Cardiac arrhythmias

Pathophysiology and Mechanisms

• Disturbance in heart rate or rhythm.
• Manifests as abnormally fast, slow, or irregular heart rates.
• Etiologies: Congenital, acquired, infectious, toxic, pharmacologic, or genetic.
• Complications: Acutely decreased cardiac output, tachycardia-induced cardiomyopathy, syncope, sudden cardiac death.
• Mechanisms divided into three main groups:
  • Re-entry: Most common mechanism. Requires two distinct pathways, zone of conduction delay, and unidirectional conduction block. Examples: Atrioventricular nodal reentry tachycardia (AVNRT), accessory pathway-mediated atrioventricular reentry tachycardia (AVRT), atrial flutter, ventricular tachycardia.
  • Abnormal Automaticity: Ectopic focus drives depolarization. Examples: Ectopic atrial tachycardia (EAT), junctional ectopic tachycardia (JET).
  • Triggered Activity: Cellular mechanism underlying specific arrhythmias.

Diagnostic Evaluation

• 12-lead Electrocardiogram (ECG): Initial noninvasive diagnostic of choice.
• Ambulatory Monitoring: 24-hour Holter monitor, event monitors, implantable loop recorders (ILR) evaluate arrhythmia burden, correlate symptoms, and assess therapy efficacy.
• Exercise Stress Testing: Elicits catecholamine-sensitive arrhythmias (e.g., Catecholaminergic Polymorphic Ventricular Tachycardia).
• Electrophysiology Study (EPS): Invasive transvenous catheter mapping. Indicated for unexplained syncope, survived sudden cardiac arrest, or pre-ablation mapping.

Bradycardias

• Characterized by slow impulse discharge or conduction failure.

Sinus Node Dysfunction

• Sinus Bradycardia: Sinoatrial node rate below lower limit for age (<90 bpm in neonates, <60 bpm in older children). Physiologic in conditioned athletes. Secondary to hypothermia, hypothyroidism, eating disorders, or high vagal tone.
• Sinus Arrhythmia: Normal physiologic variation. Heart rate slows during expiration, accelerates during inspiration. Abolished by exercise.
• Sick Sinus Syndrome: Abnormalities in sinus node or atrial conduction. Common post-surgical complication (Fontan, Senning, Mustard procedures).

Atrioventricular (AV) Block

• Disturbance in impulse conduction through AV node to ventricles.

Block Type	ECG Characteristics	Clinical Context
First-Degree	Prolonged PR interval beyond upper limit of normal for age.	Often asymptomatic. Found in ~8% of normal children secondary to increased vagal tone.
Second-Degree (Mobitz I / Wenckebach)	Progressive PR prolongation until nonconducted P-wave occurs.	Common in normal children during sleep (high parasympathetic tone) or athletes. Block at AV node level.
Second-Degree (Mobitz II)	Dropped P-waves without progressive PR prolongation.	Indicates significant conduction disease below AV node.
Third-Degree (Complete)	Complete electrical dissociation between atria and ventricles. Independent atrial and ventricular rates.	Congenital: Transplacental transfer of maternal lupus antibodies (Ro-SSA, La-SSB) disrupting fetal conduction. Acquired: Postoperative trauma, myocarditis, Lyme disease.

Narrow QRS Tachycardias (Supraventricular Tachycardias)

• QRS complex <120 ms (adolescents/adults) or <100 ms (children).
• Rapid activation of ventricles via normal His-Purkinje system; originates above or within His bundle.
• Heart rates often >180 bpm in older children, >220 bpm in infants.

Re-entrant Mechanisms

• Atrioventricular Reciprocating Tachycardia (AVRT): Utilizes muscular accessory pathway bypassing AV node. Wolff-Parkinson-White (WPW) syndrome. Orthodromic AVRT (antegrade AV node, retrograde pathway) yields narrow QRS. Antidromic AVRT (antegrade pathway, retrograde AV node) yields wide QRS.
• Atrioventricular Nodal Reentry Tachycardia (AVNRT): Utilizes dual functional pathways (fast and slow) within AV node. Rare in infancy; common in adolescence.
• Atrial Flutter: Macro-reentry circuit within atria. ECG demonstrates regular saw-tooth flutter waves with variable AV conduction. Common in neonates with normal hearts or older children with surgically stretched atria (e.g., Fontan).
• Atrial Fibrillation: Chaotic, rapid atrial excitation (400-700 bpm) producing irregularly irregular ventricular response. Rare in pediatrics. Associated with left-sided CHD, cardiomyopathies, or pre-excitation syndromes.

Automatic Mechanisms

• Sinus Tachycardia: Normal P-wave axis. Rate varies dynamically with parasympathetic/sympathetic tone. Secondary to fever, sepsis, hypovolemia, anemia.
• Ectopic Atrial Tachycardia (EAT): Abnormal focus within atrial myocardium. Abnormal P-wave axis, variable AV conduction. Exhibits gradual acceleration (warm-up) and deceleration (cool-down). Risk of tachycardia-induced cardiomyopathy if incessant.
• Junctional Ectopic Tachycardia (JET): Enhanced automaticity within AV node and proximal His bundle. Often postoperative secondary to surgical trauma, hemorrhage, or edema.

