Newer Applications of Pacemakers

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Newer Applications of Pacemakers




Permanent pacemaker technology has evolved significantly over the past 50 years. From the primitive, asynchronous, fixed-rate single-chamber devices (i.e., VOO mode) that were used initially, pacemakers have undergone a major transformation in both sophistication and complexity. Originally, permanent pacemakers were used primarily to provide rate support for symptomatic bradycardia. With technological advances and improved understanding, pacemaker therapy has been applied to a variety of additional disorders. Many of these applications are not directly related to treatment of bradycardia but rather use pacing to alter myocardial activation patterns, to suppress tachyarrhythmias, or to improve hemodynamic function. This chapter focuses on the role of permanent pacing in less common cardiac disorders, the use of pacing after novel procedures, and potential newer applications of permanent pacing technology.



Pacing for Specific Cardiac Conditions


Permanent pacing has been employed to treat a variety of disorders, in addition to traditional indications of chronotropic incompetence, sick sinus syndrome, and atrioventricular (AV) block. For example, cardiac resynchronization therapy (CRT), discussed in detail in a separate chapter, has become an important and commonly employed application for the treatment of patients with left ventricular dysfunction, congestive heart failure, and electrical dys-synchrony. Pacemakers have also played an important role in the treatment of atrial fibrillation, including providing rate support for slow ventricular response and arrhythmia suppression and after AV nodal ablation. Applications of pacemakers in the treatment of a variety of less common disorders or special situations are discussed later and are summarized in Table 119-1.




Hypertrophic Cardiomyopathy


Hypertrophic cardiomyopathy (HCM) is a condition characterized by disarray of myocardial fibers and myofibrils, producing excessive and inappropriate hypertrophy of the left ventricle (LV), most frequently of the interventricular septum. The hypertrophy interferes with ventricular relaxation and, in up to 25% of patients, causes dynamic obstruction across the left ventricular outflow tract. Early observational studies of right ventricular apical pacing suggested regression of ventricular hypertrophy and dramatic clinical improvement.1 However, this benefit was not substantiated in subsequent randomized controlled clinical trials,1 so this strategy has been largely abandoned, except when there is another indication for pacing, such as AV block after alcohol septal ablation (discussed later).



Neurocardiogenic Syncope


Neurocardiogenic syncope is a common problem that accounts for approximately 6% of hospital admissions in the United States annually. Despite improved diagnostic techniques, the cause of syncope remains undiagnosed in up to 50% of cases. It is likely that many of these cases are neurally mediated. The term neurocardiogenic syncope (also known as vasovagal syncope or neurally mediated syncope) is used to describe a group of related conditions that include carotid sinus hypersensitivity, post-tussive syncope, postmicturition syncope, and others. Because episodes are often accompanied by bradycardia, it was hypothesized that permanent pacing may be beneficial. Unfortunately, as with HCM, early uncontrolled studies suggested that permanent pacing offered major clinical benefits that have not been confirmed by subsequent randomized clinical trials.1,2 Currently, there is no clear clinical indication for permanent pacing in neurocardiogenic syncope. However, a subgroup of patients who experience very long pauses (>6 seconds) or marked bradycardia on ambulatory monitoring may benefit from pacing therapy.3



Long QT Syndrome


The long QT syndrome is a heterogeneous group of disorders characterized by QT prolongation and a tendency to develop torsades de pointes and sudden cardiac death. Congenital forms of this disorder have been linked to point mutations in specific genes associated with sodium or potassium channel activity. Acquired forms of the long QT syndrome are usually associated with use of QT-prolonging medications (e.g., sotalol, tricyclic antidepressants), the list of which continues to grow. It is also possible that congenital and acquired forms actually make up a continuum, with some individuals manifesting the abnormality in the baseline state and others requiring trigger factors to elicit QT prolongation. The pathogenesis of ventricular arrhythmias in this syndrome has not been completely elucidated but seems related to sympathetic activation and early afterdepolarizations.


