A junctional rhythm occurs when the electrical activation of the heart originates near or within the atrioventricular node, rather than from the sinoatrial node. Because the normal ventricular conduction system (His-Purkinje) is used, the QRS complex is frequently narrow. A junctional rhythm is normally slow — less than 60 beats per minute. When faster, it is referred to as an accelerated junctional rhythm.
Because the electrical activation originates at or near the AV node, the P wave is frequently not seen; it can be buried within the QRS complex, slightly before the QRS complex or slightly after the QRS complex. The morphology of the P wave will not be similar to the sinus P wave, which is normally upright in lead II and biphasic in lead V1. Often, the P wave is inverted in lead II, if it can be seen at all. A pacemaker may be needed to relieve symptoms when no reversible cause — i.e. AV blocking medications or electrolyte disturbances — is found.
The strip below shows a junctional rhythm with retrograde P waves seen just before the QRS complex. The second rhythm strip shows retrograde P waves just after the QRS complex.
1. Chou’s Electrocardiography in Clinical Practice: Adult and Pediatric, 6e
2. Surawicz B, et al. AHA/ACCF/HRS Recommendations for the Standardization and Interpretation of the Electrocardiogram. Circulation. 2009;doi:10.1161/CIRCULATIONAHA.108.191095.