Though it may feel like the heart has skipped a beat, a premature heartbeat occurs when the heart's regular rhythm is interrupted by an early or even extra heartbeat. If the beat arises from locations in the atria (upper chambers) it is called premature atrial contraction (PAC). If it arises from the ventricles (lower chambers), it is called premature ventricular complexes (PVC). In most cases, neither is considered serious.
With PAC, instead of a missed heartbeat, an extra heartbeat comes sooner than normal. This is followed by a pause that causes the next beat to be more forceful. The sensation of the skipped beat is actually this more forceful beat.
When the electrical signal from the ventricles causes an early heartbeat, it typically goes unnoticed. The heart then seems to pause until the next beat of the ventricle occurs in a regular fashion.
Premature beats are common in healthy people of all ages with most people experiencing them at some time in their lives. Caffeine, alcohol, stress, and fatigue may cause PACs to occur more frequently. Usually no special treatment is needed and no cause can be found. The premature beats may disappear, and even if they continue, most people tolerate them well.