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Features Departments Information |
 Christopher L. Johnsrude, MD
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Overview of pediatric pacemakers CHRISTOPHER L. JOHNSRUDE, MD aFall 1999 For the heart to function as an effective mechanical pump in the circulatory system, the heart must be formed normally, have adequate myocardial contractility, and cardiac contractions must be precisely coordinated. Timing of atrial and ventricular contractions is controlled by the cardiac electrical system, which ensures atrioventricular (AV) synchrony to optimize each heart beat, and adjusts heart rate to variable demands on cardiac output. This electrical system includes the sinus node, the AV node, the His-Purkinje system (HPS) or "bundle branches," and the cells of the atria and the ventricles themselves. The sinus node is the usual pacemaker of the heart due to its high frequency of automatic spontaneous depolarizations. Each electrical impulse from the sinus node spreads rapidly across the atria by cell-to-cell transmission, activating both atria in about 0.1 second to cause nearly simultaneous biatrial contraction. This same electrical impulse penetrates into the AV node, where after a brief delay (about 0.1 second), it proceeds to the ventricles via the bundle branches. This brief delay permits atrial contraction to provide an "atrial kick," which contributes to ventricular filling and cardiac output. Once the electrical impulse reaches the bundle branches, it travels rapidly across the comparatively bulky ventricles in approximately 0.1 second, causing nearly simultaneous contraction of both ventricles. Disturbances in the cardiac electrical system may result in significant bradycardia or tachycardia. Clinically important bradycardia causes symptoms of chronic fatigue, exercise intolerance, congestive heart failure, dizziness, or syncope. Rarely, abrupt and profound bradycardia may cause sudden death. Bradycardia may be due to intrinsic sinus node disease or suppression from autonomic influences, medications, or chemical imbalances. Bradycardia may also result from block of electrical conduction somewhere in the AV node or HPS. Second-degree AV block reflects intermittent conduction block, and is diagnosed when at least one atrial beat (P wave) is not followed by a ventricular beat (QRS complex). Third degree (or complete) AV block occurs when P waves are entirely independent from QRS complexes, and the ventricular rate may be very slow. AV block may be due to benign functional changes in the AV node (e.g., enhanced vagal tone), or may reflect intrinsic disease (e.g., inflammation or surgical trauma), medication effect, or abnormal blood chemistries affecting the AV node or HPS. Second-degree AV block may have clinical consequences that range from none to substantial; 3° AV block is never normal. In the pediatric population, bradycardia is most common in patients who have undergone surgery for congenital heart disease, and may have marked clinical impact in patients with impaired cardiac function. Permanent pacemakers serve an important antibradycardia function in these patients and may eliminate symptoms and allow them to participate in most physical activities. Tachycardia may result from abnormal impulse generation (i.e., enhanced automaticity) or, more commonly, from abnormal conduction of electrical impulses (i.e., reentrant circuits). Symptoms of tachycardia include palpitations, dizziness, congestive failure, dyspnea, and syncope. Malignant forms of tachycardia may also lead to sudden death. Most forms of tachycardia in children involve a "short-circuit" in the cardiac electrical system, causing electrical impulses to circle repetitively around natural anatomical obstacles (e.g., vena cavae, crista terminalis, AV valves) or surgical barriers (e.g., suture lines, patches, conduits, fibrosis). Depending on the specific mechanism and clinical circumstances, tachycardia may be treated with medications or ablation therapy using transcatheter or surgical techniques.1 Some patients with tachycardia are best managed with antitachycardia devices or cardioverter/defibrillators, which automatically detect and terminate many forms of tachycardia. ANTIBRADYCARDIA PACEMAKERS Permanent pacemakers have been used to treat pediatric patients with bradycardia since the 1970s. Currently, only 1% of pacemakers in the United States are implanted in pediatric patients, compared with roughly 85% in patients