Bradyarrhythmias

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The indications for implantation of permanent cardiac pacemakers in children are generally similar to those applied to adults, and include symptomatic bradycardia, recurrent bradycardia-tachycardia syndromes, congenital AV block, and advanced second- or third-degree AV block, either surgical or acquired. Specific recommendations for classifying pediatric indications for pacing have been recently published by working groups of the American Heart Association (AHA) and the American College of Cardiology (ACC) (46); these are summarized in Table 5.

Although pediatric pacing was formerly limited by the availability of leads and generators which were suitable with respect to size, heart-rate range, and longevity, technological development in all of these areas has rendered it possible to provide permanent transvenous or epicardial pacing to all but the smallest children (Fig. 5). Recent developments in epicardial leads are useful for children who are too small for transvenous systems, have persistent intracardiac shunts, or are undergoing lead-placement concomitant with other cardiac surgical procedures that expose the epi-cardium. Temporary pacing is occasionally necessary for transient heart block observed in the perioperative period, and in children may be accomplished by placement temporary transcutaneous pacing wires, for short periods of time, by utilization of a transcuta-neous pacing system.

Table 5

Indications for Pacing in Pediatric Patients

Class I—Evidence and consensus on usefulness/efficacy:

• Advanced 2° or 3° AV block with symptomatic bradycardia/CHF/low cardiac output

• Symptomatic sinus node dysfunction with correlation during age-inappropriate bradycardia

• Postoperative advanced second- or third-degree AV block persists > 7 d after cardiac surgery

• Congenital third-degree AV block with a wide QRS escape rhythm of ventricular dysfunction

• Congenital third-degree AV block in the infant with a ventricular rate <50 to 55 BPM or with congenital heart disease and a ventricular rate <70 BPM

• Sustained pause-dependent VT, with or without prolonged QT, in which the efficacy of pacing documented

Class Ila—Conflicting evidence and opinion, generally favoring usefulness/efficacy:

• Bradycardia-tachycardia syndrome with the need for long-term antiarrhythmic treatment other than digitalis

• Congenital third-degree AV block beyond the first year of life, with an average heart rate <50 BPM or abrupt pauses in ventricular rate two or three times the basic cycle length

• Long QT syndrome with advanced 2° or 3°AV block

• Asymptomatic sinus bradycardia in the child complex congenital heart disease with resting heart rate <35 BPM or pauses in ventricular rate >3 s

Class IIb—Conflicting evidence and opinion, usefulness/efficacy less well-established:

• Transient postoperative third-degree AV block that reverts to sinus rhythm with residual bifascicular block

• Congenital third-degree AV block, asymptomatic, with an acceptable rate, narrow QRS complex, and normal ventricular function

• Asymptomatic sinus bradycardia in the adolescent with congenital heart disease, with resting heart rate <35 BPM or passes in ventricular rate >3 s

(From: Gregoratos G, Cheitlin MD, Conill A, Epstein AE, Fellows C, Ferguson TB Jr, Freedman RA,

Hlatky MA, Naccarelli GV, Saksena S, Schlant RC, Silka MJ, Ritchie JL, Gibbons RJ, Cheitlin MD,

Eagle KA, Gardner TJ, Lewis RP, O'Rourke RA, Ryan TJ, Garson A Jr: ACC/AHA guidelines for implantation of cardiac pacemakers and antiarrhythmia devices. Journal of the American College of

Cardiology 1998;31:1175-1209.)

Congenital Heart Block

Congenital heart block is typically diagnosed prenatally during routine obstetrical evaluation or in the newborn period. Ventricular function in complete congenital heart block is typically normal (47), and as the problem is often asymptomatic, heart block may not be discovered until later in childhood. The ECG diagnosis is usually that of complete AV dissociation. More severe presentations include the occurrence of fetal CHF (hydrops fetalis), symptoms of CHF early in infancy or ventricular extrasystoles, and tachycardia. Mothers of children with congenital heart block are commonly observed to have high titers of the maternal lupus antibodies anti-Ro and anti-La, even in the absence of overt maternal systemic lupus erythematosus. The presence of these antibodies has been linked specifically to the pathogenesis of injury to the developing fetal AV node (48), and this is recognized as the most significant underlying etiology to heart block occurring in an infant with normal cardiac anatomy. This common form of congenital heart block is most often identified in the late second trimester, carries a substantial mortality in the neonatal period, and frequently requires pacing (49).

