24. Heart block and sick sinus syndrome
- Introduction. A number of conditions produce symptoms related to sudden or prolonged changes in heart rate with resultant fall in cardiac output. Some patients present with marked bradycardia; others have intermittent episodes of tachycardia followed by bradycardia. Degenerative changes or congenital defects in the conduction system or sinoatrial node are present in the vast majority of these patients who require permanent pacing. Traumatic, surgical, or inflammatory injury to the conduction system also can produce a situation demanding permanent pacing (Table 24-1). Most people who require permanent pacing have one of two conditions: heart block (HB) or sick sinus syndrome (SSS). Individuals with HB may have continuous or episodic periods of markedly slow heart rate because the electrical impulse initiated in the sinoatrial node fails to reach the ventricles because of conduction system malfunction. Consequently, the heart rate is regulated by a slow (20 to 40 beats per minute) junctional or ventricular pacemaker. Patients with SSS have dysfunction of the sinoatrial node, with either marked sinus bradycardia or alternating episodes of supraventricular tachycardia and bradycardia.
- History. Patients with HB or SSS may be asymptomatic, particularly if the resultant bradycardia is not marked. Symptomatic patients complain of episodic dizziness, giddiness, or frank syncope. Often, lightheadedness or syncopal episodes vary in length and severity because of the variable duration of episodes of bradycardia or tachycardia. Some individuals are aware of palpitations, but the majority of arrhythmic episodes are unnoticed. Individuals with HB or SSS (or both) and another form of heart disease that results in limited cardiac reserve may develop new or increased symptoms of left or right ventricular failure during episodes of bradycardia. Similarly, patients with coexisting coronary artery disease may note new or increased angina pectoris during bradycardia or tachycardia. Other symptoms that are noted occasionally include personality changes, irritability, fleeting memory losses, generalized weakness or fatigue, muscle aches, slurred speech, and insomnia.
- Physical examination. Unless HB or SSS has caused new or increased heart failure, the physical examination may be unremarkable. Because the majority of patients with HB and SSS are elderly, coexisting hypertension and coronary artery disease are common. Physical findings secondary to the two latter conditions (see Chapters 12 and 15) may be noted (e.g., a prominent fourth heart sound). Patients with permanent or prolonged episodes of HB are markedly bradycardic with a wide pulse pressure. An aortic systolic flow murmur is often heard during bradycardia. It is caused by increased stroke volume secondary to an increase in left ventricular diastolic volume, which in turn is the result of bradycardia-induced prolongation of the diastolic filling period. Large A waves (cannon waves) may be seen in the jugular pulse of patients with complete HB. They are caused by atrial contraction against a closed tricuspid valve.
An occasional patient has physical findings that result from a pacemaker. Usually such findings are limited to single or multiple systolic clicks or a high-pitched, short systolic murmur; however, perforation of the ventricle by a transvenous pacing wire can result in a pericardial rub.
- ECG. The scalar ECG or a Holter monitor ECG tape usually confirms the diagnosis of HB or SSS. Patients with persistent HB ordinarily have normal-appearing P waves with normal atrial rate and rhythm and some form of bundle branch block pattern of the QRS with a very slow rate. Supraventricular capture beats with normal QRS pattern can occur alongside the bizarre, widened complexes of the slower ventricular response. Episodes of HB may be sporadic and may be observed only during prolonged periods of ambulatory ECG monitoring (Holter monitoring). Individuals with congenital forms of HB often have QRS complexes that appear relatively normal. These patients may demonstrate prolonged periods of first- or second-degree HB with or without occasional episodes of third-degree HB. ECG findings in patients with HB and SSS are recorded in Table 24-2. Holter monitoring for 12 or 24 hours in patients with HB demonstrates persistent or paroxysmal HB (first, second, or third degree). Holter monitoring in patients with SSS may reveal a variety of abnormalities: (i) persistent unexplained sinus bradycardia, (ii) paroxysms of supraventricular tachycardia (atrial fibrillation, flutter, or tachycardia followed by long sinus pauses), (iii) episodic sinus arrest, (iv) sinoatrial exit block, or (v) marked sinus bradycardia alternating with atrial tachycardia, atrial flutter, or atrial fibrillation (the so-called bradycardia-tachycardia, or brady-tachy, syndrome).
- Chest x-ray examination. The chest x-ray picture of patients with HB or SSS may be unremarkable, or it may demonstrate findings that are the result of associated heart disease. For example, a patient with HB secondary to calcific aortic stenosis would have a chest roentgenogram with features consistent with aortic stenosis (see Chapter 16).
- Echocardiography. Echocardiography in patients with HB or SSS either is normal or shows findings of associated heart disease.
- Catheterization and angiography. Electrical events in the conduction system can be examined by intracardiac electrophysiologic study (His bundle electrocardiography). Abnormal generation of the cardiac electrical impulse in the sinus node or interruption of transmission of the impulse to the ventricles can be documented by this technique. ECG manifestations of SSS may be intermittent and, at times, difficult to elicit. For this reason, several provocative tests have been developed to test the function of the sinus node: (i) response to atropine, (ii) response to isoproterenol, (iii) response to atrial pacing, and (iv) response to premature atrial stimulation. Invasive provocative tests, however, are not always abnormal in patients with SSS.
Similarly, marked abnormalities in intracardiac conduction may be documented in individuals with first-degree atrioventricular (AV) block and bifascicular conduction system disease (right bundle branch block with left anterior or posterior hemiblock; left bundle branch block) by His bundle ECG. Despite markedly abnormal function of the conduction system, however, the majority of such persons infrequently develop high-grade (second- or third-degree) AV block. Thus, the role of His bundle ECG in diagnosis and management of patients with SSS or first-degree HB is in confirming the clinical diagnosis rather than in disclosing an unexpected entity. Other findings at catheterization depend on associated heart disease (e.g., coronary artery disease).
