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

From the Guidelines

Sinus bradycardia is caused by multiple factors, including physiological conditions and pathological states, such as good physical fitness, sleep, increased vagal tone, cardiac conditions, metabolic disorders, neurological conditions, and medication effects. The causes of sinus bradycardia can be categorized into physiological and pathological states. Physiological causes include good physical fitness, especially in athletes, sleep, and increased vagal tone. Pathological causes include cardiac conditions like sick sinus syndrome, myocardial infarction, and heart block; metabolic disorders such as hypothyroidism, hypothermia, and electrolyte imbalances (particularly hyperkalemia); neurological conditions including increased intracranial pressure and vasovagal responses; and medication effects from beta-blockers (metoprolol, propranolol), calcium channel blockers (diltiazem, verapamil), antiarrhythmics (amiodarone, digoxin), and opioids 1.

Some of the key factors that contribute to sinus bradycardia include:

• Cardiac conditions: sick sinus syndrome, myocardial infarction, and heart block
• Metabolic disorders: hypothyroidism, hypothermia, and electrolyte imbalances
• Neurological conditions: increased intracranial pressure and vasovagal responses
• Medication effects: beta-blockers, calcium channel blockers, antiarrhythmics, and opioids

According to the 2019 ACC/AHA/HRS guideline on the evaluation and management of patients with bradycardia and cardiac conduction delay, withdrawal of offending drugs or dosage reduction can improve heart rate and symptoms in patients with sinus bradycardia caused by nonessential medications 1. The guideline also highlights the importance of addressing underlying conditions, such as elevated intracranial pressure, acute MI, severe hypothermia, and obstructive sleep apnea, which can contribute to sinus bradycardia.

In terms of treatment, permanent cardiac pacing is the only effective treatment for symptomatic bradycardia, and the decision to implant a pacemaker should be made on a case-by-case basis, taking into account the patient's symptoms, underlying conditions, and overall health status 1.

From the FDA Drug Label

Bradycardia, including sinus pause, heart block, and cardiac arrest have occurred with the use of metoprolol. Patients with first-degree atrioventricular block, sinus node dysfunction, or conduction disorders may be at increased risk. Clonidine: Sinus bradycardia resulting in hospitalization and pacemaker insertion has been reported in association with the use of clonidine concurrently with diltiazem. Disopyramide is a Type I antiarrhythmic drug with potent negative inotropic and chronotropic effects. Disopyramide has been associated with severe bradycardia, asystole and heart failure when administered with beta-blockers.

Causes of sinus bradycardia may include:

• Use of beta-blockers such as metoprolol 2 and atenolol 3
• Concomitant use of clonidine and diltiazem 4
• Use of disopyramide with beta-blockers 3 It is essential to monitor heart rate and rhythm in patients receiving these medications, especially those with pre-existing conduction disorders or sinus node dysfunction. If severe bradycardia develops, the medication may need to be reduced or stopped.

From the Research

Causes of Sinus Bradycardia

• Sinus bradycardia can be a normal finding in certain individuals, such as children, individuals who exercise often, and as a physiologic response during sleep 5
• Pathologic causes of sinus bradycardia include:
  • Sinus node dysfunction
  • Medications, such as beta-adrenergic receptor antagonist drugs (beta-blockers) and non-dihydropyridine calcium-channel blockers (non-DHP CCBs) 5, 6
  • Acute myocardial infarction
  • Heart failure
  • Obstructive sleep apnea (OSA) 5, 7
  • Exaggerated vagal activity
  • Increased intracranial hypertension
  • Infection
  • Hypothyroidism
  • Hypothermia 5
  • Anorexia nervosa
  • Prolonged hypoxia

Medication-Related Causes

• The combination of beta-blockers and non-DHP CCBs, such as diltiazem and verapamil, can cause sinus arrest or severe sinus bradycardia 6
• Certain patient factors, such as age, renal or hepatic disease, and the use of other medications, can increase the risk of marked bradycardia with hypotension 6

Obstructive Sleep Apnea-Related Causes

• There is a high comorbid disease burden between OSA and bradycardia, with a pooled prevalence of daytime and nocturnal bradycardia of 25% and 69.8%, respectively, among patients with OSA 7
• The use of continuous positive airway pressure (CPAP) therapy may not significantly reduce the risk of daytime or nocturnal bradycardia 7