Bradycardia: Symptoms, Causes & Treatment

Bradycardia is defined as a heart rate below 60 beats per minute. It is physiological and benign in well-trained athletes and many healthy adults at rest (40–60 bpm during sleep). Bradycardia becomes clinically significant when the heart rate is insufficient to maintain adequate cardiac output, producing symptoms of hemodynamic compromise: dizziness, near-syncope, syncope, dyspnea, fatigue, or chest pain. Symptomatic bradycardia may represent a life-threatening emergency requiring immediate intervention.

Pathological bradycardia results from dysfunction of the cardiac conduction system: (1) Sinus node dysfunction (sick sinus syndrome — SSS): failure of the sinoatrial node to generate impulses at an adequate rate; causes include fibrosis (aging), ischemia, and medications. (2) Atrioventricular (AV) block: impaired impulse transmission from atria to ventricles — graded as first-degree (prolonged PR), second-degree (Mobitz I/Wenckebach or Mobitz II), or third-degree (complete heart block — no impulses transmitted; ventricular escape rhythm only). Third-degree AV block is a cardiac emergency requiring urgent pacing.

Common causes include: antiarrhythmic drugs (beta-blockers, non-dihydropyridine calcium channel blockers, digoxin, amiodarone), hypothyroidism, infiltrative cardiac diseases (sarcoidosis, amyloidosis), inferior MI (RCA occlusion affecting the AV node), Lyme carditis, and aging-related conduction system fibrosis. Management targets the cause; permanent pacemaker implantation is indicated for symptomatic bradycardia not reversed by treating the underlying cause.

• Dizziness and lightheadedness, especially on standing
• Syncope or near-syncope — the most serious symptom; risk of injury and cardiac arrest
• Exertional or rest dyspnea from reduced cardiac output
• Fatigue and exercise intolerance
• Chest pain or pressure in severe cases
• In extremis: loss of consciousness, cardiac arrest (complete AV block with no escape rhythm)

• Medications: beta-blockers, non-DHP calcium channel blockers (verapamil, diltiazem), digoxin, amiodarone, ivabradine
• Sick sinus syndrome: fibrotic or ischemic sinoatrial node dysfunction (most common cause of pacemaker implantation)
• AV block: first, second (Mobitz I/II), or third degree; Mobitz II and third-degree are high-risk for progression
• Hypothyroidism: slows sinus rate and conduction; reverses with thyroid replacement
• Inferior STEMI: RCA occlusion affects the AV node in 90% of cases
• Lyme carditis, infiltrative diseases (cardiac sarcoidosis, amyloidosis), cardiac surgery trauma

12-lead ECG: essential first step — identifies sinus bradycardia, AV block degree, junctional or ventricular escape rhythms. 24–48 hour Holter monitor for intermittent symptoms. Event recorder or implantable loop recorder for infrequent syncope. Exercise stress test to unmask chronotropic incompetence. Blood tests: TSH (hypothyroidism), electrolytes (hyperkalaemia, hypomagnesaemia), Lyme serology, digoxin level if applicable. Echocardiography for structural assessment and infiltrative disease.

Acute symptomatic bradycardia: IV atropine 0.5–1 mg (up to 3 mg total); transcutaneous pacing if atropine fails; dopamine or epinephrine infusion as bridge. Transvenous temporary pacing for Mobitz II or third-degree AV block pending definitive therapy. Treat reversible causes: discontinue offending drugs, thyroid replacement for hypothyroidism, reperfusion for acute inferior MI, antibiotics for Lyme carditis. Permanent pacemaker (PPM) implantation: indicated for symptomatic SSS, Mobitz II second-degree block, third-degree AV block, and chronotropic incompetence.

• Syncope with falls, head injury, and fractures
• Sudden cardiac arrest in complete AV block without escape rhythm or pacemaker
• Heart failure from chronically reduced cardiac output (particularly in SSS)

• Review and dose-adjust cardioactive medications in at-risk patients (elderly, known conduction disease)
• Prompt treatment of hypothyroidism prevents reversible bradycardia
• Regular cardiac monitoring for patients with known conduction disease or infiltrative cardiac conditions