- Protocol for the diagnosis of HB or SSS
- The diagnosis of HB and SSS usually is made by history and routine scalar ECG or Holter ECG monitoring. His bundle ECG and provo-cative tests may be helpful in making the diagnosis in an occasional patient. The following protocol is suggested (dashed arrow indicates optional step):
History and physical examination
ECG, ECG monitoring (12 to 24 hours), or both
His bundle ECG
- Criteria for making the diagnosis
- The diagnosis of HB is made in asymptomatic or symptomatic (dizzy, giddy, or syncopal) patients with first-, second-, or third-degree HB by scalar ECG or ECG monitoring. Patients with bifascicular block (right bundle branch block with left anterior or posterior hemiblock; left bundle branch block) and abnormal His bundle ECG measurements (markedly prolonged H–V interval) who have spontaneous syncopal episodes may be assumed to have transient high-grade AV block, even if such an episode does not occur during ECG monitoring. In a rare patient with unexplained syncope, occult paroxysms of AV block may be found by repeated episodes of ECG monitoring, a home event ECG recorder, or means of an implantable ECG recorder.
- The diagnosis of SSS is made in asymptomatic or symptomatic (dizzy, giddy, or syncopal) patients who demonstrate at least one of the following rhythm disturbances by ECG or ECG monitoring: (i) persistent unexplained sinus bradycardia; (ii) inability of the sinus node to resume firing after cessation of paroxysmal tachycardia, atrial pacing, or electrical cardioversion; (iii) episodic sinus arrest; (iv) sino-atrial exit block; (v) marked sinus bradycardia alternating with atrial tachycardia, atrial flutter, or atrial fibrillation with rapid ventricular response (bradycardia-tachycardia syndrome); and (vi) atrial fibrillation with a slow ventricular response in the absence of digitalis, beta-blockers, diltiazem, or verapamil. His bundle ECG or provocative tests (atropine, isoproterenol, rapid atrial pacing, premature atrial stimulation) may be required to confirm the diagnosis.
- Differential diagnosis. HB and SSS can both produce giddiness, dizziness, and frank syncope. Differentiation of the two conditions usually can be made by routine scalar ECG or Holter monitoring. Occasionally, His bundle ECG is required to distinguish these entities. HB and SSS may coexist in the same patient. The majority of differential diagnostic problems concern the identification of associated forms of heart disease or other causes of syncope or loss of consciousness (Table 24-1 and below).
Patients with a hereditary form of heart disease characterized by a prolonged Q–T interval by ECG and recurrent syncopal episodes initially may be thought to have HB or SSS. ECG monitoring reveals paroxysms of ventricular tachycardia or ventricular fibrillation. The ECG Q–T interval is markedly prolonged. Congenital deafness also may occur in patients with the prolonged Q–T interval syndrome.
- Neurologic conditions. The most common symptoms associated with HB and SSS also are noted frequently in patients with a variety of neurologic conditions (e.g., seizure disorders, labyrinthitis, cerebrovascular disease). Thus, patients who complain of dizziness or frank syncope should always have a careful neurologic evaluation to exclude central nervous system disease as the cause of their symptoms. Such an evaluation includes a careful history and neurologic examination as well as selected laboratory tests, which may include an EEG, CT scan, MRI, and/or a lumbar puncture, among others.
- Medical treatment
- Many patients with HB or SSS require no therapy because they are asymptomatic and serious bradycardia is not impending. Included in this group are (i) individuals with transient first-, second-, and even third-degree HB complicating inferior myocardial infarction; (ii) patients with asymptomatic, nonfamilial, congenital HB with narrow QRS complexes by ECG and a normal or near normal resting heart rate; (iii) asymptomatic individuals with bifascicular block with or without first-degree AV block; and (iv) asymptomatic patients with SSS.
- Patients with SSS may have an increased incidence of left atrial thrombosis with subsequent arterial embolism. If paroxysms of atrial fibrillation or flutter are noted, these patients should receive long-term oral warfarin anticoagulation.
- Some patients with SSS and marked, symptomatic bradycardia will respond to oral theophylline therapy given as 200 to 300 mg twice a day in a slow-release formulation.
- Surgery. Patients with third-degree HB or symptomatic SSS should have a right ventricular or AV demand pacemaker placed. Patients with SSS may require concomitant antiarrhythmic medication (digitalis, procainamide, beta-blockers, verapamil, propafenone, sotalol, amiodarone, etc.) to control supraventricular tachyarrhythmias. Permanent or temporary pacing should be initiated before any of these agents (except digitalis) are administered to patients with SSS to prevent the occurrence of severe bradycardia. There is increasing data favoring the implantation of an AV sequential pacemaker in these patients unless they are chronically in atrial fibrillation.
- Patients with bifascicular block and second-degree HB should have a permanent pacemaker implanted.
- Patients with congenital HB and narrow QRS complexes do not require a pacemaker despite the presence of second-degree AV block, as long as they are not symptomatic and have reasonable heart rates. Frequent ECG monitoring, exercise testing, or both (every 6 months) should be performed in such individuals to search for paroxysms of complete HB. The presence of complete HB or serious ventricular escape rhythms should induce the physician to treat these patients with permanent pacing.
- Patients with the syndrome of prolonged Q–T interval and recurrent ventricular fibrillation are treated with various combinations of implanted defibrillator, cervical sympathectomy, and/or beta-blockers.
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