From the Guidelines
Bradycardia is primarily caused by sinus node dysfunction, heart blocks, electrolyte imbalances, hypothyroidism, hypothermia, and certain medications, including beta-blockers, calcium channel blockers, digoxin, and antiarrhythmics, as well as increased intracranial pressure, sleep apnea, and vasovagal responses. The causes of bradycardia can be broadly classified into physiological and pathological categories. Physiological causes include athletic conditioning and sleep, where the heart rate slows down due to decreased metabolic demands. Pathological causes, on the other hand, include sinus node dysfunction, where the heart's natural pacemaker malfunctions, and heart blocks, where electrical signals are impeded between the atria and ventricles.
Key Causes of Bradycardia
- Sinus node dysfunction
- Heart blocks
- Electrolyte imbalances, particularly high potassium (hyperkalemia)
- Hypothyroidism, which slows metabolic processes
- Hypothermia, as cold temperatures reduce heart rate
- Medications that commonly cause bradycardia, including:
- Beta-blockers (metoprolol, propranolol)
- Calcium channel blockers (diltiazem, verapamil)
- Digoxin
- Certain antiarrhythmics (amiodarone, sotalol)
- Increased intracranial pressure, which can trigger bradycardia through the Cushing reflex
- Sleep apnea, which may cause nocturnal bradycardia due to oxygen desaturation
- Vasovagal responses, where vagal nerve stimulation slows the heart, can occur during emotional stress or pain, as noted in studies such as 1 and 1.
Treatment Considerations
Treatment of bradycardia depends on the underlying cause and may include medication adjustments, pacemaker implantation for severe cases, or simply monitoring for asymptomatic physiological bradycardia. It is crucial to distinguish between physiological bradycardia, due to autonomic conditions or training effects, and inappropriate bradycardia that requires permanent cardiac pacing, as highlighted in 1. When bradycardia is induced or exacerbated by concomitant drugs affecting sinus node function, drug discontinuation should be considered as an alternative to cardiac pacing, as suggested in 1 and 1. Reducing drug dose, however, may not resolve the bradycardia.
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 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:
- Use of beta-blockers such as metoprolol or atenolol
- Conduction disorders such as first-degree atrioventricular block or sinus node dysfunction
- Catecholamine-depleting drugs such as reserpine
- Digitalis glycosides
- Disopyramide
- Calcium channel blockers 2, 2, 3
From the Research
Causes of Bradycardia
The causes of bradycardia, which is a heart rate lower than 60 beats/min, can be attributed to various factors, including:
- Intrinsic cardiac disorders such as sick sinus syndrome or inferior myocardial infarction 4
- Metabolic and environmental causes, including hypothyroidism and electrolyte disorders 4
- Medications, such as beta-blockers and amiodarone 4
- Infections, including myocarditis 4
- Increased intracranial pressure 4
- Toxic exposure 4
- Disorders of the sinoatrial node, including failure of impulse formation, sinoatrial conduction block, and carotid sinus hypersensitivity 5
- Intraatrial block, atrioventricular nodal block, or infranodal block 5
- Paroxysmal supraventricular tachyarrhythmias 5
Classification of Bradycardia
Bradycardia can be classified into different types, including:
- Sinus bradycardia, which is a sinus rhythm with a resting heart rate of less than 60 bpm 4
- Atrial bradycardia 6
- Junctional bradycardia 6
- Bradyarrhythmias caused by a problem with the conduction system, such as an atrioventricular block 6
Diagnosis and Treatment
The diagnosis of bradycardia involves determining if the patient's symptoms are caused by the slow heart rate 6. Treatment options include: