Source of Junctional Escape Beat
A junctional escape beat originates from the AV node or His bundle. 1
Anatomical Origin
The unifying anatomical feature of all junctional rhythms, including junctional escape beats, is their origin from either:
During electrophysiological studies, each ventricular depolarization in junctional rhythm is preceded by a His bundle deflection, confirming the junctional origin. 1
Cellular Mechanism
Research suggests that the escape rhythm originates specifically from P cells (pacemaker cells) located at the junction of the AV node with the His bundle, which are thought to be the site of normal cardiac automaticity in this region. 3
Post-ablation studies demonstrate that escape rhythms originate in the distal compact AV node or proximal His bundle, as evidenced by their responsiveness to sodium channel blockade (lidocaine) but not calcium channel blockade (verapamil), and their sensitivity to sympathetic stimulation and vagal blockade. 4
Clinical Significance
Because the junctional escape mechanism arises from the AV node or His bundle region, it:
- Produces narrow QRS complexes (unless pre-existing bundle branch block is present) 1, 2, 5
- Typically fires at 40-60 bpm for escape rhythms 2, 5
- Does not require participation of either atrium or ventricle for its propagation 1
- Is more reliable and faster than ventricular escape rhythms (which originate below the His bundle at 20-40 bpm) 1, 5
The location of the junctional escape focus above the bundle branches explains why it is generally more responsive to autonomic manipulation (atropine, isoproterenol, epinephrine) compared to ventricular escape rhythms. 1