Fig. 5. Epicardial single-chamber atrial pacemaker and ICD implants in a child one year of age with refractory prolonged QT syndrome and a history of multiple cardiac arrests.

Complete AV canal and other endocardial cushion defects and L-transposition of the great arteries (L-TGA "congenitally corrected transposition") are congenital lesions which are also recognized to carry an elevated risk for the occurrence of heart block throughout life, independent of maternal antibody status. This clinical observation is believed to represent vulnerability of the specialized conduction system secondary to deformation associated with the underlying congenital defect (50).

Indications for pacing of congenital heart block in infancy are debated, but have been suggested to include: signs of CHF, extreme bradycardia (e.g., ventricular rate <55/min in isolated block or <65/min when associated with other congenital defects), prolongation of the QT interval, syncope or seizure, and frequent ventricular ectopy. Less commonly, previously asymptomatic children may present with clinical symptoms that may be secondary to bradycardia, especially exercise intolerance and fatigue, and this may prompt pacemaker placement. Although the natural history of congenital heart block has not been completely elucidated, it would appear that even in its completely asymptomatic form, long-term survival may be enhanced by provision of pacing (51). Thus, a typical recommendation for management of a patient who tolerates heart block well in infancy may be to defer pacemaker placement until symptoms occur, or until the patient has reached his or her expected adult body.

Postoperative Complete Heart Block

Postoperative heart block is most often associated with operations which involve resection, suturing, or another manipulation of the ventricular septum, such as repair of a VSD, although it may be seen after any type of cardiac procedure. Natural history studies (performed prior to the era of technically reliable and appropriate pediatric pacing systems) have demonstrated that medical management of these patients without pacemaker implantation is associated with very high mortality (52,53). Postoperative heart block is caused either by direct surgical injury to the AV node and the specialized conduction system or by indirect damage to those structures by stretch, swelling and/ or inflammatory response. Approximately two-thirds of patients who leave the operating room with newly acquired heart block will recover normal AV conduction in 7-10 d (54). Thus, it is appropriate to provide temporary pacing using transcutaneous wires over this period of time prior to implanting a permanent pacing system.

Technical Aspects of Pediatric Pacing

Several issues must be considered when planning to provide permanent cardiac pacing for pediatric patients. First, vascular caliber and infraclavicular muscle and SC tissue mass will be roughly proportional to body size (55). These will affect the choice of lead as well as the placement of the generator pocket, with the smallest patients having placement of a single epicardial lead and a generator pocket on the abdominal wall. Second, it is likely that over the life of the patient, pacemaker generators may need to be replaced several times, as may the leads themselves in the event of failure. The presence of multiple, large pacing leads in the innominate vein is associated with thrombosis and occlusion of the vessel, which adds to the desirability of epicardial leads or single, low-profile transvenous leads in small children with AV block. Finally, in the case of patients with congenital heart disease, the patient's anatomy or the surgical procedures he or she has undergone may complicate, limit, or even preclude access to the right ventricle via the transvenous route. This necessitates a careful anatomical survey to be performed prior to implantation, and may result in the use of inventive approaches to lead placement (56). Comparisons of single- and dual-chamber pacing specific to the pediatric age group have not been performed. Based on studies performed in adults, who are generally elderly and have significant underlying acquired heart disease, it would seem desirable when possible to provide dual-chamber pacing for children as well. However, for young children with otherwise well-preserved myocardial function, even in the presence of congenital heart disease, single-chamber pacing is not commonly associated with pacemaker syndrome or evidence of depressed global cardiac output (57).

A frequent problem encountered in patients in whom atrial pacing is necessary— for instance, those with congenital heart disease, sinus-node dysfunction, and atrial tachycardias—is the difficulty in placing a lead which is reliably able to sense atrial electrical activity, and differentiate it from "far-field" ventricular activity. Recognition of this issue at the time of lead placement may demand use of a bipolar lead, whether endo- or epicardial, and requires a willingness to search diligently for an implantation site which allows clear discrimination of an atrial potential much larger than the ventricular.

Residual intracardiac shunts are an important consideration, and in many cases, an absolute contraindication to transvenous lead placement. In patients with cyanosis caused by a right-to-left shunt, such leads are associated with a significant risk of thromboembolic events, and are contraindicated. Among patients with small left-to-right shunts, especially at the ventricular level (e.g., a tiny residual VSD after surgical repair), it is less clear that the risk of thromboembolism from transvenous pacing leads is elevated.

