What is the most likely underlying cause of bradycardia in an elderly patient with a history of Coronary Artery Disease (CAD), Hypertension (HTN), and hypothyroidism, presenting with intermittent palpitations, syncope, and lab results showing hypokalemia, elevated Brain Natriuretic Peptide (BNP), and telemetry findings of atrial fibrillation (afib) and bradycardia?

Sick Sinus Syndrome (Tachy-Brady Syndrome)

The most likely underlying cause of bradycardia in this patient is Sick Sinus Syndrome (Option D), specifically manifesting as tachy-brady syndrome given the telemetry findings of alternating atrial fibrillation and bradycardia. 1

Clinical Reasoning

This elderly patient presents with the classic triad that defines sick sinus syndrome:

• Alternating tachycardia and bradycardia: Telemetry shows intermittent atrial fibrillation (rates 80-88 bpm) alternating with bradycardia (rates 40-48 bpm), which is the hallmark of tachy-brady syndrome 1
• Syncope: A common manifestation present in 50% of patients who receive pacemakers for sinus node dysfunction 1
• Intermittent palpitations: Consistent with paroxysmal atrial arrhythmias that characterize this condition 1

Pathophysiology Supporting This Diagnosis

The underlying mechanism involves age-dependent degenerative fibrosis of the sinus node and surrounding atrial myocardium 1. This same fibrotic process is responsible for both the bradycardia episodes and the development of atrial arrhythmias, creating the characteristic alternating pattern 1. Notably, fibrosis of the sinus node is frequently associated with concurrent fibrosis in the atrioventricular node 1.

Why Other Options Are Less Likely

First-Degree AV Block (Option B)

• First-degree AV block causes a prolonged PR interval but does not cause bradycardia or alternating rhythms 1
• This patient's telemetry shows sinus bradycardia alternating with atrial fibrillation, not AV conduction delay 1

Heart Failure (Option C)

• While the elevated BNP (1170 pg/mL) suggests heart failure may be present, heart failure is a consequence rather than the cause of sick sinus syndrome 1
• Patients with symptomatic sinus node dysfunction have high risk of developing heart failure as a complication 1

Aortic Stenosis (Option A)

• Aortic stenosis typically causes syncope through outflow obstruction during exertion, not through bradyarrhythmias 1
• The telemetry pattern of alternating atrial fibrillation and bradycardia is not characteristic of aortic stenosis 1

Wolff-Parkinson-White Syndrome (Option E)

• WPW causes tachyarrhythmias through an accessory pathway, not bradycardia 1
• This patient's presentation of alternating bradycardia and atrial fibrillation is inconsistent with WPW 1

Critical Reversible Causes to Address

Before attributing bradycardia solely to intrinsic sick sinus syndrome, reversible causes must be evaluated and treated 1:

• Hypokalemia (2.8 mEq/L): This electrolyte abnormality can contribute to bradyarrhythmias and must be corrected 1
• Medications: Review all cardiac medications (beta-blockers, calcium channel blockers, digoxin, antiarrhythmics) that may be exacerbating the bradycardia 1, 2
• Thyroid function: Although TSH is normal (1.2 uU/mL), this appropriately excludes hypothyroidism as a reversible cause 1

Management Approach

The correlation between symptoms (syncope, palpitations) and documented bradycardia on telemetry establishes the diagnosis of symptomatic sick sinus syndrome 1. The natural history includes high risk of recurrent syncope, progression to chronic atrial fibrillation, and systemic embolism 1, 3.

Permanent pacemaker implantation is indicated for this patient given documented symptomatic bradycardia with syncope 1. Physiologic pacing (atrial or dual-chamber) is superior to ventricular pacing and helps prevent progression to chronic atrial fibrillation 1, 3.

Common Pitfall

The most critical error would be attributing the bradycardia solely to reversible causes (hypokalemia, medications) and missing the underlying sick sinus syndrome 2, 4. While reversible factors should be corrected, the alternating pattern of atrial fibrillation and bradycardia with syncope establishes intrinsic sinus node dysfunction requiring definitive treatment 1.