Bradyarrhythmias and Conduction Disturbances

Intrinsic conduction system disease and acute disorders affecting the conduction system (eg, MI, digitalis toxicity) are more common among the elderly than among younger persons. Therefore, most bradyarrhythmias and conduction disturbances (see Table 91-3) are common among the elderly. However, sinus bradycardia with < 40 beats/minute, sinus pauses of > 1.6 seconds, and high-degree AV block are rare among healthy persons > 60 and are often associated with ischemic, hypertensive, or amyloid heart disease.

Pathogenesis

Widespread age-related histologic changes in the conduction system may contribute to the striking age-related increase in the incidence of bradyarrhythmias and conduction disturbances. The number of pacemaker cells in the sinoatrial node begins to decline progressively by age 60; only about 10% of the cells are still present at age 75. The sinoatrial node becomes enveloped by fat, which may partially or completely separate the node from the atrial musculature.

Age-related changes in the His bundle include loss of cells, an increase in fibrous and adipose tissue, and amyloid infiltration. Some degree of idiopathic fibrosis may affect the left side of the cardiac skeleton (central fibrous body, mitral and aortic annuli, and proximal interventricular septum). The fibrosis may also affect the AV node, His bundle, and proximal left and right bundle branches because of their proximity. If extensive, the fibrosis may cause AV block. This fibrosis is the most common cause of chronic AV block in the elderly.

Some age-related histologic changes in the conduction system are apparent on the standard 12-lead ECG. Although resting heart rate does not change with age, respiratory variation in resting sinus rate (known as sinus arrhythmia) decreases. Heart rate variability is reduced, and PR and QT intervals are somewhat prolonged; however, QRS duration is unchanged. The increase in PR interval is due to a delay that is proximal to the His bundle; conduction time from the His bundle to the ventricle appears to be unrelated to age.

The QRS frontal plane axis shifts left with age, probably reflecting the combined effects of fibrosis in the anterior fascicle of the left bundle branch and a mild age-related increase in left ventricular wall thickness. This left axis deviation is the most common ECG abnormality among the elderly, occurring in about half. For elderly patients without organic heart disease, neither first-degree AV block nor a left axis deviation of 130° is associated with increased cardiac morbidity or mortality rates.

Symptoms, Signs, and Diagnosis

Bradycardias may produce no symptoms, mild fatigue or faintness, or significant hemodynamic upset (eg, hypotension, angina, heart failure). The diagnosis may be suspected on the basis of symptoms and signs but is confirmed by ECG.

Treatment

Patients with a bradyarrhythmia must be treated immediately if they also have hypotension, cerebral or cardiac ischemia, heart failure, and/or, in acute MI, frequent ventricular ectopic beats. Placing the patient in the supine position with the legs elevated often ameliorates hypotensive sequelae. Atropine 0.5 to 1 mg is given rapidly IV; it may be repeated at 3- to 5-minute intervals up to a total dose of 0.04 mg/kg (total, 2 to 3 mg). If atropine is ineffective or causes intolerable adverse effects, an isoproterenol infusion can be started at 1 to 4 µg/minute, then adjusted to produce a ventricular rate of 60 beats/minute. If neither drug is successful or if isoproterenol is contraindicated because of ischemia or infarction, temporary transvenous pacing should be used.

Cardiac pacemakers: More than 85% of patients who receive a pacemaker are > 64; about half of these patients are treated for high-degree AV block and half for sick sinus syndrome. Use of a permanent ventricular pacemaker has reduced the high mortality rate formerly associated with complete heart block in patients aged 65 to 79 who do not have structural heart disease; the survival rate may be lower in patients > 80 who have a pacemaker than in patients of the same age who do not.

Pacemakers that improve maximal exercise cardiac output and work capacity (eg, dual-chamber AV synchronous pacemakers as opposed to traditional fixed-rate ventricular pacemakers) may especially benefit active elderly patients. The elderly's greater dependence on the atrial contribution to ventricular filling suggests that AV synchronous pacemakers should be particularly beneficial for them. In addition, the incidence of atrial fibrillation and the mortality rate are higher for patients who have ventricular pacemakers than for patients who have atrial or dual-chamber AV synchronous pacemakers. Atrial fibrillation has been reported in 47% of patients > 70 with sick sinus syndrome who received a ventricular pacemaker and in 9% of those who received a dual-chamber pacemaker after 7 years; the mortality rates were 72% and 51%, respectively. Thus, otherwise healthy elderly patients who are in sinus rhythm and require a pacemaker for sick sinus syndrome should probably receive an AV synchronous pacemaker.

Complications with permanent pacemakers are rare but significant. Abrupt loss of pacing (due to battery failure, fibrosis around the catheter site, myocardial perforation, lead fracture, or electrode dislodgment) may result in marked bradycardia or asystole. The catheter may perforate the right ventricle, causing a pericardial friction rub or, rarely, tamponade. In patients with little overlying subcutaneous tissue, the pulse generator may extrude or the pacing wire may erode through the skin. Some patients have difficulty adjusting psychologically to pacemaker implantation. All patients with a pacemaker should have regular follow-up physical and ECG examinations.

