8.3 Pediatric Arrhythmias & Syncope Management

8.3 Pediatric Arrhythmias & Syncope Management

Cardiac arrhythmias in pediatric patients range from benign, age-appropriate physiologic fluctuations to life-threatening tachyarrhythmias and channelopathies that cause sudden cardiac arrest. Among all sustained pediatric arrhythmias, supraventricular tachycardia (SVT) is by far the most frequent, accounting for over 90% of symptomatic tachycardias in infants and children. Prompt recognition, electrophysiologic characterization, and rapid clinical management are essential components of pediatric clinical pharmacy practice.

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Electrophysiology & Mechanisms of Pediatric SVT

SVT is defined as an abnormally rapid cardiac rhythm originating from or dependent upon tissue above the bifurcation of the bundle of His. On 12-lead electrocardiography (ECG), it is classically characterized by a rapid, regular rhythm with narrow QRS complexes ($< 0.08\text{ seconds}$ in infants; $< 0.09\text{ seconds}$ in children). Ventricular rates typically exceed $220\text{ beats per minute (bpm)}$ in infants and $180\text{ bpm}$ in children.

Distinguishing Sinus Tachycardia from SVT

Electrophysiologic Mechanisms

                      Classification of Pediatric SVT Mechanisms

                                  ┌─────────────┐
                                  │Pediatric SVT│
                                  └──────┬──────┘
                                         │
               ┌─────────────────────────┴────────────────────────┐
               ▼                                                  ▼
   ┌───────────────────────┐                          ┌───────────────────────┐
   │  Reentrant Mechanisms │                          │  Automatic / Ectopic  │
   │     (~90% of cases)   │                          │    (~10% of cases)    │
   └───────────┬───────────┘                          └───────────┬───────────┘
               │                                                  │
     • Atrioventricular Reentrant                       • Ectopic Atrial Tachycardia (EAT)
       Tachycardia (AVRT): ~70-80%                      • Junctional Ectopic Tachycardia (JET)
       - Involves accessory pathway                       (common post-cardiac surgery)
       - Wolff-Parkinson-White (WPW)                    • Permanent Form of Junctional
       - Concealed accessory pathway                      Reciprocating Tachycardia (PJRT)
     • AV Nodal Reentrant Tachycardia
       (AVNRT): ~15-20%
       - Dual AV nodal pathways (fast/slow)

1. Atrioventricular Reentrant Tachycardia (AVRT):
   • Accounts for 70% to 80% of SVT in neonates and infants.
   • Utilizes an extranodal muscular connection (accessory pathway / bundle of Kent) bridging the atrium and ventricle. In orthodromic AVRT, conduction travels anterograde down the normal AV node-His-Purkinje system (yielding a narrow QRS) and retrogradely up the accessory pathway back to the atrium.
   • Wolff-Parkinson-White (WPW) Syndrome: Manifest accessory pathway exhibiting anterograde pre-excitation during normal sinus rhythm. Baseline ECG shows a short PR interval, a slurred upstroke of the QRS (delta wave), and a widened QRS. WPW occurs in 15% to 20% of pediatric SVT patients.
2. Atrioventricular Nodal Reentrant Tachycardia (AVNRT):
   • Accounts for 15% to 20% of pediatric SVT overall, becoming increasingly prevalent in adolescents.
   • Involves two functionally distinct pathways within the AV node itself: a slow pathway (short refractory period) and a fast pathway (long refractory period).
3. Automatic & Ectopic Tachycardias:
   • Driven by abnormal automaticity in an ectopic atrial or junctional pacemaker focus. Junctional Ectopic Tachycardia (JET) arises frequently in the immediate postoperative period following open-heart repair near the crux of the heart (e.g., VSD or tetralogy of Fallot repairs). Because these do not depend on an AV nodal reentrant loop, they are refractory to adenosine.

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Acute Management of Pediatric SVT

                      Pediatric Acute SVT Termination Algorithm

                       [Assess Hemodynamic Stability]
                                      │
                 ┌────────────────────┴────────────────────┐
                 ▼                                         ▼
     ┌───────────────────────┐                 ┌───────────────────────┐
     │   HEMODYNAMICALLY     │                 │   HEMODYNAMICALLY     │
     │       STABLE          │                 │      UNSTABLE         │
     │ (Normal pulses, alert,│                 │ (Hypotension, shock,  │
     │  adequate perfusion)  │                 │  altered mental state)│
     └───────────┬───────────┘                 └───────────┬───────────┘
                 │                                         │
                 ▼                                         ▼
        [Vagal Maneuvers]                     [Synchronized Cardioversion]
     • Infants: Ice water bag to face         • Initial: 0.5 - 1.0 J/kg
       (15-20 sec; dive reflex)               • Escalate to 2.0 J/kg if needed
     • Older: Valsalva (blow in syringe)      • Sedation if feasible, do not delay
                 │
        [Failed Vagal Maneuver]
                 │
                 ▼
        [Adenosine Rapid IV Push]
     • Initial: 0.1 mg/kg (max 6 mg)
     • Rapid push via proximal line
     • Immediate 5-10 mL NS flush
     • If fails (1-2 min): Repeat
       at 0.2 mg/kg (max 12 mg)

Step 1: Hemodynamically Stable SVT — Vagal Maneuvers

Vagal maneuvers stimulate the parasympathetic nervous system, releasing acetylcholine onto cardiac muscarinic $M_2$ receptors in the AV node, prolonging AV nodal refractoriness and breaking reentrant circuits.

