Bradycardia

Bradycardia refers to a resting heart rate that is slower than normal. In adults, a resting heart rate below 60 beats per minute is commonly used as a threshold, though a low rate can be normal in trained athletes or during sleep. Bradycardia can be a benign physiologic finding or a sign of an underlying condition affecting the heart’s electrical system.

There are several forms of bradycardia. Sinus bradycardia occurs when the sinoatrial node fires more slowly but produces normal P waves and a normal PR interval. Atrioventricular (AV) block involves delayed or blocked transmission of impulses from the atria to the ventricles and is classified as first-degree (prolonged PR interval), second-degree (Mobitz I or Wenckebach; intermittent dropped beats), second-degree Mobitz II (random dropped beats), or third-degree (complete heart block with no conduction between atria and ventricles).

Common causes include athletic conditioning, sleep, or increased vagal tone. Pathologic causes include hypothyroidism, electrolyte disturbances (notably hyperkalemia), hypothermia, ischemic heart disease or myocardial infarction, aging-related conduction system disease, and drugs such as beta-blockers, non-dihydropyridine calcium channel blockers, and digoxin. Structural heart disease and sleep apnea are additional contributors.

Diagnosis is typically made with an electrocardiogram, supported by ambulatory monitoring (Holter or event recorder) to correlate symptoms with rhythm. Evaluation may include blood tests and imaging to identify reversible causes and assess cardiac function.

Treatment depends on the cause and symptoms. Reversible etiologies are managed accordingly. Symptomatic bradycardia or high-grade AV block may require temporary pacing in acute settings and permanent pacemaker therapy for chronic conduction disease. In emergencies, atropine or vasoactive infusions and transcutaneous pacing may be used. Physiology-based bradycardia in healthy athletes usually does not require treatment.