Figure 2-12. Normal sinus rhythm.
b. Sinus Bradycardia.
(a) The rhythm is regular with the R-R intervals constant and the P-P
(b) The atrial and ventricular rates are less than 60 beats per minute.
The P wave is normal and upright with one P wave in front of every
(d) The P-R interval is constant between 0.12 and 0.20 seconds.
(e) The QRS complex measures less than 0.12 seconds.
(f) A heart rate of less than 60 beats per minute may indicate good
physical conditioning if the individual is young and healthy.
(g) If the person is suffering from acute myocardial infarction (AMI), this
heartbeat rate may indicate any one of the following: conduction system damage,
increased parasympathetic tone, or possible toxic levels of certain cardiac drugs
(digitalis, quinidine). If the heartbeat rate decreases to less than 50 beats per minute,
the heart may not be able to pump enough fluid through the body's vital organs.
Additionally, bradycardia leads to electrical instability in the ventricles possibly resulting
in ventricular arrhythmias.