Sinus bradycardia is a heart rhythm that originates from the sinus node and has a rate of under 60bpm.

Etiology

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• This rhythm may be caused by one of the following:
  • Increased vagal tone.
  • Intrinsic disease of the SA node.
  • An effect of drugs, such as the use of digitalis, beta-blockers, or quinidine.
  • It could also be a normal finding in a healthy, well-conditioned person.

ECG Characteristics

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• Rate: Less than 60.
• Rhythm: Regular.
• P waves: Upright, consistent, and normal in morphology and duration.
• P-R Interval: Between 0.12-0.20 seconds in duration.
• QRS Complex: Less than 0.12 seconds in width, and consistent in morphology.

Significance

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The decreased heart rate can cause a decreased cardiac output resulting in symptoms such as lightheadedness, dizziness, hypotension, vertigo, and syncope. The slow heart rate may also lead to atrial, junctional, or ventricular ectopic rhythms.

Sinus bradycardia is not necessarily a bad symptom. People who regularly practice sports may have sinus bradycardia, because their trained hearts can pump enough blood in each contraction.