Current Indications for Temporary and Permanent Cardiac Pacing

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Chapter 33 Current Indications for Temporary and Permanent Cardiac Pacing

In the past few decades, improvements in device technology and surgical techniques have made permanent pacemaker implantation a readily performed minor surgery with a low likelihood of major complications. Similarly, temporary pacemaker insertion can often be performed at the bedside quickly and safely with a relatively limited set of tools. Nevertheless, employing cardiac pacing in a given situation remains a major clinical decision, as it has the potential for life-altering consequences for the patient. In 2008, the American College of Cardiology, the American Heart Association, and the Heart Rhythm Society (ACC/AHA/HRS) jointly issued an updated set of guidelines for cardiac pacing on the basis of a comprehensive review of available clinical data and expert opinion.¹ This chapter refers to this document frequently and makes use of the ACC/AHA/HRS conventional division of indications into class I, class IIa, class IIb, and class III. The European Society of Cardiology separately released a set of guidelines for pacemaker implantation in 2007, making use of the same system for classifying indications.²

The ACC/AHA/HRS and ESC guidelines also classify the supportive data for each recommendation by assigning a level of evidence (LOE, see Appendix Table 33-1).* It is notable that of the 71 specific ACC/AHA/HRS recommendations relating to permanent cardiac pacing in adults, only 3 have LOE A, 29 have LOE B, and 39 have LOE C. In some instances, the lack of randomized trials reflects a broad consensus on the standard of care and the absence of alternatives to cardiac pacing for symptomatic bradycardia. In many other instances, however, particularly for class II indications, data from large clinical trials are scarce. The guidelines should, therefore, not be adhered to blindly but should be seen as a framework to assist with clinical decision making. Accordingly, the aim of this chapter is to review the indications for cardiac pacing in adults, including temporary pacing, permanent pacing, and multiple-site pacing, with special attention to clinical dilemmas and specific clinical situations. Device therapy in pediatric and adolescent patients and in patients with congenital heart disease will be reviewed separately.

Indications for Permanent Pacing

Bradyarrhythmias

Permanent pacing is most commonly employed to treat bradyarrhythmias, which can result from dysfunction of the specialized cardiac conduction system or dysfunction of the autonomic nervous system or from iatrogenesis. Therefore, the aim of this section is to provide a pathophysiologic framework for considering cardiac pacing in terms of the anatomic compartments required for normal cardiac rhythmicity.

Disorders of Sinoatrial Conduction

The functions of the sinus node are impulse generation, impulse transmission, and modulation of heart rate in response to autonomic inputs. Patients suffering from sinus node dysfunction manifest one or more of several distinct electrocardiographic abnormalities, including sinus bradycardia, sinus pauses, bradycardia alternating with tachyarrhythmias, and failure to augment heart rate appropriately with exercise (see Appendix Table 33-2). Typically, such patients will complain of lightheadedness, fatigue, syncope, or activity intolerance. The main diagnostic tool for establishing the diagnosis of sinus node dysfunction is continuous ambulatory electrocardiographic monitoring for a period determined by the frequency of the patient’s symptoms.

No randomized trials demonstrating that pacing improves survival in patients with sinus node dysfunction have been conducted, and at least one small series suggests that short-term prognosis is good in this patient population.³ Thus, the primary role for pacing is to treat symptoms, prevent adverse events such as syncope, and permit therapy with drugs that slow heart rate, as required to treat other medical conditions. Before considering pacemaker implantation, reversible causes of sinus node dysfunction should be ruled out (Table 33-1). The indications for permanent pacing in sinus node dysfunction are summarized in Appendix Table 33-3. When sinus pauses or bradycardia coincide temporally with the patient’s symptoms and a causal link can be established, a pacemaker is indicated; this is a class I indication. Conversely, when the patient’s typical symptoms occur during periods when monitoring reveals a normal heart rate, pacemaker implantation would not be expected to provide any benefit; this is, therefore, a class III indication.