• Infants & Toddlers (< 1–2 years): The Ice-Water Facial Immersion Technique
  • Place a watertight plastic bag filled with crushed ice and ice water (slurry) directly over the infant's forehead, eyes, and bridge of the nose for 15 to 20 seconds.
  • This stimulates the trigeminal sensory afferents, triggering the powerful mammalian diving reflex, producing intense vagal outflow that terminates the reentrant circuit in up to 60% of cases.
  • Precautions: Never occlude the mouth or nostrils; never apply direct ocular or orbital pressure (orbital compression risks retinal detachment and vitreous rupture, and is strictly prohibited).
• Older Children & Adolescents: Valsalva Maneuver
  • Have the child blow vigorously into an occluded 10-mL syringe for 15 seconds to move the plunger, followed immediately by passive leg elevation (modified Valsalva maneuver).
  • Avoid Carotid Sinus Massage in pediatric patients due to risk of carotid sinus damage and unpredictable, dangerous vagal hyperresponsiveness.

Step 2: Pharmacologic Cardioversion — Adenosine Rapid IV Push

If vagal maneuvers fail or if SVT recurs immediately, intravenous adenosine is the first-line pharmacologic intervention.

• Mechanism of Action: Adenosine binds to cardiac $A_1$ adenosine receptors in the AV node. Activation of inhibitory $G$ proteins ($G_i$) inhibits adenylyl cyclase and opens inward-rectifying potassium channels ($I_{K,Ado}$), causing profound hyperpolarization and blunting $L$-type calcium channel opening. This produces a transient, complete atrioventricular nodal conduction block that terminates reentrant circuits passing through the AV node.
• Pharmacokinetics: Adenosine has an ultra-short elimination half-life ($t_{1/2} < 10\text{ seconds}$) in circulating human blood. It is rapidly eliminated via active carrier-mediated uptake into vascular endothelial cells and erythrocytes, where it is degraded by intracellular adenosine deaminase into inosine.
• Dosing Protocol:
  • Initial Dose: $0.1\text{ mg/kg}$ rapid IV push (maximum first dose: $6\text{ mg}$).
  • Second Dose: If no conversion occurs within 1 to 2 minutes, administer $0.2\text{ mg/kg}$ rapid IV push (maximum second dose: $12\text{ mg}$).
  • Third Dose (Refractory): In persistent cases with distant venous access, some centers escalate to $0.3\text{ mg/kg}$ (max $12\text{--}18\text{ mg}$).
• Mandatory Administration Technique:
  • Must be administered via rapid IV push as close to the heart as possible (antecubital vein or central line; scalp or lower extremity veins are strongly discouraged due to drug degradation before reaching the heart).
  • Utilize a two-syringe stopcock technique: syringe 1 contains the adenosine dose; syringe 2 contains $5\text{ to }10\text{ mL}$ of normal saline. Deliver the adenosine as an explosive push over $< 1\text{ second}$, immediately turn the stopcock, and slam the saline flush in behind it.
• Counseling & Safety Warning:
  • Warn bedside clinical staff and family members: Adenosine administration causes transient asystole (a flatline pause lasting 3 to 6 seconds) on the ECG monitor before sinus rhythm resumes. The patient may exhibit flushing, dyspnea, choking sensations, or chest tightness. A continuous rhythm strip must record the rhythm throughout the event.
• Drug Interactions:
  • Methylxanthines (caffeine, theophylline, aminophylline): Competitively antagonize $A_1$ receptors; patients receiving caffeine (e.g., premature infants) require significantly higher adenosine doses.
  • Dipyridamole: Inhibits adenosine cellular reuptake; markedly potentiates adenosine action (reduce initial dose by 75%).

Step 3: Hemodynamically Unstable SVT — Synchronized Cardioversion

When SVT presents with hemodynamic instability—manifested by hypotension, altered mental status, absent peripheral pulses, or respiratory failure—pharmacologic trials must not delay electrical therapy.

• Synchronized Electrical Cardioversion:
  • Initial dose: $0.5\text{ to }1.0\text{ J/kg}$.
  • Escalate to $2.0\text{ J/kg}$ for subsequent shocks if the initial discharge fails.
  • Ensure the defibrillator is placed in synchronization mode (detecting and tracking the $R$ wave) to avoid discharging on the $T$ wave, which could trigger ventricular fibrillation ($R\text{-on-}T$ phenomenon).
  • Administer procedural sedation/analgesia (e.g., ketamine or midazolam) if venous access and time permit, but never delay cardioversion in a crashing patient.

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Chronic Maintenance Antiarrhythmic Therapy

Following acute termination, infants and children with recurrent SVT require chronic pharmacotherapy to suppress reentrant triggers and alter accessory pathway refractoriness.

