What is a defining feature of a premature atrial contraction (PAC)?

A defining feature of a premature atrial contraction (PAC) is that the P wave differs in shape and size compared to the typical sinus P waves. PACs occur when an ectopic atrial site generates an impulse before the next sinus node impulse. Because this impulse arises from a different area of the atrium, the resulting P wave will often have a distinct morphology. This difference is crucial for identifying PACs on an ECG, as it indicates the origin of the contraction.

The other features mentioned in the options do not characterize PACs accurately. For instance, the P wave is usually present during a PAC, which rules out the option discussing its absence. Heart rates associated with PACs can vary and are not restricted to being below 60 bpm; in fact, PACs can occur at faster rates, especially in the context of underlying tachycardias. Additionally, the QRS duration in PACs is typically within normal limits (generally less than 0.12 seconds) unless there is an underlying conduction abnormality. Therefore, the differential shape and size of the P wave in PACs is a key feature that helps in identifying this type of arrhythmia.