Wide QRS Tachycardias

• QRS >120 ms (adolescents), >100 ms (children).
• Originates from ventricular myocardium or Purkinje cells below His bundle bifurcation.
• Must be considered Ventricular Tachycardia (VT) until proven otherwise.

Ventricular Tachycardia (VT)

• Definition: $\ge$3 consecutive premature ventricular contractions at >120 bpm.
• Morphology: Monomorphic, polymorphic, bidirectional, or torsades de pointes.
• Diagnostic Clues: Ventriculoatrial (VA) dissociation, sinus capture beats, fusion beats.
• Etiology:
  • Structural: Postoperative CHD (Tetralogy of Fallot macro-reentry circuits).
  • Cardiomyopathies: Arrhythmogenic Cardiomyopathy (ACM), Hypertrophic Cardiomyopathy (HCM), Dilated Cardiomyopathy (DCM).
  • Channelopathies: Long QT syndrome, Brugada syndrome, Catecholaminergic Polymorphic Ventricular Tachycardia (CPVT).
  • Inflammatory: Myocarditis.
  • Idiopathic (Benign): Right ventricular outflow tract (RVOT) VT, idiopathic left ventricular (fascicular/Belhassen) tachycardia. Respond well to calcium channel blockers.

Accelerated Ventricular Rhythm

• Benign arrhythmia in infants.
• Monomorphic QRS with rate slightly faster than coexisting sinus rate (within 10%).
• Resolves spontaneously; rarely requires treatment.

Genetic Arrhythmia Syndromes

Syndrome	Pathophysiology	Clinical & ECG Features	Management
Long QT Syndrome (LQTS)	Genetic abnormality of ventricular repolarization.	Prolonged QT interval. Predisposes to torsades de pointes, syncope, sudden death. Exertional/auditory triggers depending on genotype.	Beta-blockers, Left Cardiac Sympathetic Denervation (LCSD), ICD.
Catecholaminergic Polymorphic VT (CPVT)	Calcium handling defect (RYR2 mutations).	Normal resting ECG. Exercise induces monomorphic PVCs progressing to bidirectional/polymorphic VT.	Nadolol, flecainide, LCSD, ICD.
Brugada Syndrome	Sodium channel defect (SCN5A).	Coved ST-segment elevation in V1-V3. Risk of ventricular fibrillation.	Fever avoidance, ICD.
Arrhythmogenic Cardiomyopathy (ACM)	Desmosomal protein mutations (PKP2, DSP, etc.).	Fibrofatty replacement of RV/LV myocardium. Exercise-induced ventricular arrhythmias, sudden death.	Exercise restriction, antiarrhythmics, ICD.

Management and Intervention

Acute Hemodynamic Instability

• Extreme hemodynamic instability necessitates emergency treatment.
• Synchronized Direct Current (DC) Cardioversion: 0.5–2 J/kg for hemodynamically unstable tachyarrhythmias (SVT, Atrial Flutter, VT).
• Defibrillation: For ventricular fibrillation or pulseless VT.

Acute Stable Tachyarrhythmias

• Vagal Maneuvers: Ice bag to face (infants; 15-30 seconds), Valsalva maneuver, bearing down (older children).
• Adenosine: Treatment of choice for stable narrow QRS tachycardia (AVRT, AVNRT). Rapid IV push 0.1–0.2 mg/kg (max initial 6 mg). Blocks AV node conduction.
• Amiodarone / Lidocaine / Procainamide: Initial drugs of choice for hemodynamically stable ventricular tachycardia.

Acute Bradyarrhythmias

• Atropine: 0.02 mg/kg IV bolus for severe sinus bradycardia or AV block with narrow QRS escape.
• Isoproterenol: 0.1–2 mcg/kg/min IV infusion for bradycardia unresponsive to atropine or AV block with wide QRS escape.
• Cardiac Pacing: Transcutaneous or transvenous pacing for symptomatic bradycardia or asystole.

Chronic and Interventional Therapy

• Transcatheter Ablation: Radiofrequency (tissue heating) or cryoablation (tissue freezing). Curative therapy for accessory pathways (WPW), AVNRT, typical atrial flutter, and incessant EAT/PJRT.
• Implantable Cardioverter-Defibrillator (ICD): Secondary prevention for sudden cardiac arrest survivors. Primary prevention for high-risk patients (HCM, ACM, severe dilated cardiomyopathy).
• Permanent Pacemaker: Indicated for symptomatic congenital or acquired complete AV block, and advanced sinus node dysfunction.
• Cardiac Resynchronization Therapy (CRT): Biventricular pacing for selected pediatric heart failure patients with systemic ventricular ejection fraction <35% and wide QRS (or significant pacing requirement) to improve mechanical synchrony.