What is bradycardia?

Bradycardia: Comprehensive Overview

Bradycardia is defined as a heart rate less than 60 beats per minute and can range from a benign physiological finding to a life-threatening condition requiring immediate intervention, depending on the presence of symptoms and hemodynamic compromise. 1

Definition and Classification

Bradycardia can be broadly classified into two general categories:

1. Sinus Node Dysfunction (SND) 1:

   • Sinus bradycardia: Sinus rate <50 bpm
   • Ectopic atrial bradycardia: Atrial depolarization from a non-sinus node pacemaker with rate <50 bpm
   • Sinoatrial exit block: Blocked conduction between sinus node and atrial tissue
   • Sinus pause: Sinus node depolarizes >3 seconds after last atrial depolarization
   • Sinus node arrest: No evidence of sinus node depolarization
   • Tachycardia-bradycardia syndrome: Alternating bradycardia and tachyarrhythmias
   • Chronotropic incompetence: Inability to increase heart rate with activity/demand
   • Isorhythmic dissociation: Atrial rate slower than ventricular rate

2. Atrioventricular (AV) Block 1:

   • First-degree AV block: PR interval >200 ms with 1:1 conduction
   • Second-degree AV block:
     • Mobitz type I (Wenckebach): Progressive PR prolongation before blocked P wave
     • Mobitz type II: Constant PR intervals with periodic blocked P waves
     • 2:1 AV block: Every other P wave conducts
   • Advanced/high-grade AV block: ≥2 consecutive non-conducted P waves
   • Third-degree (complete) AV block: No AV conduction

Clinical Manifestations

Symptoms of bradycardia vary widely and may include 1, 2:

• Syncope or presyncope
• Dizziness or lightheadedness
• Fatigue
• Dyspnea
• Chest pain
• Heart failure symptoms
• Confusion or altered mental status
• Morgagni-Adams-Stokes seizures

Symptomatic bradycardia refers to documented bradyarrhythmia directly responsible for symptoms resulting from cerebral hypoperfusion due to slow heart rate 1.

Etiology

Intrinsic Causes 1, 3:

• Degenerative fibrosis of the conduction system (age-related)
• Ischemic heart disease
• Infiltrative diseases (amyloidosis, sarcoidosis)
• Inflammatory conditions (myocarditis, endocarditis)
• Congenital heart disease
• Surgical trauma

Extrinsic/Reversible Causes 4, 3:

• Medications:
  • Beta-blockers (metoprolol, atenolol, propranolol)
  • Non-dihydropyridine calcium channel blockers (diltiazem, verapamil)
  • Digoxin
  • Antiarrhythmic drugs (amiodarone, sotalol, flecainide)
  • Centrally-acting agents (clonidine)
• Increased vagal tone
• Hypothyroidism
• Electrolyte abnormalities
• Hypothermia
• Increased intracranial pressure
• Sleep apnea
• Infectious diseases (Lyme disease, endocarditis)

Risk Factors 4:

• Advanced age (>70 years)
• Pre-existing cardiac disease
• Renal or hepatic dysfunction
• Electrolyte abnormalities
• Concomitant use of multiple bradycardia-inducing medications

Diagnostic Approach

Diagnostic tools include 2, 3, 5:

• 12-lead ECG
• Holter monitoring (24-48 hours)
• Event recorders
• Implantable loop recorders
• Exercise testing (for chronotropic incompetence)
• Electrophysiologic testing (rarely required)

The key diagnostic goal is establishing symptom-rhythm correlation 3.

Management

Acute Management of Symptomatic Bradycardia 4, 6, 5:

1. First-line pharmacologic therapy:

   • Atropine 0.5-1 mg IV every 3-5 minutes (maximum 3 mg)

2. Second-line options (if atropine ineffective):

   • Dopamine infusion (2-10 μg/kg/min)
   • Epinephrine infusion (2-10 μg/min)
   • Isoproterenol infusion (2-10 μg/min)

3. Temporary pacing for refractory cases:

   • Transcutaneous pacing (emergency)
   • Transvenous pacing (bridge to permanent pacing)

4. Specific antidotes for medication overdose:

   • Beta-blocker overdose: High-dose glucagon
   • Calcium channel blocker overdose: Calcium administration plus epinephrine

Long-term Management 1, 7:

1. Treat reversible causes:

   • Discontinue or reduce offending medications
   • Correct electrolyte abnormalities
   • Treat underlying conditions

2. Permanent pacemaker implantation for:

   • Symptomatic sinus node dysfunction
   • High-grade (second-degree Mobitz II or third-degree) AV block
   • Symptomatic bradycardia without reversible causes

Special Considerations

Physiologic Bradycardia 1, 3:

• Common in well-trained athletes
• Normal during sleep
• Generally asymptomatic and requires no treatment

Medication Combinations with High Risk 4:

• Beta-blockers + calcium channel blockers
• Beta-blockers + digoxin
• Calcium channel blockers + digoxin
• Multiple antiarrhythmic agents

Age-Specific Considerations 4:

• Pediatric patients require age-specific heart rate norms
• Elderly patients may have increased sensitivity to bradycardia-inducing medications

Prevention and Monitoring 4:

• Avoid combining multiple bradycardia-inducing medications when possible
• Monitor heart rate and blood pressure regularly in at-risk patients
• Adjust medication doses appropriately in elderly patients and those with renal/hepatic impairment
• Be vigilant for drug interactions that increase levels of bradycardia-inducing drugs
• Regular vital sign assessment in postoperative patients

By understanding the pathophysiology, clinical presentation, and management of bradycardia, clinicians can effectively diagnose and treat this condition, potentially preventing serious complications such as syncope, falls, and cardiac arrest.