Final answer:
The P wave is key to differentiating between Premature Junctional Contractions (PJCs) and Premature Atrial Contractions (PACs) on an electrocardiogram (ECG). PACs typically show an early and aberrant P wave, while PJCs may have an absent or retrogradely placed P wave due to the origin of the beat in the AV junction.
Step-by-step explanation:
A primary waveform used to differentiate Premature Junctional Contractions (PJCs) and Premature Atrial Contractions (PACs) is the P wave. The P wave represents atrial depolarization in an electrocardiogram (ECG). In the case of PACs, the P wave appears earlier than normal and may have a different shape compared to a normal P wave because the premature beat originates from an ectopic focus in the atria. However, with PJCs, the P wave can either be absent, or it may appear before, during, or after the QRS complex as the premature beat originates in the AV junction, which can depolarize the atria retrogradely. PJCs typically result in a P wave that has a morphology different from the P waves originating from the sinus node, which is the heart's natural pacemaker. In some cases, the P wave may be buried within the QRS complex and not visible, which can also distinguish PJCs from PACs.