Sinus Bradycardia

A sinus rate of < 60 beats/minute.

Sinus bradycardia may indicate excellent physical conditioning; however, in the elderly, it often indicates intrinsic sinus node disease. Inferior MI, hypothermia, myxedema, or increased intracranial pressure may cause this arrhythmia. For apparently healthy persons aged 40 to 80 with sinus rates < 50 beats/minute, 5-year cardiovascular morbidity and mortality rates do not appear to be increased. Treatment is not needed unless symptoms of cerebral hypoperfusion are present. If such symptoms are chronic, pacemaker implantation is the definitive treatment.

Sinoatrial Block

Inability of sinus node impulses to depolarize the atria.

Sinoatrial block is often 2:1, resulting in atrial and ventricular rates that are exactly one half the sinus rate. In the elderly, common causes are intrinsic conduction system disease, ischemia, digitalis toxicity, and toxicity from class Ia antiarrhythmics. The diagnosis is made when an ECG shows a pause in the sinus rhythm equal to a multiple of the underlying PR interval. Treatment is similar to that for sinus bradycardia.

Atrioventricular Block

Partial or complete interruption of electrical conduction from the atria or sinus node to the AV node and ventricles.

First-degree atrioventricular block: The PR interval is prolonged (>= 0.22 seconds). First-degree block may occur in healthy persons with high vagal tone, or it may be caused by intrinsic conduction system disease or by various drugs (eg, digoxin, -blockers, calcium channel blockers, class Ia antiarrhythmic drugs). Because cardiac morbidity and mortality rates are not increased in patients with this disorder, no treatment is required.

Second-degree atrioventricular block: There are three patterns. In Mobitz type I (Wenckebach) block, the PR interval is progressively prolonged until a ventricular complex is dropped. Because this block is usually proximal to the His-Purkinje system, the QRS complex typically appears normal. Digitalis toxicity and acute inferior MI are common precipitating factors. Mobitz type I block is usually transient and rarely requires specific treatment.

In Mobitz type II block, the PR interval is fixed, but QRS complexes are dropped. Because the block occurs at or below the His bundle, the QRS complex is often wide. Mobitz type II block is most often associated with acute anterior MI, myocarditis, or advanced sclerodegenerative conduction system disease. Patients with this block are usually symptomatic and often present with syncope due to inadequate cerebral perfusion (Stokes-Adams attack). Because of the symptomatic presentation and frequent progression to complete heart block, patients usually receive a permanent pacemaker.

In high-grade block, there is a mathematical relationship between the P waves and QRS complexes (eg, 2:1, 3:1). High-grade block may progress to complete heart block. Patients are often symptomatic and usually receive a permanent pacemaker.

Third-degree (complete) atrioventricular block: Atrial depolarizations cannot activate the ventricle. Block within the AV node is usually associated with normal QRS complexes and an escape rate of almost 60 beats/minute. Common causes are acute inferior MI and digitalis toxicity. This block is usually transient, and no specific treatment is required. If digitalis toxicity is the cause, the drug is withdrawn. Pacemakers may be used when hypotension or symptoms of cerebral hypoperfusion are present.

Block within the ventricles is accompanied by wide QRS complexes and a slow escape rate, often < 40 beats/minute. Such block may occur in patients with severe sclerodegenerative conduction system disease or extensive acute anterior MI. Because these patients usually respond poorly to atropine and isoproterenol, pacemaker implantation is necessary.

Sick Sinus Syndrome

A variety of rhythm disturbances that reflect sinoatrial node dysfunction and are often associated with dysfunction elsewhere in the conduction system.

CAD and a primary sclerodegenerative process are the most common precipitating disorders, although many heart diseases may precipitate the syndrome. Patients may present with bradyarrhythmias (sinus bradycardia, sinus pauses or arrest, sinoatrial exit block, or atrial fibrillation with a slow ventricular response) or with the bradycardia-tachycardia syndrome, in which a supraventricular tachycardia terminates in a long period of asystole. Therefore, symptoms may consist of palpitations or chest pain during tachycardia and dizziness or syncope during bradycardia.

The long-term prognosis for patients with sick sinus syndrome depends primarily on the presence and severity of the underlying heart disease.

For tachycardia (paroxysmal supraventricular tachycardia, atrial flutter, or atrial fibrillation), digoxin, other antiarrhythmic drugs, or cardioversion is used. For bradycardia associated with syncope, a permanent pacemaker is needed.

Bundle Branch Block

Partial or complete interruption of electrical conduction in the bundle branches.

Left bundle branch block is usually associated with ischemic or hypertensive heart disease in elderly men and women. In contrast, complete right bundle branch block often occurs in apparently healthy elderly men and appears to be benign, although it is highly predictive of underlying heart disease in elderly women.

Pacemakers are not warranted for asymptomatic elderly patients with chronic bifascicular block (either left or right bundle branch block plus left anterior-superior or left posterior-inferior division block), with or without a prolonged PR interval, because complete heart block rarely occurs (see Table 91-3).