The rate for sinus bradycardia is less than 60 beats per minute. Each P wave is followed by a QRS and each QRS is preceded by a P wave. Each P wave is constant in morphology and upright in lead II. The PR interval is constant and of normal duration. It can be a physiologic response to a variety of stimuli, including enhanced vagal tone and various drugs such as beta blockers. The following animation illustrates what we have just described.