Complex sentence: Premature Atrial Contractions (PACs) are a type of premature heart beat, irregular heart beat, or benign arrhythmia which starts in the upper two chambers of the heart called the atria.
Split into simpler sentences: Premature Atrial Contractions (PACs) are one type of premature heart beat, irregular heart beat or benign arrhythmia. These ectopoic beats occur in the upper two chambers of the heart, called the atria.