What are the causes of bradycardia (abnormally slow heart rate)?

From the Guidelines

Bradycardia, or a heart rate below 60 beats per minute, can be caused by various factors, including medications, hypothyroidism, electrolyte imbalances, and heart conditions, and treatment should focus on addressing the underlying cause, as recommended by the 2018 ACC/AHA/HRS guideline on the evaluation and management of patients with bradycardia and cardiac conduction delay 1.

Causes of Bradycardia

The causes of bradycardia can be broadly classified into physiological and pathological categories. Physiological causes include:

• Being physically fit, as athletes often have lower resting heart rates due to efficient cardiovascular systems
• Normal sleep patterns Pathological causes include:
• Heart conditions such as sick sinus syndrome, heart block, myocardial infarction, and cardiomyopathy
• Medications that can induce bradycardia, such as:
  • Beta-blockers (metoprolol, propranolol)
  • Calcium channel blockers (diltiazem, verapamil)
  • Certain antiarrhythmics (amiodarone, digoxin)
• Other causes, including:
  • Hypothyroidism
  • Electrolyte imbalances (particularly hyperkalemia)
  • Increased intracranial pressure
  • Hypothermia
  • Sleep apnea
  • Vasovagal responses
  • Infections like Lyme disease and certain viral infections

Treatment of Bradycardia

Treatment of bradycardia depends on identifying and addressing the underlying cause. According to the 2018 ACC/AHA/HRS guideline, patients presenting with symptomatic sinus bradycardia attributable to reversible causes should first be managed by directing therapy at eliminating or mitigating the offending condition 1. This may involve:

• Medication adjustments, such as withdrawing or reducing the dose of negative chronotropic drugs
• Treating underlying medical conditions, such as hypothyroidism or electrolyte imbalances
• Pacemaker implantation for persistent cases of symptomatic bradycardia It is essential to distinguish between physiological bradycardia due to autonomic conditions or training effects and circumstantially inappropriate bradycardia that requires permanent cardiac pacing, as noted in the 2012 ACCF/AHA/HRS focused update incorporated into the ACCF/AHA/HRS 2008 guidelines for device-based therapy of cardiac rhythm abnormalities 1.

From the FDA Drug Label

Because digoxin slows sinoatrial and AV conduction, the drug commonly prolongs the PR interval. The drug may cause severe sinus bradycardia or sinoatrial block in patients with pre-existing sinus node disease and may cause advanced or complete heart block in patients with pre-existing incomplete AV block Therapeutic doses of digoxin may cause heart block in patients with pre-existing sinoatrial or AV conduction disorders; heart block can be avoided by adjusting the dose of digoxin High doses of digoxin may produce a variety of rhythm disturbances, such as first-degree, second-degree (Wenckebach), or third-degree heart block (including asystole); atrial tachycardia with block; AV dissociation; accelerated junctional (nodal) rhythm; unifocal or multiform ventricular premature contractions (especially bigeminy or trigeminy); ventricular tachycardia; and ventricular fibrillation. In children, the use of digoxin may produce any arrhythmia The most common are conduction disturbances or supraventricular tachyarrhythmias, such as atrial tachycardia (with or without block) and junctional (nodal) tachycardia. Ventricular arrhythmias are less common. Sinus bradycardia may be a sign of impending digoxin intoxication, especially in infants, even in the absence of first-degree heart block

The causes for bradycardia include:

• Pre-existing sinus node disease: Digoxin may cause severe sinus bradycardia or sinoatrial block in these patients 2
• Pre-existing incomplete AV block: Digoxin may cause advanced or complete heart block in these patients 2
• High doses of digoxin: May produce a variety of rhythm disturbances, including sinus bradycardia 2
• Digoxin intoxication: Especially in infants, even in the absence of first-degree heart block 2

From the Research

Causes of Bradycardia

• Bradycardia can be caused by certain medications, including:
  • Beta-adrenergic receptor antagonist drugs (beta-blockers) 3
  • Non-dihydropyridine calcium-channel blockers (non-DHP CCBs), such as diltiazem and verapamil 3
• The combination of beta-blockers and non-DHP CCBs can increase the risk of sinus arrest or severe sinus bradycardia 3
• Other factors that can contribute to bradycardia include:
  • Age 3, 4
  • Presence of renal or hepatic disease 3
  • Use of other medications, such as digoxin, diuretics, angiotensin-converting enzyme inhibitors, and calcium channel blockers 4

Medication-Related Bradycardia

• Beta-blockers and calcium channel blockers are commonly used to treat conditions such as hypertension, coronary artery disease, and heart failure 4
• However, these medications can also cause bradycardia, particularly in older patients or those with certain underlying medical conditions 3, 4
• The prevalence of beta-blocker and calcium channel blocker use in older patients with previous myocardial infarction or coronary artery disease is significant, with 19% and 45% of patients prescribed these medications, respectively 4