Sinus bradycardia. This strip meets the criteria of: regular rhythm, rate less than 60 bpm (40 bpm in this case), regular P waves before every QRS. Sinus bradycardia can have many causes from a completely normal variation to a malfunction of the sinus node. In some cases, enhanced parasympathetic tone causes sinus bradycardia. Well-conditioned athletes typically have sinus bradycardia. Treatment depends upon the cause and the patient's response to the rate. If the rate does not cause hemodynamic impairment, treatment may not be necessary.