What does a premature atrial contraction (PAC) involve?

A premature atrial contraction (PAC) is characterized by an early contraction of the atria that occurs due to ectopic firing. This means that instead of the normal electrical impulses originating from the sinoatrial (SA) node, the electrical activity is initiated from another site within the atria. As a result, the atrial muscle contracts prematurely, causing an irregular heartbeat.

This phenomenon highlights how these contractions are specific to the atria rather than involving other parts of the heart, such as the ventricles. Understanding PACs is crucial in the context of cardiac function and rhythm disturbances, as they can indicate underlying atrial excitability or other cardiac conditions. The other choices describe events or conditions that do not accurately reflect the nature of PACs, focusing instead on contractions that involve the ventricles or a rapid atrial rate, which are not characteristics of a PAC.