Cardiology

Background 

Sick sinus syndrome (SSS), also known as sinus node dysfunction, is a collection of arrhythmias arising from impaired impulse generation and/or conduction from the sinoatrial (SA) node. It encompasses persistent sinus bradycardia, sinus pauses or arrest, sinoatrial (SA) exit block, and tachycardia-bradycardia (tachy-brady) syndrome. The hallmark of SSS is an inability of the SA node to maintain appropriate heart rate responses, particularly during activity, often resulting in symptoms such as dizziness, fatigue, or syncope. 

II) Classification/Types 

By Rhythm Abnormality: 

• Sinus bradycardia: Persistent slow rate due to SA node dysfunction 

• Sinus arrest: Intermittent failure of impulse initiation 

• SA exit block: Impulses generated but fail to exit the SA node 

• Tachy-brady syndrome: Alternation between bradyarrhythmias and supraventricular tachycardias (e.g., atrial fibrillation) 

By Chronicity: 

• Intermittent SSS: Paroxysmal symptoms, often captured on Holter/Event monitor 

• Persistent SSS: Continuous bradycardia or conduction failure evident on ECG 

Pathophysiology 

SSS results from intrinsic disease of the SA node or surrounding atrial tissue, most commonly age-related fibrosis, ischemia, or infiltration. Dysfunctional impulse formation or conduction causes bradycardia or pauses. Compensatory atrial tachyarrhythmias may occur (tachy-brady syndrome), with prolonged pauses after tachycardia termination due to suppressed automaticity. 

Epidemiology 

• Primarily affects older adults (>60 years) 

• Leading cause of pacemaker implantation in the developed world 

• Prevalence increases with structural heart disease, especially atrial remodeling 

• May be exacerbated by medications like beta-blockers or digoxin 

Etiology 

I) Causes 

• Idiopathic age-related fibrosis of SA node 

• Ischemic heart disease, especially involving the right coronary artery 

• Surgical trauma (e.g., post-CABG or valve surgery) 

• Infiltrative diseases (e.g., amyloidosis, sarcoidosis) 

• Inflammatory diseases (e.g., myocarditis, rheumatic fever) 

• Medications: beta-blockers, digoxin, calcium channel blockers, amiodarone 

• Hypothyroidism 

• Sleep apnea 

• Autonomic dysfunction (e.g., diabetic neuropathy) 

II) Risk Factors 

• Age >65 years 

• Structural heart disease or cardiomyopathy 

• History of atrial fibrillation or flutter 

• Use of AV nodal blocking agents 

• Post-cardiac surgery or ablation procedures 

• Systemic diseases (e.g., hypothyroidism, amyloidosis) 

Clinical Presentation 

I) History (Symptoms) 

• Fatigue, lethargy 

• Dizziness or lightheadedness 

• Syncope or near-syncope 

• Palpitations (particularly in tachy-brady syndrome) 

• Dyspnea or worsening heart failure symptoms 

• Intermittent chest discomfort 

II) Physical Exam (Signs) 

• Bradycardia at rest 

• Irregular rhythm (especially with tachy-brady syndrome) 

• Variable intensity of heart sounds 

• Signs of low cardiac output (cool extremities, altered mentation) 

• Often normal unless advanced disease or coexistent heart failure 

Differential Diagnosis (DDx) 

• Physiologic sinus bradycardia (e.g., athlete’s heart) 

• AV block (first-degree, Mobitz I or II, complete heart block) 

• Junctional escape rhythm 

• Hypothyroidism 

• Neurocardiogenic syncope 

• Carotid sinus hypersensitivity 

• Atrial fibrillation with slow ventricular response 

Diagnostic Tests 

Initial Work-Up 

• 12-lead ECG: May show sinus bradycardia, SA pauses/arrest, tachy-brady pattern 

• Holter monitor or Event recorder: Essential to document intermittent arrhythmias 

• Exercise stress testing: Evaluate chronotropic incompetence 

• TSH, electrolytes, glucose: Rule out reversible causes 

• Echocardiography: Assess structural heart disease 

• Sleep study: If obstructive sleep apnea is suspected 

Advanced Testing 

• Electrophysiology study (EPS): Rarely needed; used in unclear or complex cases 

Treatment 

I) Acute Management 

• Symptomatic bradycardia or pauses: 

• Atropine 0.5 mg IV every 3–5 min (max 3 mg) 

• Dopamine or epinephrine infusion if unresponsive 

• Temporary transcutaneous or transvenous pacing 

II) Chronic Management 

• Eliminate reversible causes (e.g., adjust AV nodal blockers, correct hypothyroidism) 

• Permanent pacemaker: 

• Symptomatic bradycardia not reversible 

• Tachy-brady syndrome with prolonged pauses 

• Chronotropic incompetence with activity limitation 

Medications 

Device Therapy 

• Permanent Pacemaker (PPM): Cornerstone of therapy for symptomatic SSS 

• Dual-chamber pacing: Preferred in tachy-brady syndrome to maintain AV synchrony 

• ICD: Not indicated unless concurrent ventricular arrhythmias are present 

Patient Education, Screening, Vaccines 

Education 

• Recognize symptoms (dizziness, palpitations, syncope) 

• Adhere to follow-up and device checks 

• Avoid unnecessary AV nodal blockers 

• Keep a symptom log to correlate with device interrogation 

Screening/Prevention 

• Periodic ECG in elderly and high-risk patients 

• Review medications regularly, especially in polypharmacy 

• Screen for sleep apnea if unexplained bradycardia or fatigue 

Vaccinations 

• Follow age-appropriate immunization schedules 

• No vaccine specifically indicated for SSS 

Consults/Referrals 

• Cardiology: For initial evaluation and pacemaker placement 

• Electrophysiology: For complex rhythm assessment or device programming 

• Endocrinology: If thyroid or adrenal causes are suspected 

• Sleep Medicine: For suspected sleep apnea work-up 

Follow-Up 

Short-Term 

• Monitor for symptom resolution after interventions 

• Repeat ECG or Holter monitoring post-medication adjustment 

• Temporary pacemaker removal after stabilization 

Long-Term 

• Pacemaker checks every 3–6 months 

• Reassessment of symptoms and progression of conduction disease 

• Repeat echocardiography if structural abnormalities evolve 

Prognosis 

• Excellent with appropriate pacing in symptomatic patients 

• Risk of syncope or sudden death minimized by pacemaker 

• Natural history may involve progression to more severe conduction disease 
  
• Prognosis influenced by underlying cardiac comorbidities