premature atrial contraction
A premature atrial contraction is an early extra beat that springs from a spot in the atria other than the normal pacemaker. Like its ventricular cousin, it arrives ahead of schedule, but because it still travels down the heart's proper conduction system, it produces a normal-looking narrow beat — just one that comes too soon. Most people feel nothing, while some notice a brief flutter or skip.
On the ECG a PAC shows an early P wave of unusual shape, followed by a normal QRS, because the impulse reaches the ventricles through the usual route. They are exceedingly common and almost always benign, provoked by the same everyday culprits as other extra beats: caffeine, alcohol, fatigue, stress and nicotine.
PACs rarely need treatment on their own. Their main significance is that frequent atrial extra beats can act as triggers for atrial fibrillation in susceptible people, so a heavy burden of PACs is sometimes a clue worth noting rather than a disease in itself.
After a sleepless night and too much coffee, a man feels his heart “flutter”; his ECG captures frequent early narrow beats with abnormal P waves — benign PACs that settle with rest.
PACs are usually benign but a heavy burden can foreshadow atrial fibrillation.
The key contrast with a PVC is the QRS width: a PAC conducts through the normal pathway and looks narrow, whereas a PVC starts in the ventricle and looks wide.