Pacemaker Follow-Up and Lifestyle

Pacemaker and device therapy in children has different implications for long-term follow-up, both because of the significantly increased life expectancy of pediatric patients compared to the older adult populations and their physically active lifestyle. Higher programmed heart-rate ranges and the frequent use of unipolar ventricular epicardial leads may significantly reduce the expected longevity of the generator battery, especially in very young children. Somatic growth over time may also result in problems with lead length, especially in transvenous systems. Thus, routine clinical follow-up at a maximum interval of 6-12 mo and in combination with transtelephonic monitoring is mandatory.

Because many children have a strong desire to participate in sports, and because there may be considerable peer pressure for them to do so, reasonable judgment must be used in determining whether any constraints on physical activity are indicated. Factors which should be considered in this decision include the nature of the activity being considered; whether it is competitive or recreational, whether it likely to result in an impact injury to the pacemaker pocket in the event of a mishap, whether appropriate protective clothing or equipment can be worn, and whether the activity may interact in any adverse way with the patient's underlying hemodynamic and/or arrhythmia problems. Guidance for participation in physical activities for patients with cardiac disease in general is available (58,59).

SPECIAL TOPICS IN PEDIATRIC ARRHYTHMIA Fetal Arrhythmias

Irregular fetal heart sounds are often observed during prenatal care; less commonly, fetal tachycardia or bradycardia may present as a clinically significant arrhythmia. Either may result in fetal hydrops when severe, with a constellation of findings of CHF including polyhydramnios and effusions by fetal ultrasonography and an increased risk of fetal demise (Fig. 6). Although normal cardiac anatomy is likely, concomitant structural congenital heart disease has been observed, and necessitates echocardiograhic evaluation of these children at the time of diagnosis (60).

Fetal tachycardia most commonly represents a SVT, and may be either an AV reciprocating tachycardia associated with an accessory pathway or atrial flutter. The presence of sustained tachycardia (61) and/or marked elevation of fetal heart rate (62) increases the risk of development of hydrops, particularly if the fetus is of early gestational age. Because these tachycardias are frequently characterized by incessant recurrence and spontaneous termination, techniques used postnatally for acute termination of arrhythmia are not likely to be useful in clinical management of these patients. Given the difficulties both of ECG monitoring and of drug delivery to the fetus, by the most effective method of management is to defer specific arrhythmia therapy until after the infant is delivered. A variety of different agents have been used to attempt

Fig. 6. Ultrasonographic appearance of fetal hydrops with effusions and polyhydramnios resulting from fetal SVT.

to establish chronic control of these tachycardias, with the goal being to allow the fetus to remain in utero for as much of its normal gestation as possible. The first intervention attempted is usually transplacental digoxin therapy administered to the mother. In refractory cases, various drugs have been administered to the mother. In the many case reports and short series which describe the outcomes of these drug trials, some response may be attributed to almost every agent which has been tried. It is clear that a major issue in drug therapy for these patients is the difficulty in achieving therapeutic levels of drug in the fetal circulation (63), which is presumably caused by both the buffering effects of the placental circulation and to the alteration of normal pharmacokinetics in pregnancy. Successful direct intraperitoneal (IP) and intravascular therapy, using long-acting antiarrhythmic agents such as amiodarone and guided by fetal ultrasonography, has also been reported (64).

Fetal bradycardia is usually associated with congenital complete heart block; this is typically, although not universally, associated with a maternal lupus syndrome and maternal positivity for SSA/Ro and SSA/La antibodies. Other associations of fetal complete heart block include anatomical malformations such as corrected transposition of the great arteries (L-TGA) and complete AV canal defects. Profound fetal bradycardia (<55 BPM) and observation of fetal hydrops are ominous prognostic signs. Although attempts have been made experimentally to develop intrauterine cardiac pacing as a surgical technique to rescue severely ill infants, at present this does not appear to be a viable clinical option. Similarly, maternal steroid and plasmapheresis therapy has been of uncertain benefit. Thus, bradycardic fetuses must be monitored closely, and delivered electively if signs of CHF evolve so that they may be paced.

Postoperative Acquired Atrial Arrhythmias

Atrial flutter in its common form or other atypical reentrant tachycardias are a common problem after repair of many forms of congenital heart disease (65-67).

