From the Guidelines
Severe bradycardia is most commonly caused by certain medications, underlying heart conditions, and electrolyte imbalances, as well as other factors such as hypothyroidism, increased vagal tone, sleep apnea, severe infections, increased intracranial pressure, and hypothermia, as stated in the 2019 guideline on the evaluation and management of patients with bradycardia and cardiac conduction delay 1. The causes of severe bradycardia can be broadly categorized into several groups, including:
- Medications: beta-blockers, calcium channel blockers, and digoxin, especially when taken in excessive doses, can cause severe bradycardia, as noted in the 2019 guideline 1.
- Underlying heart conditions: sick sinus syndrome, heart block, and myocardial infarction can disrupt the heart's electrical conduction system, leading to severe bradycardia, as discussed in the 2013 focused update incorporated into the 2008 guidelines for device-based therapy of cardiac rhythm abnormalities 1.
- Electrolyte imbalances: hyperkalemia and hypokalemia can affect cardiac conduction, leading to severe bradycardia, as mentioned in the 2019 guideline 1.
- Other factors: hypothyroidism, increased vagal tone, sleep apnea, severe infections, increased intracranial pressure, and hypothermia can also cause severe bradycardia, as stated in the 2019 guideline 1. It is essential to identify the underlying cause of severe bradycardia to provide appropriate treatment, which may include medication adjustments, temporary or permanent pacemaker placement, thyroid hormone replacement, or addressing electrolyte abnormalities, as recommended in the 2019 guideline 1.
From the Research
Causes of Severe Bradycardia
- Severe bradycardia can be caused by a failure of the sinus node to generate normal impulses or due to a defect in cardiac conduction that in turn causes a delay or failure of impulse propagation 2
- Intrinsic conditions causing damage to the conduction system, such as sinus node dysfunction or atrioventricular conduction disturbances, can lead to bradyarrhythmias 3
- Extrinsic conditions, including certain medications like beta-blockers, glycosides, and calcium channel blockers, can also cause bradycardia 3, 2
- Underlying treatable diseases, such as hyperkalemia or cardiac conduction defects post cardiac bypass or acute myocardial infarction, can also result in bradycardia 2, 4
- Pathology that produces bradycardia may occur within the sinus node, atrioventricular (AV) nodal tissue, and the specialized His-Purkinje conduction system 5
Normal Physiologic Variants
- Bradycardia can be a normal physiologic reaction under certain circumstances, such as during sleep or in trained athletes 6, 2
- A heart rate decrease to 30 beats per minute (bpm) with episodes of sinoatrial block, junctional rhythms, and first and second degree atrioventricular block can occur often enough to be considered normal variants 2