What are the causes and treatments of sinus bradycardia?

Causes and Treatment of Sinus Bradycardia

Sinus bradycardia has numerous reversible and irreversible causes, and treatment should focus on addressing underlying causes first before considering permanent pacing for symptomatic patients who don't respond to conservative management.

Causes of Sinus Bradycardia

Medications and Toxins

• Beta blockers, non-dihydropyridine calcium channel blockers, digoxin, antiarrhythmic drugs, lithium, methyldopa, risperidone, cisplatin, and interferon are common medication causes 1, 2
• Toxins such as toluene, organophosphates, tetrodotoxin, and cocaine can induce bradycardia 1

Cardiovascular Causes

• Acute myocardial ischemia or infarction 1, 3
• Atrial fibrillation 1
• Post-cardiac surgery (valve replacement, maze procedure, coronary artery bypass graft) 1
• Heart transplant (acute rejection, chronic rejection, remodeling) 1

Metabolic and Endocrine Causes

• Electrolyte abnormalities (hyperkalemia, hypokalemia, hypoglycemia) 1, 3
• Hypothyroidism 1, 2
• Hypoxemia, hypercarbia, acidosis 1

Other Medical Conditions

• Hypervagotonia 1
• Hypothermia (therapeutic post-cardiac arrest cooling or environmental exposure) 1, 3
• Hypovolemic shock 1
• Sleep apnea and respiratory insufficiency 1, 4
• Elevated intracranial pressure 3
• Infections (Lyme disease, legionella, psittacosis, typhoid fever, typhus, listeria, malaria, leptospirosis, Dengue fever, viral hemorrhagic fevers, Guillain-Barre) 1

Physiological Causes

• Athletic training 1
• Normal variant during sleep 5

Clinical Presentation

• Symptoms range from mild fatigue to frank syncope, with severity generally correlating with heart rate or pause duration 1
• Common symptoms include syncope (present in 50% of patients receiving pacemakers for SND), dyspnea on exertion due to chronotropic incompetence, lightheadedness, and chronic fatigue 1, 6
• Asymptomatic sinus bradycardia is common and not associated with adverse outcomes, especially in trained athletes or during sleep 1, 5

Treatment Approach

Evaluation of Reversible Causes

• In symptomatic patients, evaluation and treatment of reversible causes is recommended before considering permanent pacing 1, 2
• Laboratory tests should include thyroid function, electrolytes, and other tests based on clinical suspicion 3

Management of Asymptomatic Bradycardia

• Asymptomatic sinus bradycardia does not require treatment, especially in young individuals, athletes, or during sleep 2, 5

Acute Management of Symptomatic Bradycardia

• Atropine (0.5-1 mg IV) is reasonable for patients with SND associated with symptoms or hemodynamic compromise 1, 2
• Atropine blocks muscarinic actions of acetylcholine, preventing or abolishing vagal cardiac slowing 7
• In patients at low likelihood of coronary ischemia, isoproterenol, dopamine, dobutamine, or epinephrine may be considered to increase heart rate and improve symptoms 1, 5
• Important caveat: Atropine should not be used to treat sinus bradycardia in patients who have undergone heart transplant without evidence for autonomic reinnervation 1

Chronic Management of Symptomatic Bradycardia

• Permanent pacemaker implantation is indicated when symptoms directly correlate with bradycardia and reversible causes have been excluded 2, 8
• Physiological pacing (atrial or dual-chamber) is superior to VVI pacing for sick sinus syndrome 8
• For symptomatic bradycardia due to necessary guideline-directed medical therapy with no alternative treatment, permanent pacing is recommended 2
• Despite adequate pacing, syncope recurs in approximately 20% of patients during long-term follow-up due to associated vasodepressor reflex mechanisms 8

Special Considerations

• In patients with sleep apnea and bradycardia, treatment with nasal continuous positive airway pressure (nCPAP) should be considered before pacemaker implantation 4
• Elderly patients (around 80 years) with organic heart disease and severe chronotropic incompetence are more prone to develop heart failure with sinus bradycardia 9
• In some cases, traditional Chinese medicine approaches have been reported as alternative therapies, though these require further validation 10

Treatment Algorithm for Sinus Bradycardia

1. Determine if patient is symptomatic (syncope, dizziness, fatigue, dyspnea)

   • If asymptomatic → observation only 2
   • If symptomatic → proceed to step 2

2. Evaluate and treat reversible causes:

   • Review and adjust medications 1, 2
   • Correct metabolic/electrolyte abnormalities 1
   • Treat hypothyroidism if present 2
   • Address sleep apnea if present 4
   • Manage other underlying conditions 1

3. For acute symptomatic bradycardia with hemodynamic compromise:

   • Administer atropine 0.5-1 mg IV 1, 2
   • Consider isoproterenol, dopamine, dobutamine, or epinephrine if no response to atropine 1
   • Consider temporary pacing as bridge to definitive treatment 5

4. For persistent symptomatic bradycardia after addressing reversible causes:

   • Permanent pacemaker implantation (preferably physiological pacing) 2, 8, 5