Initial therapy for the long QT disorders traditionally has been pharmacologic (β-blockers), although left stellate ganglion ablation and permanent pacing have also been advocated. The exact mechanism by which pacing exerts its beneficial effect is unclear but probably is related to preventing pauses that characteristically precede episodes of torsade, with elimination of early afterdepolarizations and decreased dispersion of refractoriness. In addition, overdrive pacing to increase heart rate decreases the QT interval. Currently, it is standard to use an implantable cardioverter-defibrillator (ICD) in any individual with documented cardiac arrest, sustained ventricular tachycardia, or recurrent syncope refractory to β-blockers, with pacing reserved for symptomatic bradycardia.



Chronic Disorders of the Neuromuscular System


The muscular dystrophies are a diverse group of inherited, chronic degenerative conditions primarily affecting skeletal muscle. It has long been appreciated that these disorders can also affect cardiac muscle, resulting in fibrosis and fatty replacement of the myocardium. Awareness is growing that the prevalence of arrhythmias, conduction system disease, and sudden cardiac death in these patients may warrant early intervention.4 Because these conditions are infrequent, the therapeutic guidelines are necessarily nonspecific. In general, it can be recommended that unexplained syncope, presyncope, and other transient neurologic symptoms should be aggressively investigated using ambulatory electrocardiographic recordings, long-term event recorders and loop recorders, and, in more worrisome situations (e.g., unexplained sudden syncope precipitating a motor vehicle accident), hospitalization with inpatient monitoring. In most cases, the finding of second-degree atrioventricular block, even when asymptomatic, may warrant permanent pacemaker implantation. It should be noted that ventricular tachyarrhythmias are also common in this population, so, depending on the clinical scenario, an ICD should also be considered.



1. Duchenne’s muscular dystrophy—A progressive neuromuscular disease becoming clinically manifest in the mid-teens and usually fatal by the end of the third decade, generally from heart failure. Although cardiac involvement is the rule, the conduction system is not always involved. Permanent pacing is warranted for second- or third-degree AV block, especially in the setting of a widened QRS complex.


2. Becker muscular dystrophy—Similar to Duchenne’s muscular dystrophy but more slowly progressive and with less frequent cardiac involvement. The indications for permanent pacing appear similar to those of Duchenne’s muscular dystrophy.


3. Myotonic muscular dystrophy—Cardiac involvement is common and usually affects the conduction system. Published data indicate that permanent pacing is warranted with an HV interval >70 milliseconds, even in the absence of high-degree AV block.5 It has been demonstrated that patients with myotonic dystrophy who underwent an invasive approach with electrophysiological study and prophylactic pacing had a longer 9-year survival than those treated conservatively.6


4. Emery-Dreyfuss muscular dystrophy—Conduction system disease is frequent. Sudden cardiac death due to high-grade AV block has been documented, and permanent pacing should be offered with the development of any unexplained transient neurologic symptoms or second- or third-degree AV block.


5. Limb girdle muscular dystrophy—A heterogeneous group of disorders characterized by weakness in the pelvic musculature and upper legs. The familial form has a high incidence of conduction system disease, and permanent pacing should be considered in anyone with a family history of heart block or sudden death.


6. Kearns-Sayre syndrome—A multisystem disorder of the mitochondria with diverse clinical manifestations. Progressive conduction system disease is common, and permanent pacing is probably warranted for marked first-degree AV block.



Infiltrative Diseases of the Myocardium


The myocardium is infiltrated and replaced by nonmyocardial tissue in a diverse variety of disorders. Although the prognosis is usually determined more by type and severity of the underlying disease, cardiac and conduction system involvement is not unusual and may be accompanied by potentially life-threatening bradyarrhythmias or tachyarrhythmias. In these situations, permanent pacemakers may be helpful symptomatically, although they may not necessarily prolong life.


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Jun 5, 2016 | Posted by in CARDIAC SURGERY | Comments Off on Newer Applications of Pacemakers

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