more than 65 years old. The clinical circumstances of children with bradycardia are diverse, and general guidelines are available to facilitate decisions regarding pacemaker implantation.2 The most common indications for pacemaker implantation in children include advanced AV block associated with symptomatic bradycardia, symptomatic sinus bradycardia, postoperative AV block that persists beyond the first week after intracardiac surgery, and bradycardia-tachycardia syndrome. In addition, pacemakers may be useful in some patients with other conditions, including familial long QT syndrome, hypertrophic or dilated cardiomyopathy, and postoperative heart transplantation. To address problems of significant bradycardia, pacemaker technology takes advantage of the heart’s reliance on electrical impulses for control of heart rate and AV synchrony. The basic function of a cardiac pacing system is to detect intrinsic cardiac electrical impulses and to deliver impulses when and where they are lacking. The original cardiac pacemakers simply provided a 'safety net' so that a patient’s heart rate would never be too low (e.g., not below 60 beats/min); this remains an important pacing function today. Later designs allowed re-establishment of more physiological cardiac activation patterns, including AV synchrony for patients with AV block, and heart rates that vary with level of physical exertion for patients with chronotropic incompetence due to sinus node dysfunction. The optimal pacing system for each patient depends on patient age and associated cardiac and non-cardiac conditions. For example, an infant with 3° AV block and tricuspid atresia requires an epicardial dual-chamber pacing system, whereas an adolescent with repaired atrial septal defect and sinus bradycardia may require a transvenous single chamber system with rate-response feature. COMPONENTS OF A PACING SYSTEM The basic components of a pacemaker are shown in an example of a patient with a single chamber pacemaker implanted for significant sinus bradycardia (Figure 1a). This pacing system consists of a pacing lead connected to an implantable pulse generator (IPG). The pacing lead is composed of multiple metallic filaments insulated by a coating of polyurethane or silicone material. The lead in this example has been passed from the subclavian vein, through the superior vena cava, to the atrium. The electrode tip of the lead is modified for the lead-tissue interface, permitting detection (or "sensing") of intrinsic electrical impulses from subjacent atrial myocardium. This lead tip may anchor into atrial myocardium with either passive fixation ("tined" lead tip) or active fixation ("screw-in" tip). More recently, lead tips have been designed to elute small amounts of steroid to reduce the local inflammatory response and subsequent fibrosis, thereby enhancing lead function and prolonging longevity of the IPG battery.   FIGURE 1A (top). Chest radiograph of a young teenager with an atrial pacing system implanted via the transvenous approach. The pulse generator has been placed in an infraclavicular pocket. In the above example, the IPG was implanted in the right infraclavicular region near the site of subclavian vein puncture. The IPG is the most complex part of the pacing system, containing multiple microprocessors, circuits, and switches that permit sensing of intrinsic cardiac impulses and coordination of an appropriate response to that signal. Most of the IPG mass consists of the battery that generates electrical current for pacing the heart and driving the IPG electronic circuitry. These components are hermetically sealed, allowing the IPG to be implanted beneath the skin in a subcutaneous or submuscular pocket without risk of subsequent deterioration. In addition, the IPG and pacing leads are essentially physiologically inert, which limits significant inflammation, scarring, or “rejection” by the body. Significant attention by pacemaker manufacturers has led to substantial reduction in the size of IPGs currently being implanted in adults and children (Figure 2b).   