What are the causes of bradycardia in older patients?

Causes of Bradycardia in Older Patients

In older patients, age-dependent degenerative fibrosis of the sinus node and specialized conduction system is the predominant intrinsic cause of bradycardia, typically manifesting in the seventh and eighth decades of life. 1

Primary Age-Related Degenerative Causes

Sinus node dysfunction (SND) is the most common bradyarrhythmia in elderly patients, caused by progressive fibrotic degeneration of sinus nodal tissue and surrounding atrial myocardium. 1 This degenerative process:

• Affects impulse formation and propagation, resulting in sinus bradycardia, sinus pauses, chronotropic incompetence, and ectopic atrial or junctional escape rhythms 1
• Frequently coexists with atrial arrhythmias, creating "tachy-brady syndrome" where paroxysmal atrial fibrillation alternates with bradycardia 1
• Progresses slowly to involve the atrioventricular conduction system, with complete heart block developing at an annual incidence of 0.6% (range 0-4.5%) 1

Atrioventricular conduction disease represents the second major degenerative pattern, accounting for 75% of pacemaker implantations. 2 The fibrotic process affects:

• The AV node, His bundle, and bundle branches, producing varying degrees of heart block 1
• Progressive conduction delays that are part of normal aging but become pathologic when symptomatic 1

Reversible and Secondary Causes Requiring Immediate Evaluation

Medications are the most common reversible cause and must be identified before considering permanent pacing. 3 Critical offenders include:

• Beta-blockers, calcium channel blockers (verapamil, diltiazem), and digoxin 3, 4
• Antiarrhythmic drugs (amiodarone, sotalol, flecainide) and ivabradine 3
• Digoxin specifically causes bradycardia and heart block at therapeutic doses in patients with pre-existing conduction disorders 4

Metabolic and endocrine disorders are immediately reversible causes that must be excluded: 3

• Hypothyroidism (check thyroid function tests in all patients with new bradycardia) 1
• Electrolyte abnormalities (hyperkalemia, hypercalcemia, hypomagnesemia) 3
• Hypoxemia and hypothermia 3

Acute cardiac events account for 14% of emergency bradycardia presentations: 3

• Inferior myocardial infarction (affects AV nodal blood supply, often transient) 1, 5
• Anterior myocardial infarction (affects His-Purkinje system, worse prognosis, often requires permanent pacing) 5
• Acute myocarditis 3

Infectious and Infiltrative Causes

Lyme disease (Borrelia burgdorferi) causes AV block in 0.3-8% of infected patients in endemic areas, though persistent heart block is rare and usually self-limiting with antibiotics. 5, 6 Consider in younger patients with new AV block in endemic regions. 1

Infective endocarditis with perivalvular abscess, particularly involving the aortic valve, can erode into the septum causing complete heart block. 5, 6 New-onset heart block in endocarditis is highly specific (95% positive predictive value) for abscess formation and warrants urgent transesophageal echocardiography and surgical evaluation. 6

Infiltrative diseases affecting the conduction system include: 1

• Cardiac sarcoidosis (diagnose with fluorodeoxyglucose PET imaging) 1
• Cardiac amyloidosis (wild-type transthyretin common in elderly, diagnose with nuclear imaging) 1
• Cardiac tumors 1

Chagas disease (Trypanosoma cruzi) commonly causes progressive conduction defects in endemic areas of Central and South America, with prevalence up to 30% in some regions. 6

Structural Heart Disease

Ischemic heart disease beyond acute MI includes chronic coronary disease with fibrosis affecting the conduction system. 3

Valvular heart disease, particularly calcific aortic stenosis and mitral annular calcification, can extend into the conduction system. 3 Transcatheter aortic valve replacement carries significant risk of new conduction abnormalities. 1

Cardiomyopathies of various types (dilated, hypertrophic, restrictive) produce progressive conduction disease through myocardial fibrosis and disarray. 3

Critical Clinical Pitfalls to Avoid

Do not assume isolated sinus bradycardia or a single pause >3 seconds alone indicates sinus node dysfunction—multiple factors including resting heart rate, exercise response, and symptom correlation must be assessed. 1

Do not rush to permanent pacing without excluding reversible causes—in emergency settings, 45% of compromising bradycardia cases have reversible etiologies including drugs, MI, intoxication, and electrolyte abnormalities. 3

Do not assume bifascicular block will rapidly progress to complete heart block—progression is slow with only 0.6% annual incidence, and prophylactic pacing in asymptomatic patients shows no benefit. 1, 6

Recognize that vagally-mediated AV block during sleep with concomitant sinus slowing (P-P prolongation) may be physiologic and asymptomatic, not requiring intervention. 1

In patients with bradycardia on digoxin, check serum digoxin levels and potassium—therapeutic doses cause heart block in patients with pre-existing conduction disorders, and toxicity manifests as progressive bradyarrhythmias. 4