Sinus bradycardia:

Etiology:

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. Sleep Seizure. It could also be a normal finding in a healthy, well-conditioned person. It may be secondary to infections like Diphtheria, acute rheumatic fever, viral myocarditis. It is physiologic in trained athletes.

ECG CharacteristicsRate:

Less than 60 per minute. 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:

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 problematic. People who regularly practice sports may have sinus bradycardia, because their trained hearts can pump enough blood in each contraction to allow a low resting heart rate.

Sinus Bradycardia can aid in the sport of Freediving, which includes any of various aquatic activities that share the practice of breath-hold underwater diving.[citation needed] Bradycardia aids in this process due to drop in blood rate pulse.[citation needed] These adaptations enable the human body to endure depth and lack of oxygen far beyond what would be possible without the mammalian diving reflex.