FIGURE 2a (top). Earlier model of a cardiac pacemaker, which remained external and was connected to implantable pacing leads. (Courtesy of Medtronic, Inc.) Many sensing and pacing parameters of the IPG are programmable and can be reprogrammed after implantation using non-invasive methods. Basic pacemaker parameters include pacing mode, pacing output, and sensitivity. The pacing mode is represented by at least 3 letters, such as AAI, VVI, or DDD-R. The first position reflects which cardiac chamber is paced (A=atrium, V=ventricle, and D=both chambers). The second position reflects which chamber is sensed, again using letters A, V, or D. The third position reflects the IPG response to a sensed cardiac event (I=inhibit a pacing impulse, T=deliver a pacing impulse, and D=either inhibit or deliver an impulse). The positions after the first 3 letters denote special functions, such as "R" for rate-response and "T" for antitachycardia pacing. The pacing output is the amount of current delivered by the IPG to the lead tip to activate the underlying myocardium. The IPG sensitivity is adjusted so the tiny intrinsic cardiac impulses (<1 to 10 mV) are appropriately detected, thereby permitting an appropriate pacemaker response. In addition, many IPGs record some elements of the patient’s intrinsic and paced rhythms, allowing further optimization of the pacing system during a follow-up visit. PACEMAKER FUNCTION Sinus bradycardia Figure 1b shows an ECG tracing that demonstrates the function of an AAI pacemaker in a young patient with bradycardia due to sinus node disease. In this example, the programmed lower rate limit of the IPG is 90 beats/min. An atrial beat that is intrinsic or paced constitutes an “event” for the IPG; a timer is started with each event. When the patient’s own heart rate is above 90 beats/min (i.e., timing interval between consecutive events < 0.67 second), the IPG is inhibited from delivering a pacing impulse. However, when the patient’s heart rate drops below 90 beats/min, 0.67 second elapses on the timer from the last sensed beat, and the IPG delivers a pacing impulse. The ECG shows a pacing artifact and resultant P wave on the 4th beat, indicating the atrium has been electrically activated or "captured." The P wave is followed by a QRS complex because the patient has normal AV conduction. As the IPG did not sense another intrinsic event within 0.67 second after the last paced beat, another impulse was delivered. In this way, this pacing system will prevent the patient’s heart rate from dropping below 90 beats/min. AV block Patients with significant AV block often have diminished cardiac output due to significant bradycardia and no AV synchrony. These patients may receive a VVI or DDD pacemaker. The VVI pacer requires that only one pacing lead be implanted (usually in the right ventricle), and this system functions as a ventricular equivalent to the AAI system described above. Although VVI pacing provides adequate ventricular rates, there is no AV synchrony or benefit from the atrial kick. DDD or "dual-chamber" pacing is an excellent option for patients with AV block, but requires pacing leads to be implanted in both atrium and ventricle. When the IPG is programmed in the DDD mode, the system will sense intrinsic atrial beats (if the sinus node functions normally) and soon thereafter pace the ventricle if the programmed AV delay elapses and an intrinsic ventricular event does not occur. This is called "atrial tracking," as shown in Figure 3b. Of course, these devices also have a programmable lower rate limit, and so may pace both the atrium and the ventricle if the patient’s sinus node is also sluggish.   FIGURE 3a (top). Chest radiograph of a young patient with a dual chamber pacing system implanted using the epicardial approach. The pulse generator has been placed in an epigastric position, and is not well seen on this film. Rate response As mentioned above, pacemakers may provide heart rates that vary with level of patient’s exertion. The IPG in these rate-response devices contain an activity sensor (e.g., accelerometer or piezoelectric crystal) which senses motion of the body and IPG. This sensor then permits continuous adjustments to the programmed lower rate limit, such as would occur during exercise. Thus, patients with significant sinus node disease may have paced rates during activity that closely approximate those of normal children. Patients with rate-response devices have fewer symptoms and perform better on exercise tests than those with fixed-rate devices. IMPLANTATATION AND FOLLOW-UP Surgical implantation of a permanent cardiac pacemaker is usually performed under general anesthesia by a cardiac surgeon or cardiologist, or both. Figure 1a shows a pacing system implanted via the transvenous approach, commonly used in adults and older children. In infants, small children, and patients with intracardiac shunts or complex congenital