# Pediatric Arrhythmias and Syncope

## Overview

Arrhythmias are common in pediatric practice, ranging from benign conditions like sinus arrhythmia to life-threatening events like ventricular fibrillation. Supraventricular tachycardia (SVT) is the most common pathologic arrhythmia in children. Syncope is extremely common in adolescents, with 15-25% experiencing at least one episode; the vast majority is vasovagal in origin. The critical clinical skill is distinguishing benign arrhythmias and vasovagal syncope from cardiac causes of sudden death.

## Supraventricular Tachycardia (SVT)

### Mechanism

SVT presents as a narrow complex tachycardia (QRS less than 120 ms) with heart rates typically 220-300 bpm in infants and 150-250 bpm in older children. The most common mechanism is atrioventricular reentrant tachycardia (AVRT) via an accessory pathway, accounting for 60-70% of pediatric SVT. The second most common is AV nodal reentrant tachycardia (AVNRT), which is more prevalent in older children and adolescents. Less common mechanisms include ectopic atrial tachycardia, atrial flutter, and junctional ectopic tachycardia.

### Clinical Presentation

Infants with SVT present with irritability, poor feeding, pallor, diaphoresis, and tachypnea. If the arrhythmia is sustained beyond 24 hours, heart failure may develop. The diagnosis is often discovered incidentally or after heart failure has already occurred. Older children report palpitations, chest discomfort, dizziness, and a sensation of the heart racing with characteristically abrupt onset and termination. Hemodynamically unstable patients present with hypotension, altered consciousness, and signs of shock.

### Acute Management

#### Stable Patient

Vagal maneuvers are the first-line intervention. In infants, an ice bag applied to the face for 15-20 seconds triggers the diving reflex (with care not to cover the nose and mouth simultaneously). Older children can perform Valsalva maneuvers, bear down, blow through a straw, or do a handstand. Modified Valsalva with leg elevation has shown improved conversion rates. Success rates range from 20-40%.

If vagal maneuvers fail, adenosine is given at 0.1 mg/kg IV as a rapid push (maximum first dose 6 mg), which can be increased to 0.2 mg/kg (maximum 12 mg). Adenosine must be given as a rapid bolus followed by an immediate saline flush because of its extremely short half-life (10-15 seconds). The IV closest to the heart should be used, and a two-syringe stopcock technique is helpful. The drug briefly blocks AV node conduction, terminating reentrant circuits. Older children should be warned about transient chest tightness and flushing. A rhythm strip should be recorded during administration for diagnostic purposes.

#### Unstable Patient

Synchronized cardioversion at 0.5-1 J/kg is performed, increasing to 2 J/kg if the initial attempt fails. Sedation with ketamine or midazolam should be provided if time permits, but cardioversion should not be delayed for sedation in a critically unstable patient.

### Chronic Management

Infants with a first episode receive prophylactic therapy for 6-12 months (due to high recurrence rates, though many outgrow the arrhythmia by 1 year). Propranolol at 1-3 mg/kg/day divided three to four times daily is first-line, with flecainide or sotalol as alternatives offering higher efficacy but more side effects. Older children with recurrent SVT are best treated with catheter ablation, which is the preferred definitive treatment with success rates exceeding 95% for both AVRT and AVNRT. The complication rate is low (1-3%, including AV block and perforation), and the procedure is generally deferred until the child weighs more than 15-20 kg to reduce complication risk.

<image>Comparison of ECG tracings showing normal sinus rhythm, narrow complex SVT at 260 bpm in an infant, Wolff-Parkinson-White pattern with delta wave and short PR interval during sinus rhythm, and the response to adenosine with transient AV block terminating SVT</image>

## Wolff-Parkinson-White (WPW) Syndrome

### Definition

WPW is defined by a pre-excitation pattern on ECG: short PR interval (less than 120 ms in children), delta wave (slurred upstroke of QRS), and widened QRS. This pattern is caused by an accessory pathway (bundle of Kent) that bypasses the AV node. The WPW pattern refers to ECG findings without symptoms, while WPW syndrome indicates ECG findings with symptomatic tachycardia.

### Risk of Sudden Death

The risk of sudden death is rare (less than 0.5% per year) but feared. The mechanism involves atrial fibrillation conducting rapidly over the accessory pathway to the ventricles, degenerating into ventricular fibrillation. Risk is stratified by electrophysiology study, with a short antegrade refractory period of the accessory pathway (less than 250 ms) indicating high risk. Digoxin and verapamil must be avoided in WPW because they can enhance conduction over the accessory pathway and precipitate ventricular fibrillation.

### Management

Symptomatic WPW is definitively treated with catheter ablation. Asymptomatic WPW undergoes risk stratification with electrophysiology study, with ablation recommended for high-risk pathways. Exercise testing can provide useful information: intermittent pre-excitation (delta wave disappearing with exercise) suggests low risk.

