Bradyarrhythmias are arrhythmias in which the heart rate is slower than normal. Meaning not enough blood reaches the brain and can cause unconsciousness. Bradyarrhythmias can be caused by a heart attack conditions that change the heart's electrical activity such as an under active thyroid gland, aging and an imbalance of chemicals in the blood. In bradyarrhythmias the ventricles don't contract at exactly the same time as they should. Thus, the heart has to work harder to pump blood to the body.
Attribution: Straight Talk Foundation