From the Guidelines
Bradycardia is primarily caused by sinus node dysfunction (SND) and atrioventricular block, which can be attributed to various factors including certain medications, heart conditions, electrolyte imbalances, and increased vagal tone. According to the 2019 ACC/AHA/HRS guideline on the evaluation and management of patients with bradycardia and cardiac conduction delay 1, the clinical manifestations of bradycardia can vary widely from insidious symptoms to episodes of frank syncope. The guideline highlights that bradycardia can be broadly classified into two general categories: SND and atrioventricular block.
Some of the key causes of bradycardia include:
- Certain medications like beta-blockers, calcium channel blockers, and antiarrhythmics which slow heart rate as part of their therapeutic effect
- Heart conditions such as sick sinus syndrome, heart block, and myocardial infarction can disrupt normal electrical conduction
- Electrolyte imbalances, particularly high potassium (hyperkalemia) or low potassium (hypokalemia), can affect cardiac rhythm
- Hypothyroidism slows metabolic processes including heart rate
- Sleep apnea, hypothermia, and increased vagal tone (often seen in well-conditioned athletes) can also cause bradycardia
- Increased intracranial pressure may trigger a reflex bradycardia as the body attempts to maintain cerebral perfusion
- Infections like Lyme disease or endocarditis can affect the heart's electrical system
It's essential to note that bradycardia isn't always pathological—it's normal during sleep and can be a sign of good cardiovascular fitness in athletes. The 2013 ESC guidelines on cardiac pacing and cardiac resynchronization therapy 1 and the 2012 ACCF/AHA/HRS focused update incorporated into the ACCF/AHA/HRS 2008 guidelines for device-based therapy of cardiac rhythm abnormalities 1 also support the idea that bradycardia can be caused by various factors, including SND and atrioventricular block. Treatment depends on identifying and addressing the underlying cause, with medication adjustments, electrolyte correction, or pacemaker implantation in severe cases.
From the FDA Drug Label
Atropine-induced parasympathetic inhibition may be preceded by a transient phase of stimulation, especially on the heart where small doses first slow the rate before characteristic tachycardia develops due to paralysis of vagal control Adequate doses of atropine abolish various types of reflex vagal cardiac slowing or asystole The drug also prevents or abolishes bradycardia or asystole produced by injection of choline esters, anticholinesterase agents or other parasympathomimetic drugs, and cardiac arrest produced by stimulation of the vagus.
Bradycardia causes are not directly stated in the label, but it can be inferred that vagal stimulation or choline esters can cause bradycardia. Atropine is used to prevent or abolish bradycardia caused by these factors. 2
From the Research
Causes of Bradycardia
- Physiologic and pathologic conditions, including sinoatrial node dysfunction and atrioventricular (AV) conduction disturbances 3
- Pathology within the sinus node, atrioventricular (AV) nodal tissue, and the specialized His-Purkinje conduction system 4
- Myocardial ischemia or infarction with necrosis of the cardiac pacemaker sites and/or conduction system 5
- Altered autonomic influence, systemic hypoxia, electrolyte disturbances, acid-based disorders, and complications of various medical therapies 5
- Normal aging or disease 4
- Sinoatrial node dysfunction or AV block 3