What are the different types of bradycardia?

Types of Bradycardia

Bradycardia is broadly classified into two main categories: Sinus Node Dysfunction (SND) and Atrioventricular (AV) Block, with several specific subtypes within each category. 1

Sinus Node Dysfunction (SND)

SND encompasses various abnormalities of impulse formation in the sinus node and includes:

• Sinus Bradycardia: Sinus rate <50 bpm
• Ectopic Atrial Bradycardia: Atrial depolarization from an atrial pacemaker other than the sinus node with a rate <50 bpm
• Sinoatrial Exit Block: Blocked conduction between the sinus node and adjacent atrial tissue, often manifesting as "group beating" of atrial depolarization and sinus pauses
• Sinus Pause: Sinus node depolarizes >3 seconds after the last atrial depolarization
• Sinus Node Arrest: No evidence of sinus node depolarization
• Tachycardia-Bradycardia ("Tachy-Brady") Syndrome: Alternating periods of bradycardia and tachycardia; the tachycardia may suppress sinus node automaticity causing a pause when it terminates
• Chronotropic Incompetence: Inability of the heart to increase rate appropriately with increased activity or demand, often defined as failure to attain 80% of expected heart rate reserve during exercise
• Isorhythmic Dissociation: Atrial depolarization is slower than ventricular depolarization 1

Atrioventricular (AV) Block

AV blocks involve impaired conduction between the atria and ventricles:

• First-Degree AV Block: P waves with 1:1 AV conduction but PR interval >200 ms (technically an AV delay rather than a block)
• Second-Degree AV Block:
  • Mobitz Type I (Wenckebach): Periodic single nonconducted P wave with progressively lengthening PR intervals
  • Mobitz Type II: Periodic single nonconducted P wave with constant PR intervals
  • 2:1 AV Block: Every other P wave conducts to the ventricles
• Advanced/High-Grade AV Block: ≥2 consecutive P waves that don't conduct to the ventricles, with some AV conduction present
• Third-Degree AV Block (Complete Heart Block): No evidence of AV conduction
• Vagally Mediated AV Block: AV block caused by heightened parasympathetic tone
• Infranodal Block: AV conduction block occurring distal to the AV node 1

Conduction Tissue Disease

These involve impaired conduction through the bundle branches:

• Right Bundle Branch Block (RBBB):
  • Complete RBBB: QRS ≥120 ms with characteristic morphology in leads V1/V2 and I/V6
  • Incomplete RBBB: Similar morphology but QRS 110-119 ms
• Left Bundle Branch Block (LBBB):
  • Complete LBBB: QRS ≥120 ms with characteristic morphology in leads I, aVL, V5, V6
  • Incomplete LBBB: Similar morphology but QRS 110-119 ms 1

Clinical Significance

Bradyarrhythmias can range from benign physiological findings to life-threatening conditions requiring immediate intervention 2:

• Symptomatic bradycardia refers to documented bradyarrhythmia directly responsible for symptoms resulting from cerebral hypoperfusion
• Common symptoms include syncope, presyncope, dizziness, fatigue, dyspnea, chest pain, heart failure symptoms, and confusion
• Severe bradycardia can lead to Morgagni-Adams-Stokes seizures and cardiac arrest 3, 4

Common Pitfalls in Diagnosis

• Pseudobradycardias: Slow peripheral pulse rates can result from frequent nonconducted early atrial premature beats, ventricular bigeminy, or mechanical alternans 5
• Physiologic bradycardia: Common in trained athletes or during sleep and may not require treatment 6
• Medication-induced bradycardia: Always review medications as beta-blockers, calcium channel blockers, digoxin, and antiarrhythmics are common culprits 2

Understanding the specific type of bradycardia is crucial for determining appropriate management, which may range from observation for asymptomatic cases to temporary pacing and eventual permanent pacemaker implantation for symptomatic patients with persistent bradycardia 6, 7.