Table 33-1 Reversible Causes of Sinus Bradycardia and Atrioventricular Block

TYPE OF CAUSE	SPECIFIC CAUSE
Cardiac	Myocardial ischemia
	Viral myocarditis
	Lyme disease
	Rheumatic fever
	Endocarditis
	Post–cardiac surgery
	Post–cardiac transplant
	Athletic training
Autonomic nervous system	Neurocardiogenic syncope
	Carotid hypersensitivity
	Vagal stimulation
Drugs and medications	β-Adrenergic blockers
	Calcium channel blockers
	Clonidine
	Digitalis
	Ivabradine
	Reserpine, methyldopa
	Class IA antiarrhythmic drugs
	Class IC antiarrhythmic drugs
	Class III antiarrhythmic drugs
	General anesthetics
	Tricyclic antidepressants
	Phenothiazines
	Lithium
	Phenytoin
	Cholinesterase inhibitors
Hypothermia
Hypothyroidism
Neurologic	Elevated intracranial pressure
	Spinal cord injury
Electrolyte imbalances	Hypokalemia
	Hyperkalemia
	Hypercalcemia
Pulmonary	Obstructive sleep apnea
	Prolonged hypoxia

It should be emphasized that when the vagal tone is high, particularly during sleep, episodes of sinus bradycardia are common in healthy individuals. Thus, in inpatients or outpatients with cardiac monitors, pauses and periods of slow heart rate are frequently seen and can be of uncertain clinical significance. Similarly, symptoms of lightheadedness, syncope, and activity intolerance are not specific and can occur for a number of reasons. Thus, the decision to implant a pacemaker in a patient with evidence of sinus bradycardia and suggestive symptoms but without a clear causal link can be a difficult one. In such situations, we favor a thorough workup for other cardiac and noncardiac causes of the patient’s symptoms. Particularly in older adults, we have found that polypharmacy is an often-overlooked cause for lightheadedness and syncope and should be ruled out before pacemaker implantation is considered. When no other cause can be identified, pacemaker implantation is a class IIa indication. Such patients should be advised that when a causal link has not been established between bradycardia and their symptoms, a pacemaker might not provide any benefit.

In patients with suspected sinus node dysfunction, particularly in those manifesting some degree of activity intolerance, exercise testing can be a very helpful adjunctive assessment of sinus node function. Chronotropic incompetence is often defined as failure to achieve a heart rate of 80% of age-adjusted maximum predicted heart rate with exercise.⁴ However, the critical issue in such cases is whether heart rate is sufficient to meet physiologic demands. The clinician should, therefore, not place too much importance on the specific rate achieved but should determine whether sinus node function is the main factor limiting the patient’s ability to exercise. Where this is the case, pacemaker implantation is warranted; this is a class I indication, even when heart rate is normal at rest and sinus pauses are not identified on monitoring.

The usefulness of invasive electrophysiology testing in determining the need to implant a pacemaker is limited, primarily because abnormalities in electrophysiologic parameters are not, in general, predictive of clinical events. Nevertheless, for patients with syncope who are undergoing electrophysiology testing and in whom other causes have not been identified, profound abnormalities of sinus node function may tip the balance of considerations in favor of pacemaker implantation; this is a class IIa indication.

Finally, a clinical situation frequently encountered is evidence of sinus node dysfunction manifested by sinus pauses or resting bradycardia in the absence of symptoms. At present, data suggesting that implantation of a pacemaker in such patients prevents adverse events or prolongs life are not available. Pacemaker implantation in the asymptomatic patient with sinus node dysfunction is therefore not recommended (class III). However, when sinus node dysfunction is marked (resting heart rate persistently less than 40 beats/min while awake), the ACC/AHA/HRS guidelines do recognize a class IIb indication for pacing in those who are at least minimally symptomatic. Often, a simple exercise test is sufficient in these patients to establish whether clinically significant sinus node dysfunction is present. However, in patients who cannot exercise because of comorbidities, pacemaker implantation can be considered.

Disorders of Atrioventricular Conduction

The function of the atrioventricular (AV) node is to delay impulse propagation from atria to ventricles to the extent necessary to optimize ventricular filling. This delay varies physiologically over a large range and is heavily influenced by autonomic tone and heart rate. The function of the bundle of His, which passes from the AV node to the ventricle through the central fibrous body, is to transmit the electrical impulse rapidly from the atrium to the ventricle. Conduction time through the bundle of His is rapid and does not vary with heart rate under normal conditions. Conduction slowing or block can occur in the AV node, the bundle of His, or both, and characteristic electrocardiogram (ECG) patterns are present, as well as differences in prognosis, depending on the location and severity of the block. In general, when the block is above the level of the His bundle (supra-His), prognosis for progression to a high-grade block is better than when the block is at or below the level of the His bundle (intra-His or infra-His).⁵ Symptoms caused by an AV block can include fatigue, lightheadedness, and syncope; a persistent high-grade AV block can result in heart failure caused by bradycardia and loss of AV synchrony and can predispose to ventricular arrhythmias.

