What are the causes of junctional tachycardia and Adenomatous Polyposis Coli (APC)?

Causes of Junctional Tachycardia

Focal Junctional Tachycardia

Focal junctional tachycardia in adults is typically exercise or stress-related and may occur in structurally normal hearts or in patients with congenital abnormalities such as atrial or ventricular septal defects. 1

Primary Mechanisms

• Enhanced automaticity of a high junctional focus is the underlying mechanism, distinguishing it from reentrant arrhythmias 1, 2
• The arrhythmia originates from the AV node or His bundle, with each ventricular depolarization preceded by a His bundle deflection 3

Clinical Context

• This form appears to be an adult extension of pediatric "congenital junctional ectopic tachycardia" but presents more benignly than the pediatric form 1
• Usually presents in young adulthood with rates of 110-250 bpm 3
• Patients are often highly symptomatic and may develop heart failure if the tachycardia is incessant and untreated 1

Associated Structural Abnormalities

• Structurally normal hearts (most common in adults) 1
• Congenital heart defects including atrial septal defects 1
• Ventricular septal defects 1

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Nonparoxysmal Junctional Tachycardia

The most critical feature of nonparoxysmal junctional tachycardia is that it serves as a marker for serious underlying conditions requiring immediate identification and correction. 1

Primary Causes (in order of clinical importance)

Digitalis Toxicity

• The most important reversible cause to identify, as withholding digitalis when junctional tachycardia is the only manifestation is usually adequate 1, 3
• May present with anterograde AV-nodal Wenckebach conduction block in the setting of digitalis toxicity 1

Post-Cardiac Surgery

• Occurs within 72 hours after cardiac surgery, termed postoperative JET (POJET) 4
• Caused by direct trauma, ischemic injury, or stretch injury to the AV conduction tissues during surgical repair of congenital heart defects 4
• Most frequently occurs after tetralogy of Fallot repair (21.9% incidence) 5
• Resection of muscle bundles is significantly more arrhythmogenic than simple division (P <0.0001) 5
• Relief of right ventricular outflow tract obstruction through the right atrium increases risk (P <0.05) 5
• Higher bypass temperatures independently predict postoperative junctional ectopic tachycardia (P <0.03) 5

Electrolyte Abnormalities

• Hypokalemia is a major precipitant 1, 3
• Hypomagnesemia increases risk, particularly in the postoperative setting 6

Myocardial Ischemia

• Altered automaticity from ischemic injury to the AV junction 1, 3

Other Metabolic/Inflammatory Conditions

• Chronic obstructive lung disease with hypoxia 1, 3
• Inflammatory myocarditis affecting the conduction system 1, 3

Sinus Node Dysfunction

• In rare cases, sympathetic stimulation of AV-junction automaticity leads to a junctional rhythm that supersedes the sinus rhythm 1
• This can cause symptoms mimicking "pacemaker syndrome" due to retrograde conduction and resultant cannon A waves with possible hypotension 1, 7

Mechanism

• Enhanced automaticity arising from a high junctional focus or triggered mechanism 1
• Characterized by typical "warm-up" and "cool-down" patterns that cannot be terminated by pacing maneuvers 1, 3
• Heart rates of 70-120 bpm distinguish it from the more rapid focal form 1, 3

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Postoperative Risk Factors

Patient-Related Factors

• Young age (neonatal and infant age groups) carries the highest risk 6
• Congenital JET (CJET) has a 35% mortality rate in pediatric cases 4

Surgical Factors

• Higher-complexity surgical procedures, especially those near the atrioventricular node or His bundle 6
• Relief of right ventricular outflow tract obstruction appears more important in causation than ventricular septal defect closure alone 5
• Excessive traction through the right atrium during surgery 5

Pharmacologic Factors

• Use of exogenous catecholamines such as dopamine and epinephrine increases risk 6

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Common Pitfalls

• Do not assume all narrow complex tachycardias are AVNRT or AVRT—the clinical setting (particularly recent cardiac surgery, digitalis use, or electrolyte abnormalities) should raise suspicion for junctional tachycardia 1
• Failure to identify digitalis toxicity early can lead to progression to ventricular arrhythmias or high-grade heart block 1
• In postoperative patients, recording the atrial electrogram using temporary epicardial pacemaker leads is especially useful for diagnosing junctional arrhythmias in children after congenital heart surgery 3
• When junctional rhythm results from sinus node dysfunction, atrial pacing is the definitive treatment, not suppressive antiarrhythmic therapy 1, 7