## Long QT Syndrome (LQTS)

### Definition

LQTS is an inherited channelopathy causing prolonged ventricular repolarization, predisposing to polymorphic ventricular tachycardia (Torsades de Pointes) and sudden cardiac death. A corrected QT (QTc) greater than 470 ms in males or greater than 480 ms in females is considered prolonged (using the Bazett formula). A QTc exceeding 500 ms indicates high risk.

### Types

| Type | Gene | Frequency | Trigger | ECG T-Wave Pattern | Channel Defect |
|------|------|-----------|---------|-------------------|----------------|
| LQT1 | KCNQ1 | 40-55% | Exercise (swimming) | Broad-based | K+ channel loss-of-function |
| LQT2 | KCNH2 (HERG) | 30-40% | Emotional stress, auditory stimuli | Notched/bifid | K+ channel loss-of-function |
| LQT3 | SCN5A | 5-10% | Rest/sleep | Flat/late onset | Na+ channel gain-of-function |

LQT1 (KCNQ1 mutations) is the most common type (40-55%), triggered by exercise (especially swimming), with broad T-waves on ECG. LQT2 (KCNH2/HERG mutations) is triggered by emotional stress and sudden auditory stimuli (alarm clocks), showing notched or bifid T-waves. LQT3 (SCN5A mutations) involves events occurring at rest or during sleep, with flat or late T-waves, caused by sodium channel gain-of-function. All types follow autosomal dominant inheritance with variable penetrance.

### Clinical Presentation

Patients may present with syncope (especially with exertion, emotional stress, or during sleep), seizures (often misdiagnosed as epilepsy), cardiac arrest, or sudden death. A family history of unexplained drowning, sudden death in young relatives, or "seizure disorder" is an important clue. Some patients are asymptomatic with LQTS discovered incidentally on ECG.

### Management

Beta-blockers are first-line for all symptomatic LQTS and asymptomatic LQT1/LQT2 patients. Nadolol (preferred) at 1-2 mg/kg/day reduces cardiac events by 60-70%. Activity restriction varies by genotype: LQT1 patients should avoid competitive swimming, LQT2 patients should avoid alarm clocks and sudden loud noises, and LQT3 patients should avoid drugs that prolong QT. ICD implantation is indicated for cardiac arrest survivors, recurrent syncope despite beta-blockers, and QTc greater than 500 ms with additional risk factors. Left cardiac sympathetic denervation is adjunctive for ICD-refractory cases. QT-prolonging drugs (complete list at CredibleMeds.org, including macrolides, fluoroquinolones, high-dose ondansetron, methadone, and many antipsychotics) must be avoided.

### Drug-Induced QT Prolongation

Many common medications prolong the QT interval. QTc should always be checked before prescribing QT-prolonging agents, especially in patients with a family history of sudden death. Electrolyte abnormalities (hypokalemia, hypomagnesemia, hypocalcemia) exacerbate QT prolongation.

## Other Important Arrhythmias

### Premature Atrial Contractions (PACs)

PACs are extremely common in neonates and children and are almost always benign, requiring no treatment unless they cause sustained tachycardia.

### Premature Ventricular Contractions (PVCs)

PVCs are common and usually benign if unifocal, disappearing with exercise, without structural heart disease, and with normal LV function. Worrisome features include multifocal morphology, couplets or triplets, increase with exercise, and association with structural heart disease. If PVC burden exceeds 10-15%, there is risk of PVC-induced cardiomyopathy, and ablation should be considered.

### Ventricular Tachycardia (VT)

VT is a wide complex tachycardia (QRS 120 ms or greater) with 3 or more consecutive ventricular beats. Causes include channelopathies (LQTS, CPVT, Brugada), HCM, myocarditis, ARVC, drug toxicity, and electrolyte abnormalities. Stable VT is treated with amiodarone 5 mg/kg IV over 20-60 minutes, with lidocaine as an alternative. Pulseless VT or VF requires CPR plus defibrillation at 2 J/kg then 4 J/kg, with epinephrine and amiodarone per PALS protocols.

### Complete Heart Block

Congenital complete heart block is associated with maternal anti-Ro/La antibodies (neonatal lupus) and structural CHD (L-TGA). Acquired causes include post-cardiac surgery, myocarditis, and Lyme disease. A permanent pacemaker is indicated if symptomatic, if resting heart rate is below 55 bpm in infants, if the escape rhythm has a wide QRS, or if ventricular dysfunction develops.

