Depolarization that arise from the atria can be triggered by alcohol ingestion, tobacco, or adrenergic stimulants.
They usually don't lead to symptoms and pass unnoticed. However, in some they can lead to palpitaions.
P waves differ in contour and axis from the normal sinus P wave. Depending on the degree of prematurity the PR interval of the PAC may be prolonged, and usually a compensatory pause occurs prior to the next sinus P wave. The compensatory pause here may not be complete (i.e. the interval between conducted sinus beats that bracket the VPC is not equal to two basic RR intervals, see premature ventricular complexes).
PACs typically require no therapy. If symptomatic, therapy should be directed toward correction of underlying abnormalities. Beta-adrenergic antagonists or calcium channel antagonists may be useful.