First-Degree Atrioventricular Block

First-Degree AV block is defined as P-R interval exceeding 0.2 seconds and is often reflective of incipient AV nodal disease, though it can be a normal finding in patients at rest with high vagal tone. Recent data from the Framingham cohort suggest that in the long term, patients with first-degree AV block have a significantly higher risk of requiring pacemaker implantation and have a higher mortality rate.⁶ However, with a few exceptions (discussed below), the short-term prognosis for those with isolated first-degree AV block is excellent, so pacemaker implantation is, in general, not indicated in asymptomatic patients as a prophylactic measure (class III). However, a few indications for pacing are recognized for first-degree AV block (summarized in Appendix Table 33-4).

Some patients with marked first-degree AV block (P-R interval >0.3 seconds) have symptoms that may warrant pacemaker implantation.⁷ In these patients, ventricular activation is so delayed that the subsequent atrial activation occurs shortly after the QRS complex on the surface ECG, resulting in atrial contraction against closed AV valves (Figure 33-1). Patients with severe left ventricular dysfunction and markedly prolonged P-R intervals may experience hemodynamic compromise caused by impaired ventricular filling and diastolic AV valvular regurgitation.⁸ Often, reducing the dosages of AV nodal blocking medications in these patients can shorten the P-R interval and improve symptoms. Where this is not possible, restoration of AV synchrony with dual-chamber pacing and a normal AV delay can solve the problem, and this is a class IIa indication. However, ventricular pacing under these circumstances carries with it the potential deleterious effects of intraventricular dyssynchrony, so consideration should be given to biventricular pacing in such instances, especially when left ventricular function is not normal (see below).

FIGURE 33-1 Twelve-lead electrocardiogram showing sinus rhythm with a prolonged P-R interval of 400 ms. This can lead to symptoms caused by atrioventricular dyssynchrony that improve after implantation of a dual-chamber pacemaker.

Finally, patients with several neuromuscular diseases (see Appendix Table 33-5) often have rapidly progressive AV conduction system disease. Even in asymptomatic patients with these disorders, marked first-degree AV block should prompt consideration of pacemaker implantation as a prophylactic measure.

Type I Second-Degree Atrioventricular Block

Second-degree AV block is divided into type I and type II on the basis of the ECG pattern. In type I, the P-R interval lengthens before the nonconducted beat and shortens on the first beat after the nonconducted beat (Wenckebach conduction); in type II, the P-R interval is fixed. In general, type I second-degree AV block suggests a supra-His level of block and is associated with a more benign prognosis, while type II occurs with intra-His or infra-His conduction system disease and has a higher rate of progression to complete heart block.⁵ It should be remembered that in pathologic conditions, it is possible for any component of the conduction system, even the structures below the AV node, to exhibit Wenckebach conduction. Thus, type I second-degree AV block has a benign prognosis only when it is caused by supra-His block, which is usually the case when the QRS is narrow.

Regardless of the level of block, patients with symptomatic bradycardia resulting from second-degree AV block from irreversible causes or essential drug therapy should undergo permanent pacemaker implantation (class I). In the setting of high vagal tone such as sleep or in highly conditioned athletes at rest, type I second-degree AV block is frequently observed and is a normal finding.⁹ For asymptomatic patients, pacemaker implantation is therefore not recommended (class III). However, as with first-degree AV block, patients with type I second-degree AV block can have symptoms caused by loss of AV synchrony, and pacemaker implantation should be considered for this indication (class IIa) as well as for symptomatic bradycardia. Indications for pacing in type I second-degree AV block are summarized in Appendix Table 33-4.

High-Grade Atrioventricular Block

Patients with type II second-degree AV block (Figure 33-2) have a higher rate of progression to third-degree AV block, particularly in the setting of intraventricular conduction abnormalities. From the point of view of cardiac pacing, advanced second-degree AV block and third-degree AV block should be grouped together in a single high-risk category, and pacemaker implantation is the only reasonable treatment when reversible causes cannot be identified (see Appendix Table 33-6). Randomized clinical trials comparing pacing with conservative management are not available, but considerable nonrandomized evidence exists that pacing for patients with high-grade AV block results in a prolongation of life and reduction in morbidity.¹⁰