## Approach to Pediatric Syncope

### Vasovagal Syncope (Most Common — 75-80%)

Vasovagal syncope has a neurocardiogenic mechanism triggered by prolonged standing, heat, emotional stress, or dehydration. The prodrome includes lightheadedness, nausea, diaphoresis, vision changes, and warmth. Loss of consciousness is brief (seconds to 1-2 minutes) with rapid recovery without prolonged confusion. Brief myoclonic jerks (convulsive syncope) may occur but do not indicate epilepsy. The cardiac exam and ECG are normal. Management consists of reassurance, education about triggers, adequate hydration (2-3 L/day), salt supplementation, counter-pressure maneuvers (leg crossing, squatting), and slow position changes.

### Cardiac Syncope (RED FLAGS)

Red flags for cardiac syncope include syncope during exertion (not just after), absence of prodrome (sudden loss of consciousness), chest pain or palpitations preceding syncope, family history of sudden death under age 40, known channelopathy or cardiomyopathy, abnormal cardiac examination (murmur of HCM or AS), and abnormal ECG (prolonged QTc, pre-excitation, Brugada pattern, hypertrophy).

### Workup

All syncope patients require a detailed history, orthostatic vitals, cardiac examination, and 12-lead ECG. Additional testing for patients with red flags includes echocardiography, Holter monitor, exercise stress test, and event monitor. Tilt table testing has low sensitivity and specificity with limited utility in pediatrics. EEG should be obtained only if seizure is strongly suspected clinically; syncope does not require routine EEG.

<image>Diagnostic algorithm for pediatric syncope showing initial assessment with history, orthostatic vitals, and ECG, branching into vasovagal syncope (reassurance, hydration, salt) versus cardiac syncope red flags (exertional, no prodrome, family history of SCD, abnormal ECG) requiring echocardiography, Holter monitoring, and possible EP study</image>

## ECG Screening in Young Athletes

Sudden cardiac death in young athletes is rare (1-3 per 100,000 athlete-years) but devastating. Causes include HCM (36%), anomalous coronary arteries (17%), ARVC, LQTS, WPW, and commotio cordis. The Italian experience in the Veneto region showed that mandatory ECG screening reduced SCD by 89% over 25 years.

Arguments for screening include that ECG is inexpensive and detects most channelopathies and HCM, identification allows potentially life-saving intervention, and the Italian data demonstrates dramatic SCD reduction. Arguments against include high false-positive rates (5-25%, especially from training-related ECG changes in athletes), low positive predictive value given the rarity of SCD, unclear cost-effectiveness in the US healthcare system, psychological impact of false positives, and insufficient infrastructure for follow-up. The AHA recommends a 14-element history and physical screening but not universal ECG screening, while the ESC recommends ECG for competitive athletes.

## Clinical Pearls

SVT in infants can present as heart failure if sustained, and SVT should always be in the differential for an irritable, poorly feeding infant. Ice to the face is the preferred vagal maneuver in infants and should never involve ocular pressure. In WPW with atrial fibrillation, verapamil, diltiazem, and digoxin must never be given because they enhance accessory pathway conduction and can cause ventricular fibrillation. A family history of "seizures" or drowning in a young relative should raise suspicion for LQTS. Syncope with exertion is cardiac until proven otherwise, while syncope after exertion or prolonged standing is usually vasovagal. An ECG should be obtained in every child presenting with syncope.

## Key Controversy: ECG Screening in Young Athletes

No randomized controlled trial has been performed. Italian data is compelling but comes from a single-payer system with established infrastructure. US implementation faces logistical, financial, and follow-up challenges. Current US practice uses comprehensive history and physical per the AHA 14-element screening, with ECG added at some institutions. The likely future direction involves targeted ECG screening in higher-risk populations or with improved ECG interpretation algorithms including AI-assisted analysis.

## References
- Brugada J, et al. Pharmacological and Non-Pharmacological Therapy for Arrhythmias in the Pediatric Population (ESC Guidelines). Europace. 2013;15(9):1337-1382.
- Priori SG, et al. HRS/EHRA/APHRS Expert Consensus Statement on the Diagnosis and Management of Patients with Inherited Primary Arrhythmia Syndromes. Heart Rhythm. 2013;10(12):1932-1963.
- Saarel EV, et al. Evaluation and Management of Syncope in Children and Adolescents. Pediatr Rev. 2020;41(11):579-592.
- Maron BJ, et al. Assessment of the 12-Lead ECG as a Screening Test for Detection of Cardiovascular Disease in Athletes. Circulation. 2014;130(16):1303-1314.
- Corrado D, et al. Trends in Sudden Cardiovascular Death in Young Competitive Athletes After Implementation of a Preparticipation Screening Program. JAMA. 2006;296(13):1593-1601.
- Schwartz PJ, et al. Inherited Cardiac Arrhythmias. Lancet. 2012;380(9852):1520-1529.