From the Guidelines
Bradycardia can be caused by a variety of factors, including physiological conditions such as good physical fitness and sleep, as well as pathological conditions like sick sinus syndrome, heart block, and cardiomyopathy, with the most recent guidelines from 2019 emphasizing the importance of distinguishing between symptomatic and asymptomatic bradycardia 1.
Causes of Bradycardia
The causes of bradycardia can be broadly classified into two categories: physiological and pathological. Physiological causes include:
- Good physical fitness, especially in athletes
- Sleep, when the parasympathetic nervous system dominates Pathological causes include:
- Heart conditions such as sick sinus syndrome, heart block, myocardial infarction, and cardiomyopathy
- Medications like beta-blockers (metoprolol, propranolol), calcium channel blockers (diltiazem, verapamil), and certain antiarrhythmics (amiodarone, digoxin)
- Electrolyte imbalances, particularly hyperkalemia and hypocalcemia
- Hypothyroidism, which reduces metabolic demands
- Increased intracranial pressure, triggering bradycardia through the Cushing reflex
- Infections like Lyme disease and certain viral infections, affecting the heart's electrical system
- Sleep apnea, causing bradycardia during apneic episodes
- Vasovagal responses to pain, emotional stress, or specific triggers, temporarily slowing heart rate
- Hypothermia, progressively slowing heart rate as body temperature drops
Diagnosis and Treatment
The diagnosis of bradycardia involves correlating symptoms with electrocardiographic findings, and treatment depends on the underlying cause and whether the bradycardia is symptomatic, with permanent cardiac pacing being the only effective treatment for symptomatic bradycardia 1. It is crucial to distinguish between physiological bradycardia due to autonomic conditions or training effects and circumstantially inappropriate bradycardia that requires permanent cardiac pacing. The decision to implant a pacemaker for symptomatic bradycardia should be based on the presence of symptoms directly related to the bradycardia, such as syncope, presyncope, or heart failure symptoms.
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. Catecholamine-depleting drugs (e. g., reserpine) may have an additive effect when given with beta-blocking agents, or monoamine oxidase (MAO) inhibitors. Observe patients treated with metoprolol plus a catecholamine depletor for evidence of hypotension or marked bradycardia, which may produce vertigo, syncope, or postural hypotension Digitalis Glycosides and Beta-Blockers Both digitalis glycosides and beta-blockers slow atrioventricular conduction and decrease heart rate. Concomitant use can increase the risk of bradycardia Calcium Channel Blockers Concomitant administration of a beta-adrenergic antagonist with a calcium channel blocker may produce an additive reduction in myocardial contractility because of negative chronotropic and inotropic effects 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.
The causes of bradycardia include:
- Metoprolol use, especially in patients with first-degree atrioventricular block, sinus node dysfunction, or conduction disorders 2
- Catecholamine-depleting drugs (e.g., reserpine) when given with beta-blocking agents (e.g., metoprolol) 2
- Digitalis glycosides and beta-blockers (e.g., metoprolol) when used concomitantly 2
- Calcium Channel Blockers when administered with beta-adrenergic antagonists (e.g., metoprolol) 2
- Disopyramide when administered with beta-blockers (e.g., atenolol) 3
- Concomitant use of prostaglandin synthase inhibiting drugs (e.g., indomethacin) with beta-blockers 3
From the Research
Causes of Bradycardia
- Bradycardia can be due to sinus, atrial, or junctional bradycardia or to a problem with the conduction system (eg, an atrioventricular block) 4
- It can be classified into disorders of impulse formation and conduction, with impulse formation disorders being diseases of the sinus node, and conduction disturbances including sinoatrial (SA), atrioventricular (AV) block, and bundle branch block 5
- Conditions resulting in bradyarrhythmic disorders can be divided into intrinsic and extrinsic conditions causing damage to the conduction system 6
- Bradycardia can also be a normal physiologic reaction under certain circumstances, such as in young athletic individuals or as part of normal aging 7
- Reversible or preventable causes of bradycardia, such as drugs (most often beta-blockers, glycosides, and/or calcium channel blockers) or underlying treatable diseases, should be investigated and treated 5, 6
Underlying Causes
- Sinoatrial node dysfunction 6, 8
- Atrioventricular conduction disturbances 6, 8
- Damage to the conduction system 6
- Extrinsic causes such as drugs or underlying treatable diseases 5, 6
- Intrinsic conditions causing damage to the conduction system 6