Premature beats often feel like a skipped heartbeat, it's actually an extra beat between 2 normal heartbeats. It occurs in the ventricles before the ventricles have had time to fill with blood after a regular heartbeat. A premature beat can trigger an arrhythmia especially in people with heart disease. Premature atrial contractions (PACs) occur in the atria and premature ventricular contractions (PVCs) in the ventricle. Certain heart diseases, stress, exercise or caffeine can cause premature beats.
Attribution: Straight Talk Foundation