What are the causes and treatments of bradycardia?

Bradycardia: Causes and Treatment

Causes of Bradycardia

Bradycardia (heart rate <50-60 bpm) results from either intrinsic cardiac dysfunction or extrinsic reversible causes, and identifying reversible causes is the critical first step before considering permanent interventions. 1

Medication-Induced Causes

• Beta-blockers, non-dihydropyridine calcium channel blockers (diltiazem, verapamil), and digoxin are the most common medication culprits and should be discontinued or dose-reduced when causing symptomatic bradycardia 2, 3, 1
• Antiarrhythmic drugs (particularly sotalol), lithium, methyldopa, risperidone, cisplatin, and interferon can all cause bradycardia 3
• Drugs transmitted transplacentally or through breast milk can cause bradycardia in neonates 1

Cardiac Causes

• Sinus node dysfunction characterized by abnormal impulse formation and conduction within the sinoatrial node 2
• Acute myocardial ischemia or infarction affecting sinus node function 2, 3
• AV blocks (first-, second-, and third-degree) caused by structural problems from AMI or other myocardial diseases 4
• Infiltrative diseases such as lymphoma or amyloidosis affecting the conduction system 2
• Myocarditis causing direct inflammation of the conduction system 1

Metabolic and Electrolyte Causes

• Hypothyroidism is a key reversible metabolic cause requiring thyroxine replacement 3, 1
• Electrolyte abnormalities: hyperkalemia, hypokalemia, severe hypoglycemia, and systemic acidosis 2, 3, 1

Neurologic and Vagal Causes

• Increased intracranial pressure from any cause triggers reflex bradycardia through vagal stimulation 1
• Increased vagal tone during feeding, sleep, defecation causes physiologic bradycardia in neonates 1
• Gastrointestinal distress and acute abdominal pain trigger vagal reflexes causing transient bradycardia 1
• Carotid sinus hypersensitivity 5

Environmental and Toxic Causes

• Hypothermia causes dose-dependent bradycardia and requires active rewarming 1
• Toxins including toluene, organophosphates, tetrodotoxin, and cocaine 3
• Hypoxemia is a common cause requiring immediate evaluation 4

Treatment Algorithm

Step 1: Initial Assessment and Stabilization

Assess if bradycardia is causing symptoms and hemodynamic compromise—heart rate typically <50 bpm if clinically significant. 4

• Identify signs of increased work of breathing (tachypnea, intercostal retractions, suprasternal retractions) and check oxyhemoglobin saturation 4
• Provide supplementary oxygen if hypoxemic or showing increased work of breathing 4
• Attach cardiac monitor, evaluate blood pressure, establish IV access 4
• Obtain 12-lead ECG to define rhythm (don't delay therapy) 4

Step 2: Identify and Treat Reversible Causes (CRITICAL FIRST STEP)

The most important clinical error is failing to identify reversible causes before considering permanent pacing. 1

• Discontinue offending medications (beta-blockers, calcium channel blockers, digoxin) 2, 3, 1
• Correct electrolyte abnormalities (hyperkalemia, hypokalemia, hypoglycemia) 2, 3, 1
• Treat hypothyroidism with thyroxine replacement 3, 1
• Rewarm if hypothermic 1
• Investigate for increased intracranial pressure, infections, and myocarditis 1

Step 3: Determine if Symptoms are Due to Bradycardia

Look for acute altered mental status, ischemic chest discomfort, acute heart failure, hypotension, or other signs of shock directly attributable to bradycardia. 4

• Asymptomatic or minimally symptomatic patients do not require treatment unless the rhythm is likely to progress (e.g., Mobitz type II second-degree AV block in setting of AMI) 4
• Asymptomatic sinus bradycardia has not been associated with adverse outcomes and typically does not require treatment 2

Step 4: Acute Pharmacologic Treatment for Symptomatic Bradycardia

Atropine 0.5-1 mg IV is reasonable to increase sinus rate in patients with symptomatic bradycardia associated with hemodynamic compromise. 3, 6

• Atropine abolishes reflex vagal cardiac slowing or asystole and prevents bradycardia produced by choline esters or anticholinesterase agents 6
• Atropine's effects on heart rate are delayed by 7-8 minutes after IV administration 6
• Common pitfall: Atropine may cause transient initial bradycardia before characteristic tachycardia develops due to paralysis of vagal control 6
• Occasionally large doses may cause AV block and nodal rhythm 6

Step 5: Temporary Pacing

• Consider transcutaneous or transvenous pacing as a bridge to definitive treatment in unstable patients 4, 7
• Expert consultation should be obtained 4

Step 6: Permanent Pacemaker Implantation

Permanent pacemaker implantation is recommended for persistent symptomatic bradycardia despite addressing reversible causes. 2, 3

• Indicated when symptoms directly correlate with bradycardia and reversible causes have been excluded 3
• Physiological pacing (atrial or dual-chamber) is superior to VVI pacing for sick sinus syndrome 3
• Symptomatic patients with sick sinus syndrome and high second- or third-degree AV blocks require permanent pacemakers 7
• For patients with infrequent symptoms suspected to be caused by bradycardia, long-term monitoring with an implantable cardiac monitor is reasonable if initial non-invasive evaluation is non-diagnostic 2

Special Considerations and Pitfalls

High-Risk Populations

• Symptomatic patients with sinus node dysfunction have high risk of cardiovascular events including syncope, atrial fibrillation, and heart failure 2
• Chronotropic incompetence with age is associated with increased risk of cardiovascular death and overall mortality 2
• The elimination half-life of atropine is more than doubled in children under 2 years and elderly (>65 years) 6

Pregnancy and Lactation

• Severe or life-threatening muscarinic events are medical emergencies in pregnancy; life-sustaining therapy should not be withheld due to concerns about fetal effects 6
• Atropine crosses the placenta but is not found in amniotic fluid 6
• To minimize infant exposure, women may pump and discard milk for 24 hours after atropine use before resuming breastfeeding 6

Contraindications and Cautions

• Avoid theophylline in bradycardia-tachycardia manifestations of sick sinus syndrome or when ventricular ectopy is frequent 8
• Patients with complete heart block may have variable responses to atropine—some accelerate idioventricular rate, others stabilize 6