 weeks after surgery. The risk of death from surgically acquired heart block is as high as 35% in unpaced patients. Most children with congenital heart block will eventually require pacemaker implantation. Those patients with additional CHDs will require pacemaker therapy at an earlier age than those without heart defects.
Temporary pacing is indicated for (1) patients with advanced second-degree or complete heart block secondary to overdose of certain drugs, myocarditis, or myocardial infarction and (2) certain patients immediately after cardiac surgery.
Types of Pacing Devices
The North American Society of Pacing and Electrophysiology and the British Pacing and Electrophysiology Group devised a generic letter code to describe the types and functions of pacemakers, and it was updated in 2002 ( Table 26-1 ). The first three letters are used exclusively for antibradyarrhythmia functions.
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The letter in the first position identifies the chamber paced (O, none; A, atrium; V, ventricle; D, dual chamber; or both A and V).
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The second is the chamber sensed (O, none; A, atrium; V, ventricle; D, dual).
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The third letter corresponds to the response of the pacemaker to an intrinsic cardiac event (O, none; I, inhibited; T, triggered; D, dual [I + T]).
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The fourth letter indicates both programmability and rate modulation.
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The fifth position of the code is used to indicate whether multisite pacing is present.