Factors that predispose these patients to arrhythmia include atrial scarring and hypertrophy, abnormal atrial anatomy, and sinus-node dysfunction. These tachycardias are most prevalent in patients who have undergone procedures involving significant alterations of the atrium, especially the Mustard and Senning operations (68) and the Fontan procedure (67). These atrial tachycardias frequently recur, and are a cause of significant morbidity and even mortality in these patients (65,69).

Management of atrial tachycardias after repair of congenital heart disease is challenging and often problematic. The underlying congenital heart lesions are often associated with a marginal hemodynamic status, which not only exacerbates the effects of the tachycardia, but may also limit the options for arrhythmia management. Although the use of antiarrhythmic agents from every class has been reported, none have been clearly efficacious (18,65), and most agents carry the risk of pro-arrhythmia. Recent studies have suggested that radiofrequency ablation for the treatment of atrial reentrant tachycardia in the setting of postoperative congenital heart disease may be feasible (70-72). However, although early results using this approach have been encouraging, more extended follow-up of these patients has demonstrated recurrence rates of over 50% at 2 yr (72), only marginally better than the results of conventional therapy. Although discouraging, the results of both arrhythmia surgery and catheter ablation procedures for AF (73,74) suggest that a combination of more accurate identification of the reentry circuits and improved techniques for creation of radiofrequency lesions may successfully address this problem. Surgical modification of the atria—either as a curative procedure for patients with established arrhythmia or prophylactically performed at the time of congenital heart repair—has also been proposed, and studies of the safety and efficacy of those techniques are in progress.

Evaluation of Syncope in Pediatric Patients

Syncope is a frequent presenting complaint in the pediatric age range (75). Loss of consciousness as a primary complaint, with or without associated seizure activity, demands careful consideration of possible cardiac etiologies by the evaluating physician. The most common causes of syncopal and near-syncopal episodes in children are the various expressions of neurocardiogenic syncope (75-77). However, potentially malignant arrhythmic causes of syncope that must be carefully excluded include abnormalities of repolarization such as LQTS, Brugada syndrome, and metabolic, toxic or drug-induced abnormalities of repolarization, pre-excitation (WPW), idiopathic ventricular tachycardia and acquired heart block (e.g., secondary to infectious causes such as Lyme disease). Syncope may also be a presenting symptom of neurological syndromes, as well as several less serious arrhythmias, such as AV reciprocating tachycardias and breath-holding spells in toddlers (Fig. 7).

Basic outpatient evaluation of syncope in children includes several elements. A detailed history of the event is needed, including its associations with exercise, cardiovascular symptoms, and any witnessed objective cardiovascular signs. Past medical history must include prior occurrences of similar syncopal or presyncopal events, migraines and/or seizure disorders, prior cardiac evaluations, and current medications (including over-the-counter medicines). A family history should document occurrence of sudden death, syncope, seizure disorder, "named arrhythmia" such as LQTS or WPW, hypertrophic cardiomyopathy, use of pacemakers, ICDs, and antiarrhythmic medications in the extended as well as the immediate family. Complete cardiac and

neurological physical examination should be performed, and the ECG should be carefully examined for evidence of pre-excitation, LQTS, or other abnormalities.

If the screening evaluation is negative, certain historical characteristics of the syncopal event may allow the physician to assign a diagnosis of neurocardiogenic syncope with a high degree of certainty. A typical history for neurocardiogenic syncope in children notably includes an association with standing or sitting posture, presyncopal warmth, nausea and diaphoresis, brief duration of loss of consciousness, and an observation of pallor and fatigue after the event. The patient will often be able to draw a parallel with a similar and more frequent but less severe symptom complex experienced in association with presyncope.

When the clinical evaluation of a syncopal pediatric patient does not clearly lead to exclusion of malignant arrhythmia and a positive diagnosis of neurocardiogenic syncope by historical features, additional work-up may be needed. Evaluation of atypical syncope in the child is a significant diagnostic challenge, given the a priori low risk of major cardiac disease in this age group and the often low sensitivity and specificity of the diagnostic tests available. Tests often used to screen for occult cardiac and arrhythmic disease in greater depth include echocardiography, Holter and/or ambulatory event monitoring, and tilt-table testing. Neurological consultation and electroencephalography may also be indicated. Less commonly, and in response to specific positive findings on history, physical, or electrocardiography, it may be appropriate to consider exercise testing, cardiac catheterization, and programmed electrical stimulation, use of an implantable loop recorder (ILR), drug testing, and/or cardiac magnetic resonance imaging.

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