• A premature junctional contraction (PJC) is generated by an irritable impulse from a junctional automaticity focus that is a bit earlier than the expected sinus impulse. As a result, the expected sinus impulse cannot be conducted because His bundle will enter refractory period once it reaches it.
• Because the junctional automaticity focus is located in the lower part of the AV junction (a distant way from atrial cells), its impulse might not reach the atria and this often times result in no P wave.
• When impulse from junctional focus reaches the atria in a retrograde fashion, it causes atrial depolarization and retrograde P waves (negatively deflected in inferior leads I, III, aVF and positively deflected in lead V1).
• Since conduction down the His-Purkinje pathway remains intact, QRS complex is normal and narrow.
• The next sinus impulse that follows the premature junctional beat will resume normal sinus rhythm.
• Coupling of a premature junctional beat to one or two normal sinus beat(s) are called a junctional bigeminy or junctional trigeminy, respectively.
• Each coupling is called “couplet”.