heart disease, an epicardial approach may be preferred. With this technique, pacing leads are implanted on the outer surface of the heart, and the IPG is placed in an epigastric or abdominal position (Figure 3a). Because this approach requires sternotomy, thoracotomy or subxiphoid incision, epicardial leads are usually placed at the time of concomitant cardiac surgery. Epicardial lead implantation reduces the chance that significant body growth will affect lead function, avoids potential venous obstruction in patients with small caliber vessels, and avoids the risk for systemic embolic events if thrombus forms on the lead in patients with intracardiac shunts. At Children’s Memorial Hospital, we have seen that the newer steroid-eluting epicardial pacing leads in pediatric patients with diverse forms of heart disease have excellent performance, comparable to the more traditional transvenous leads.3 Children with complex congenital heart disease and difficult arrhythmias may benefit from an alternative transmural approach to permanent cardiac pacing. We recently described our experience with this technique, whereupon the pacing lead is passed directly through the atrial wall at the time of heart surgery.4 This approach allows for an endocardial implantation site (which may be preferred) but bypasses complex systemic venous connections common in these patients. Following pacemaker implantation, the patient remains hospitalized for one to several days, depending on surgical technique and associated clinical concerns. Patients are typically followed soon after hospital discharge and then every 6 to 12 months. At each visit, the pacing system is interrogated non-invasively to evaluate lead performance, IPG function relative to the patient’s intrinsic rhythm, and to estimate remaining battery longevity. In addition to these visits, periodic transtelephonic transmissions from home permit additional assessment of pacer function. At Children’s Memorial Hospital, Maggie Fischer, RN, coordinates this follow-up to ensure early detection of potential pacing system failure and to optimize individual outcomes. IPG battery longevity varies substantially depending on lead performance, programmed mode, and patient use, often ranging from 2 years to more than 10 years (average 5 to 9 years). In the absence of mechanical dysfunction, pacing leads often last at least twice as long as the IPG. With close attention to implantation techniques, optimal system selection, and careful follow-up, many pacemaker patients enjoy few restrictions to daily activities from the pacing systems per se. Rarely, pacemaker interference may occur due to interaction with environmental factors, such as cellular telephones, metal detectors, large magnets, arc welding, and electrocautery. Typically, the pacemaker has a transient response to these stimuli, including inhibition (placing the patient at risk for profound bradycardia) or triggering asynchronous pacing (which may rarely induce a tachyarrhythmia). ANTITACHYCARDIA DEVICES Some pediatric patients develop abrupt malignant tachycardia that may result in syncope, aborted cardiac arrest with profound neurological sequelae, or sudden death. Such tachycardia may be due to primary cardiac electrical disease (e.g., long QT syndrome), cardiomyopathy (e.g., hypertrophic cardiomyopathy), or postoperative congenital heart disease (e.g., Mustard operation for transposition of the great arteries). When tachycardia is dangerous and/or refractory to medical therapy, electrical therapy may be required to terminate it. Electrical therapy involves delivery of electrical impulses to atrial or ventricular myocardium to interrupt reentrant arrhythmias. Rapid pacing with temporary transesophageal or transvenous pacing leads and/or transthoracic cardioversion/defibrillation can terminate most forms of reentry tachycardia. An additional option exists for patients with a permanent pacemaker implanted for tachycardia-bradycardia syndrome. Most current IPGs have non-invasive programmed stimulation capabilities, which allow the cardiologist to terminate atrial reentry tachycardia painlessly, obviating the need for sedation required for other electrical therapies. Several implantable antitachycardia devices provide electrical therapy automatically, the moment malignant tachycardia occurs. Atrial antitachycardia pacemakers have been implanted in many patients at our institution, most commonly after complex atrial surgery.5 These AAI-T devices automatically