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.