Junctional rhythm results from impulses generated from a locus of tissue in the area of the AV node, otherwise known as the "junction", located between the atria and ventricles. With normal sinus rhythm, the signal originates in the SA node, and depolarizes/contracts the atria (seen as a P-wave). The signal then passes through the AV node, the bundle of His, then traveling along the Purkinje fibers to depolarize the ventricles (QRS). The atria in regular fashion, contracts in sequence before the ventricles. However, with junctional rhythm the SA node does not control the heart's pacing. This abnormal rhythm can happen from a block in conduction somewhere along the pathway. When this happens, the heart's AV node takes over as the pacemaker, generating a Junctional escape rhythm usually at a rate of 40 to 60 bpm. Why does the P-wave follow the QRS? Why would it sometimes be hidden in the QRS? It's because when the AV node takes over, it causes "retrograde conduction". The AV node (Junction) sends the impulse to the ventricles creating this Junctional escape rhythm. However, the atria will actually still contract before the ventricles. Although this does not happen by the normal pathway of activation, and instead is due to conduction coming from the AV node back up through the atria, known as "retrograde conduction" In scope, the AV node paces the ventricles and also sends an impulse backwards up to contract the atria in the same sequence. The ventricles contract (QRS) at the same time the atria contract (P-wave), thus seen as a hidden P-wave on a graph. Or the atria contract slightly following the ventricles, seen as a retrograde P-wave which follow the QRS. Causes and symptoms: The heart rate of junctional rhythm determines if the patient has symptoms (too slow, too fast). Junctional rhythms are common in patients with sick sinus syndrome. Severe bradycardia. If the sinus node slows, or the intrinsic rate of the AV node elevates. Exit blocks or SA blocks. Digitalis toxicity may cause junctional or accelerated junctional rhythms to occur. Hyperkalemia Drugs such as beta blockers or calcium channel blockers AV disassociation can lead to symptoms and possible 3rd degree heart block. Periods of junctional rhythm are not necessarily associated with an increase in mortality. If an obvious cause is present, such as complete heart block or sick sinus syndrome, then the morbidity or mortality is directly related to that and not to the junctional rhythm mechanism, which is serving as a "backup rhythm" during the periods of bradycardia.

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