detect and terminate atrial tachycardia, thereby decreasing the need for antiarrhythmic medications and reducing emergency room visits for recurrent tachycardia. This device works by continuously sensing intrinsic atrial impulses and comparing the pattern (e.g., rate and interbeat variability) with a programmable template to facilitate rapid recognition of tachycardia. When an episode of tachycardia is detected, the device delivers rapid impulses to the atrium to terminate tachycardia (so-called "antitachycardia pacing"). Therapies delivered by the device may or may not be felt by the patient. Implantable cardioverter-defibrillators (ICDs) represent a different class of antitachycardia devices and were originally developed for adult heart patients at high risk for recurrent malignant ventricular tachycardia. Currently, pediatric patients account for 1 to 2% of total ICD implants in the United States. These devices have undergone remarkable evolution over the last decade, enhancing reliability, diagnostic and therapeutic capabilities, and reducing ICD can size. The original devices utilized large ICD cans to hold the battery, capacitor, and circuitry, and thoracotomy or midline sternotomy was required to place large electrical patches on the epicardial surface of the heart. Most ICDs are now implanted using a non-thoracotomy approach, much the same way as the transvenous pacing systems described above (Figure 4a).   FIGURE 4a (top). Chest radiograph of a patient with a transvenous implantable cardioverter-defibrillator. In general, ICDs are used in pediatric and adult patients with documented or presumed malignant ventricular arrhythmias not due to otherwise treatable or reversible conditions. In many of these patients, ICDs have been shown to effectively decrease mortality. Approximately 50% of children with these devices receive appropriate shocks for life-threatening tachycardia within the first 2 years after implantation, but mortality rates are only about 5%.6 These devices use sensing methods similar to atrial antitachycardia devices, but may be connected to the ventricle or atrium, or both. Again, cardiac rhythm is constantly monitored, and when tachycardia occurs and matches a pre-programmed template, the device responds either with overdrive impulses, low-energy cardioversion, or high-energy defibrillation shocks (Figure 4b). These patients are seen routinely every 3 to 4 months, and evaluation focuses on tachycardia recurrence and system performance. SUMMARY Antibradycardia pacemakers are used successfully in pediatric patients of diverse ages and medical conditions. These devices provide patients with substantial symptom relief, improved lifestyle, and do not themselves impose significant restrictions on activities. Antitachycardia devices and implantable cardioverter-defibrillators afford an important and reliable safety net for patients with recurrent malignant tachycardia. Technological improvements, careful implantation techniques, and continued outpatient surveillance of these systems limit complications and enhance longevity of these systems to reduce the frequency of revisions. REFERENCES 1. Deal BJ: Current concepts and treatment of supraventricular tachycardia in childhood. The Child’s Doctor 1998;16(1):13–18. 2. Gregoratos G, Cheitlin MD, Conill A, Epstein AE, Fellows C, Ferguson TB Jr., et al.: ACC/AHA guidelines for implantation of cardiac pacemakers and antiarrhythmia devices: Executive summary—a report of the American College of Cardiology/American Heart Association Task Force on Practice Guidelines (Committee on Pacemaker Implantation). Circulation 1998; 97(13):1325–35. 3. Dodge A, Johnsrude CL, Backer CL, Mavroudis C, Deal BJ. Performance of steroid-eluting epicardial versus transvenous pacing leads in children with congenital heart disease. (Submitted). 4. Johnsrude CL, Backer CL, Deal BJ, Strasburger JF, Mavroudis C. Transmural atrial pacing in patients with postoperative congenital heart disease. J Cardiovasc Electrophysiol 1999;10:351–357. 5. Johnsrude CL, Deal BJ, Backer CL, Mavroudis C, Fischer M, Strasburger JF: Clinical experience with antitachycardia devices in patients with postoperative congenital heart disease. Pacing and Cardiac Electrophysiology 1997;20 (Part II):1052. 6. Case CL, Soloski MC, Gillette PC: Device therapy for arrhythmias. In Current Concepts In Diagnosis And Management Of Arrhythmias In Infants And Children, Deal, Wolff, Gelband (eds.), Armonk, NY: Futura Publishing Co., Inc., 1998. |