Final answer:
Rapid focal ectopic activity in the heart is the mechanism of premature contraction. It occurs when an area of the heart other than the SA node initiates an impulse that leads to a premature contraction. This can be caused by various factors and if it becomes chronic, it can result in arrhythmia or fibrillation.
Step-by-step explanation:
Rapid focal ectopic activity is the mechanism of premature contraction in the heart. It occurs when an ectopic focus or ectopic pacemaker, which is an area of the heart other than the SA node, initiates an impulse that is followed by a premature contraction. This can be caused by factors such as localized ischemia, exposure to certain drugs, elevated stimulation by the autonomic nervous system, or certain disease conditions. If this condition becomes chronic, it may lead to arrhythmia or fibrillation.