<P> An' arrhythmia' refers to an abnormal rhythm or speed of rhythm of the heartbeat . An abnormal rhythm or speed is defined as one which is not physiological . </P> <P> A resting heart that beats slower than 60 beats per minute, or faster than 100 beats per minute, is regarded as having an arrhythmia . A heartbeat slower than 60 beats per minute is known as bradycardia, and a heartbeat faster than 100 is known as a tachycardia . </P> <P> Some individuals, for example trained athletes, may have heart beats slower than 60 beats per minute when not exercising . If the SA node fails to initialize, the AV junction can take over as the main pacemaker of the heart . The AV junction "surrounds" the AV node (the AV node is not able to initialize its own impulses) and has a regular rate of 40 to 60 bpm . These "junctional" rhythms are characterized by a missing or inverted P - Wave . If both the SA node and the AV junction fail to initialize the electrical impulse, the ventricles can fire the electrical impulses themselves at a rate of 20 to 40 bpm and will have a QRS complex of greater than 120 ms . </P> <P> In the event of arrhythmia, a pacemaker may be surgically inserted into the conduction system . </P>

Where do depolarization and repolarization of both atria and ventricles occur