Description Medical Cardiac Arrhythmias and

Conduction Disorders with CC

Cardiac rhythms that are generated from the area around the atrioventricular (AV) junction node are termed junctional dysrhythmias. For a variety of reasons, the area that surrounds the AV node may generate impulses and become the cardiac pacemaker. Impulses produced in the junction do not necessarily result in an atrial contraction that precedes the ventricular contraction. This lack of coordination leads to a loss of ventricular filling during the last part of diastole; this loss of what is termed the atrial kick may reduce cardiac output by about 20% to 25%.

The inherent rate of the junctional tissue is 40 to 60 beats per minute. When the junctional pacemaker paces at its inherent rate, it produces what is called a passive junctional rhythm or a junctional escape rhythm. When it paces between 60 and 100 beats per minute, the term accelerated junctional rhythm is used. Junctional tachycardia occurs when the junctional pacemaker paces the heart at a rate between 100 and 160 beats per minute. Isolated complexes that arise from the junctional tissue are called premature junctional complexes (PJCs) if they come earlier than the expected sinus beat or junctional escape beats if they come later.

Junctional tissue may take over as the heart's pacemaker if the sinus node fails to produce an impulse or if that impulse is blocked in its conduction through the AV node. Junctional escape rhythms may be caused by digitalis toxicity, acute infections, oxygen deficiency, inferior wall myocardial infarction, or stimulation of the vagus nerve.

If the junctional tissue becomes irritable or increasingly automatic, it may override the sinus node and pace at a faster rate. Nonparoxysmal junctional tachycardia is often the result of enhanced automaticity, usually called irritability, which can be the result of digitalis toxicity, damage to the AV junction after an inferior myocardial infarction or rheumatic fever, or excessive administration of catecholamines or caffeine. Paroxysmal junctional tachycardia (a rapid rhythm that starts and stops suddenly) is usually the result of a re-entry mechanism.

PJCs may be found in healthy individuals, or they may be the result of excessive intake of stimulants such as caffeine, tobacco, or sympathomimetic drugs. Digitalis toxicity or use of alcohol may also cause PJCs. Junctional escape beats occur after pauses in the heart's rhythm. When the sinus node fails to fire, the junctional pacemaker should take over impulse initiation.

Studies are ongoing into the inheritable origins of cardiac rhythm. No information specifically targeting the genetics ofjunctional rhythms is currently available. AV conduction block has been associated with mutations in the sodium channel SCN5A and transcription factors NKX 2.5 and TBX5.

Because of the common causes ofjunctional dysrhythmias, they occur more often in the elderly patient with cardiac disease. They can occur in any age group and in all races and ethnicities.

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