There are a few types of bradycardia. The most common is sinus bradycardia. Common causes for sinus bradycardia include hypothermia (low body temperature), deeper levels of anesthesia, athletic conditioning, and anything impacting the vagus nerve.
The vagus nerve is involved with many body systems, including the GI tract, respiratory tract, and urinary tract. Diseases affecting these systems might cause what is called high vagal tone, and lead to bradycardia if the disease is prolonged or severe.
Other causes of bradycardia are usually related to arrhythmias (abnormal heart rate). Common causes include urinary obstruction and underlying heart disease affecting electrical conduction through the heart.