A malfunction in the sinus node in the heart may cause sinus bradycardia, according to the Heart Rhythm Society. The sinus node, located in the right atrium of the heart, may misfire, wear out or slow down over time, and these problems can lead to an abnormally slow heart rate. Slow heart rates generally go below 60 beats per minute. Doctors examine a patient thoroughly to determine if a slow heart rate represents a dangerous condition.
Good physical fitness, medications and heart blocks may also cause sinus bradycardia, notes MedicineNet. Fainting may occur if a heartbeat goes below 50 beats per minute. Choking may trigger a slow heartbeat due to the vagus nerve's response to a sudden change in the throat. Doctors call this type of fainting the vasovagal reflex.
A heart block can occur when the biological processes that carry electrical signals to the heart muscle breaks down, says the Heart Rhythm Society. In this case, the sinus node may send out the correct electrical impulses, but somehow the heart muscle does not receive it properly. A first-degree heart block happens when impulses reach lower chambers of the heart slower than normal, but each signal arrives through the upper chambers. A second-degree heart block indicates some signals do not reach the lower areas, while a third-degree heart block means all signals cannot reach the lower parts of the heart. Third-degree heart blocks generally show the most symptoms in patients with this form of sinus bradycardia.