Premature atrial contractions are benign. The atrium part of your heart is the UPPER part. Premature ventricular contractions are also usually benign . However, sometimes, they can develop into ventricular tachycardia which can be serious (mostly only if you have structural heart problems). Ventricular refers to the LOWER part of your heart, the ventricles. Atrial fibrillation is a rapid - regularly irregular - arrhythmia that is not life-threatening but can make you feel awful ( or you may not notice it, depending on the person ) but raises the risk fo rstroke. It is NOT the same as PACs . Ventricular fibrillation is a different story and life-threatening - however, it is far less common that atrial fibrillation and associated usually with serious heart disease. Please don't confuse PACs with some kind of serious arrhythmia. They are premature beats of no consequence the vast amount of the time - but they can be frightening. PVCs are also part of a normal heart rhythm - only some people may have more than others.. but occasional PVCs in a normal heart are just part of life.