Extrasystole is a cardiac arrhythmia that occurs when the heart beats abnormally fast or with extra beats. It is characterized by an early, extra heartbeat that can feel like a skipped beat, fluttering, or a thud in the chest. It may develop gradually or suddenly, and can be caused by stress, physical activity, or certain medications. It can manifest as palpitations, dizziness, fainting, chest pain, or shortness of breath.