From the Guidelines
Bradycardia, defined as a heart rate below 60 beats per minute, has numerous potential causes, including physiological conditions such as being physically fit or normal sleep patterns, and pathological conditions like sick sinus syndrome, heart block, myocardial infarction, and heart valve disease, as well as medications and other medical conditions. The causes of bradycardia can be broadly classified into two categories: sinus node dysfunction (SND) and atrioventricular block, with various clinical manifestations depending on the underlying cause and timing, as noted in the 2018 ACC/AHA/HRS guideline on the evaluation and management of patients with bradycardia and cardiac conduction delay 1. Some of the key causes include:
- Physiological conditions: being physically fit, normal sleep patterns
- Pathological conditions: sick sinus syndrome, heart block, myocardial infarction, heart valve disease
- Medications: beta-blockers, calcium channel blockers, digoxin, and some antiarrhythmics
- Other medical conditions: hypothyroidism, electrolyte imbalances, increased intracranial pressure, sleep apnea, and hypothermia
- Infections: Lyme disease and certain viral infections affecting the heart
- Vasovagal responses triggered by pain, stress, or other stimuli The underlying mechanism typically involves disruption of the heart's electrical conduction system, either through direct damage to cardiac tissue, medication effects on cardiac receptors, or autonomic nervous system influences that alter the heart's pacemaker function, as discussed in the 2018 ACC/AHA/HRS guideline 2. In terms of management, the guideline recommends evaluating and treating reversible causes of bradycardia, such as discontinuing or reducing offending medications, and considering pacing therapy in certain cases, as noted in the 2013 ESC guidelines on cardiac pacing and cardiac resynchronization therapy 3. Therefore, it is essential to identify and address the underlying cause of bradycardia, whether physiological or pathological, to provide appropriate management and improve patient outcomes, as emphasized in the 2019 ACC/AHA/HRS guideline on the evaluation and management of patients with bradycardia and cardiac conduction delay 4. Overall, the management of bradycardia requires a comprehensive approach, taking into account the underlying cause, clinical manifestations, and individual patient factors, as discussed in the 2012 ACCF/AHA/HRS focused update incorporated into the ACCF/AHA/HRS 2008 guidelines for device-based therapy of cardiac rhythm abnormalities 5.
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.
- Causes of bradycardia associated with metoprolol use include: + Sinus pause + Heart block + Cardiac arrest + First-degree atrioventricular block + Sinus node dysfunction + Conduction disorders If severe bradycardia develops, reduce or stop metoprolol 6.
From the Research
Causes of Bradycardia
- Bradycardia can be caused by conditions affecting the natural pacemakers of the heart, such as sick sinus syndrome (SSS) and atrioventricular (AV) blocks 7
- Pathology that produces bradycardia may occur within the sinus node, atrioventricular (AV) nodal tissue, and the specialized His-Purkinje conduction system 8
- It can be due to sinus, atrial, or junctional bradycardia or to a problem with the conduction system (eg, an atrioventricular block) 9
- Bradycardia can also be observed as a normal phenomenon in young athletic individuals, and in patients as part of normal aging or disease 8
Types of Bradycardia
- Sinus bradycardia: a heart rate lower than 60 beats/min due to a problem with the sinus node 9
- Atrial bradycardia: a heart rate lower than 60 beats/min due to a problem with the atria 9
- Junctional bradycardia: a heart rate lower than 60 beats/min due to a problem with the atrioventricular (AV) node 9
- Sick sinus syndrome (SSS): a condition that affects the natural pacemakers of the heart, leading to bradycardia 7