Sinus bradycardia occurs on an ECG when there is a normal upright P wave in lead II ― sinus P wave ― preceding every QRS complex with a ventricular rate of less than 60 beats per minute.
The causes of sinus bradycardia include the following:
- AV blocking medications (beta-blockers, nondihydropyridine calcium channel blockers, digoxin)
- Heightened vagal tone (i.e. well-trained athlete)
- Sick sinus syndrome
- Obstructive sleep apnea
Sinus bradycardia is frequently asymptomatic. Heart rates in the 40s and 50s can be tolerated well. When symptoms arise, they manifest as fatigue, dizziness and dyspnea on exertion (including chronotropic incompetence). Severe sinus bradycardia is a form of sick sinus syndrome, and a pacemaker would relieve any associated symptoms.
1. Chou’s Electrocardiography in Clinical Practice: Adult and Pediatric, 6e
2. Surawicz B, et al. AHA/ACCF/HRS Recommendations for the Standardization and Interpretation of the Electrocardiogram. Circulation. 2009; doi:10.1161/CIRCULATIONAHA.108.191095.