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.
Causes of sinus bradycardia include:
AV blocking medications (beta-blockers, non-dihydropyridine calcium channel blockers, digoxin).
Heightened vagal tone (i.e. well trained athlete)
Sick sinus syndrome
Obstructive sleep apnea
Frequently, sinus bradycardia is asymptomatic. Heart rates in the 40’s and 50’s can be well tolerated. When symptoms arise they manifest as fatigue, dizziness and dyspnea on exertion (chronotropic incompetence).
Sinus Bradycardia (Example 1)
Sinus Bradycardia (Example 2)
Sinus Bradycardia (Example 3)
Sinus Bradycardia (Example 4)
Note that an ectopic atrial rhythm occurs when the P wave is not upright in lead II. This indicates that the action potential is arising from a location in the atrium that is not the sinus node.