The premature junctional complex is an ectopic beat, not a rhythm. An irritable focus in the AV junction stimulates a premature beat that interrupts the underlying rhythm. 

Since it is junctional in origin, the P wave is inverted.

The P wave may:

• Manifest before or after the QRS complex
  • If the P wave manifests before the QRS complex, the PR interval is < 120 milliseconds—shorter than normal. 

• Be hidden within the QRS complex (see Figure 5.2)

Figure 5.2. Premature Junctional Complex

A Premature QRS Complex Without a Preceding P Wave

Characteristics of Premature Junctional Complex 

1. Rhythm regularity dependent on the underlying rhythm
2. Rate dependent on the underlying rhythm
3. An inverted P wave that manifests before, after, or hidden within the QRS complex
4. A shortened PR interval if the P wave is seen before the QRS complex (< 120 milliseconds)
5. A normal QRS complex (< 120 milliseconds)