Premature Junctional Contractions: How to Differentiate Them from PACs and PVCs

Premature junctional contractions (PJCs) are often confused with premature atrial contractions (PACs) because both typically produce a narrow QRS complex. However, the key difference lies in where the impulse originates and what happens to the P wave.

What Is a PJC?

A PJC originates from an ectopic focus within the AV junction. Because ventricular depolarization still travels through the normal His-Purkinje system, the QRS complex is usually narrow.

The distinguishing feature is the P wave.

Since the impulse begins in the AV junction, the atria are activated retrogradely (backward) rather than from the SA node. As a result, the P wave may be:

• Absent (hidden within the QRS)
• Inverted before the QRS (if the atria depolarize first)
• Inverted after the QRS (if the ventricles depolarize first)

Recognizing these abnormal P-wave relationships is often the easiest way to identify a PJC.

How Is It Different from a PAC?

Like a PJC, a PAC also produces a narrow QRS complex because ventricular conduction still occurs through the His-Purkinje system.

The difference is that a PAC originates within the atria, so it is almost always preceded by an early P wave. 

In contrast, a PJC lacks a normal P wave, making the AV junction the likely origin.

How Is It Different from a PVC?

A PVC is usually much easier to recognize.

Because it originates within the ventricular myocardium, ventricular depolarization spreads slowly from myocyte to myocyte rather than through the His-Purkinje system. The result is a wide, bizarre-appearing QRS complex that is typically not preceded by a P wave.

Unlike PJCs and PACs, the wide QRS immediately suggests a ventricular origin.