Cardiac arrhythmias

Pathophysiology and Mechanisms

Cardiac arrhythmias in the pediatric population arise from three primary mechanisms: reentry, abnormal automaticity, and triggered activity.
Reentry is the most common mechanism underlying the majority of supraventricular and ventricular tachycardias.
For a reentrant circuit to establish, there must be two distinct functional or anatomical pathways separated by a zone of conduction block, a region of conduction delay, and unidirectional conduction block within one limb of the circuit.
Reentrant tachycardias are characterized by a sudden, abrupt onset and termination, a regular tachycardia cycle length, and the ability to be induced or terminated with programmed electrical stimulation.
Automatic tachycardias (such as ectopic atrial tachycardia) originate from an abnormal focus remote from the sinus node, demonstrating a gradual acceleration ("warm-up") at onset and a gradual deceleration ("cool-down") prior to termination.

Classification and Differential Diagnosis

Arrhythmia Category	Key Etiologies and Associations	Specific Diagnostic Features
Bradycardias	Sinus bradycardia (high vagal tone, hypothyroidism, long QT syndrome); First-degree AV block; Second-degree AV block (Mobitz I/II); Complete (Third-degree) AV block.	Congenital complete AV block is heavily associated with maternal Ro-SSA/La-SSB antibodies (neonatal lupus) or L-looped transposition of the great arteries (L-TGA). Acquired AV block often follows surgical repair near the AV node (e.g., VSD or AV canal repair).
Narrow QRS Tachycardias	Atrioventricular Reentrant Tachycardia (AVRT); AV Nodal Reentrant Tachycardia (AVNRT); Ectopic Atrial Tachycardia (EAT); Atrial Flutter; Atrial Fibrillation; Junctional Ectopic Tachycardia (JET).	QRS duration <100 ms in children (<120 ms in adolescents). Arises above or within the His bundle.
Wide QRS Tachycardias	Ventricular Tachycardia (VT); Antidromic AVRT; SVT with aberrancy.	QRS duration >100 ms in children (>120 ms in adolescents). Pathologies include repaired Tetralogy of Fallot, channelopathies (Long QT, Brugada, CPVT), and cardiomyopathies (ARVC, HCM).

Characteristics of Specific Arrhythmias

Atrioventricular Reentrant Tachycardia (AVRT): Uses an accessory pathway and is the most common mechanism of supraventricular tachycardia (SVT) in infants. In orthodromic AVRT, the impulse travels antegrade down the AV node and retrograde up the accessory pathway, producing a narrow QRS complex, a retrograde P-wave with a negative axis in leads II, III, and aVF, and a ventriculoatrial (VA) interval >70 ms. Wolff-Parkinson-White (WPW) syndrome is a subset of AVRT where the resting ECG shows a short PR interval and a slurred QRS upstroke (delta wave) due to antegrade preexcitation.
Atrioventricular Nodal Reentrant Tachycardia (AVNRT): Utilizes dual functional pathways (slow and fast) within the AV node itself, typically presenting in adolescence. The ECG demonstrates a narrow QRS with retrograde P waves either buried within the QRS or manifesting with a short VA interval (<70 ms).
Atrial Flutter: A macroreentrant circuit often rotating around the tricuspid valve annulus. It is exceptionally rare in structurally normal hearts outside the neonatal period, but highly prevalent following congenital heart surgeries involving extensive atrial scarring (Mustard, Senning, and Fontan procedures). The ECG classically shows a "saw-tooth" flutter wave pattern best visualized in leads II, III, and aVF (and positive in V1) at an atrial rate of approximately 300 bpm with variable AV conduction block.
Ventricular Tachycardia (VT): Defined as three or more consecutive beats of ventricular origin at a rate >120 bpm. The hallmark diagnostic features on the ECG include atrioventricular (VA) dissociation, where the atrial rate is completely independent of the faster ventricular rate, and the presence of occasional sinus capture beats or fusion beats. Torsades de Pointes is a specific polymorphic VT pattern associated with long QT syndrome characterized by an irregular QRS rotating around an imaginary baseline.

Clinical Manifestations

Clinical presentations vary dramatically based on the patient's age, ventricular function, and the specific arrhythmia rate.
Infants frequently present with non-specific signs of congestive heart failure, such as poor feeding, feeding refusal, extreme irritability, tachypnea, pallor, and diaphoresis with feeds.
If fetal tachycardia occurs in utero, prolonged episodes can lead to hydrops fetalis (the fetal manifestation of severe heart failure) and intrauterine demise.
Older children and adolescents are more likely to complain of distinct episodic palpitations, chest discomfort, fatigue, exercise intolerance, or lightheadedness.
Extreme presentations include sudden unresponsiveness, syncope, generalized seizures due to cerebral hypoperfusion, cardiogenic shock, and sudden cardiac death.
Incessant tachycardias (such as ectopic atrial tachycardia or permanent junctional reciprocating tachycardia) maintained for weeks or months can induce profound, but often reversible, left ventricular systolic dysfunction known as tachycardia-induced cardiomyopathy.

Diagnostic Investigations

Modality	Specific Clinical Findings and Utility
Electrocardiogram (ECG)	Essential for determining the rate, P wave axis/morphology, QRS duration, PR interval, and QTc interval. Can reveal specific substrates like WPW (delta waves), Brugada syndrome (coved ST-segment elevation in V1-V2), or Long QT syndrome (QTc > 0.47 seconds, notched T waves).
Ambulatory ECG (Holter/Event Monitor)	Captures ephemeral, paroxysmal arrhythmias and quantifies premature ventricular contraction (PVC) burden over 24-48 hours. Implantable loop recorders (ILRs) are utilized for highly infrequent syncope presentations.
Echocardiography	Required to evaluate for underlying structural congenital heart disease, assess biventricular systolic and diastolic function, and screen for tachycardia-induced cardiomyopathy. Essential to rule out intracardiac thrombus via transesophageal echo (TEE) prior to cardioverting atrial flutter/fibrillation lasting >48 hours.
Invasive Electrophysiology (EP) Study	Utilized for precise arrhythmia mapping and risk stratification. Intracardiac catheters record specific conduction intervals, including the AH interval (conduction time over the AV node) and the HV interval (conduction time over the His-Purkinje system). Programmed electrical stimulation is used to induce tachycardias or assess the shortest preexcited R-R interval in WPW patients.
Exercise Stress Testing	Instrumental for diagnosing adrenergically mediated arrhythmias, specifically Catecholaminergic Polymorphic Ventricular Tachycardia (CPVT), which classically presents with a progression from PVCs to bidirectional or polymorphic VT during peak exertion.

Management

Acute Management of Hemodynamic Instability

Extreme hemodynamic instability (e.g., hypotension, altered mental status, signs of shock) necessitates immediate emergency intervention.
Synchronized direct current (DC) cardioversion (0.5 to 2 J/kg) is the absolute treatment of choice for hemodynamically unstable patients with tachyarrhythmias, preceded by sedation with a short-acting benzodiazepine if vascular access and clinical status permit.
For hemodynamically unstable bradycardia, management follows Pediatric Advanced Life Support (PALS) algorithms, utilizing intravenous epinephrine, atropine (0.02 mg/kg, for primary AV block or high vagal tone), or isoproterenol infusions.
Transcutaneous pacing or the placement of temporary transvenous pacing wires must be rapidly instituted if profound symptomatic bradycardia is unresponsive to pharmacotherapy.

Acute Management of Stable Tachycardias

In stable infants with SVT, vagal stimulation can be achieved by placing an ice bag over the upper face for 15 to 30 seconds to elicit the diving reflex.
Older children can be instructed to perform Valsalva maneuvers, bearing down, or blowing into a syringe; ocular pressure is strictly contraindicated.
If vagal maneuvers fail, rapid intravenous push of Adenosine (0.1 mg/kg, maximum initial dose 6 mg; followed by 0.2 mg/kg if needed) is highly effective at terminating AV node-dependent tachycardias (AVRT, AVNRT) by inducing transient potassium-mediated AV nodal block.
Intravenous Amiodarone, Procainamide, or Lidocaine are the primary pharmacological therapies for hemodynamically stable ventricular tachycardia.
Verapamil is strictly contraindicated in infants younger than 1 year due to the risk of profound negative inotropy, severe hypotension, and cardiovascular collapse.

Long-Term and Interventional Management

Medical prophylaxis for recurrent arrhythmias frequently relies on beta-blockers (e.g., Propranolol, Metoprolol), Class III agents (Amiodarone, Sotalol), or Class IC agents (Flecainide, Propafenone).
Transcatheter ablation (using radiofrequency energy or cryoablation) is the definitive curative therapy of choice for recurrent AVRT, AVNRT, ectopic atrial tachycardia, and typical atrial flutter, demonstrating high success rates (>90%) in pediatric EP laboratories.
Permanent epicardial or transvenous pacemaker implantation is indicated for symptomatic sinus node dysfunction or complete, irreversible AV block (persisting >10-14 days following cardiac surgery or secondary to congenital heart block).
Implantable Cardioverter-Defibrillator (ICD) placement is indicated for both secondary prevention in survivors of sudden cardiac arrest and primary prevention in high-risk patients with hypertrophic cardiomyopathy, long QT syndrome, arrhythmogenic right ventricular cardiomyopathy (ARVC), and specific repaired congenital heart defects (e.g., Tetralogy of Fallot with extensive RV scarring and prolonged QRS >180 ms).