Clinical Pharmacology Pearls for Maintenance Agents

• Propranolol & Hypoglycemia in Infants: In infants receiving maintenance propranolol, fasting or viral gastroenteritis can trigger profound hypoglycemia and hypoglycemic seizures. Because $\beta$-blockers mask adrenergic warning symptoms (tachycardia, diaphoresis, tremulousness), parents must be counseled to feed the infant regularly and hold doses during acute gastrointestinal illnesses with poor caloric intake.
• Sotalol Inpatient Initiation Protocol: Sotalol delays ventricular repolarization ($I_{Kr}$ blockade), prolonging the QT interval. Inpatient hospital admission with continuous ECG telemetry for at least 3 to 4 days (five elimination half-lives) is required. A 12-lead ECG must be obtained at baseline and 2 to 4 hours post-dose at steady state. If the corrected QT interval (QTc) exceeds $500\text{ milliseconds}$, the dose must be decreased or discontinued to prevent Torsades de Pointes.
• Flecainide in Wolff-Parkinson-White (WPW) Syndrome: Flecainide profoundly depresses conduction velocity across the accessory pathway, making it highly effective for suppressing orthodromic and antidromic AVRT. However, it carries an FDA Black Box Warning: it is strictly contraindicated in patients with structural or ischemic heart disease (e.g., tetralogy of Fallot, d-TGA, cardiomyopathy) due to high risks of fatal ventricular proarrhythmia (originating from the Cardiac Arrhythmia Suppression Trial [CAST]). Pre-treatment echocardiography confirming a structurally normal heart is mandatory.
• Amiodarone Organ Toxicities & Interactions: Amiodarone has an immense volume of distribution ($V_d > 60\text{ L/kg}$) and a terminal elimination half-life of 40 to 60 days. Each 200 mg tablet contains approximately 75 mg of elemental iodine. Systemic toxicities include:
  • Thyroid Dysfunction: Hypothyroidism (more frequent, treated with levothyroxine) or hyperthyroidism (destructive thyroiditis) occurs in 15% to 20% of children.
  • Corneal Microdeposits: Occur in virtually 100% of patients on therapy $> 6\text{ months}$; typically benign but can cause halos.
  • Cytochrome P450 & Transporter Interactions: Amiodarone is a potent inhibitor of CYP3A4, CYP2C9, and P-glycoprotein. Co-administration doubles serum digoxin concentrations (empirically decrease digoxin dose by 30% to 50%) and prolongs the INR in patients taking warfarin (empirically decrease warfarin dose by 33% to 50%).

[!CAUTION] Intravenous Verapamil is Absolutely Contraindicated in Infants < 1 Year of Age. In immature neonatal and infant myocytes, intracellular calcium recycling by the sarcoplasmic reticulum is rudimentary; myocardial contractility depends almost entirely upon extracellular calcium influx via $L$-type calcium channels. Administration of IV verapamil (a non-dihydropyridine calcium channel blocker) in infants $< 1\text{ year}$ precipitates profound, irreversible electromechanical dissociation, severe refractory hypotension, and fatal cardiovascular collapse. Oral verapamil is similarly avoided in young infants.

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Pediatric Syncope: Vasovagal vs. Cardiogenic Workup

Syncope (transient loss of consciousness accompanied by loss of postural tone with spontaneous, complete recovery) is a common pediatric complaint, affecting up to 15% to 20% of children and adolescents prior to adulthood.

                  Pediatric Syncope Differential & Red Flags

                 ┌─────────────────────────────────────────┐
                 │            Pediatric Syncope            │
                 └────────────────────┬────────────────────┘
                                      │
                 ┌────────────────────┴────────────────────┐
                 ▼                                         ▼
   ┌───────────────────────────┐             ┌───────────────────────────┐
   │   Neurocardiogenic        │             │   Cardiogenic Syncope     │
   │   (Vasovagal: ~80%)       │             │   (High-Risk: ~2-5%)      │
   └─────────────┬─────────────┘             └─────────────┬─────────────┘
                 │                                         │
     • Triggered by prolonged standing,        • OCCURS DURING EXERTION / EXERCISE
       warm environment, dehydration           • OCCURS WHILE SUPINE OR SLEEPING
     • Prolonged prodrome (>10-30 sec):        • Sudden collapse WITHOUT prodrome
       pallor, warmth, nausea, tunnel vision   • Preceding chest pain or palpitations
     • Quick recovery in recumbency            • Family history: sudden unexplained
     • Structurally normal heart                 death <40 yr, channelopathies, HCM
     • Normal 12-lead ECG                      • ABNORMAL 12-LEAD ECG (QTc > 460 ms,
                                                 delta waves, RV strain, Brugada)

Neurocardiogenic (Vasovagal) Syncope

• Accounts for $> 80%$ of pediatric syncope presentations.
• Pathophysiology: Venous pooling in lower extremities during prolonged standing leads to reduced venous return. In response to vigorous ventricular contraction against an underfilled chamber, intracardiac mechanoreceptors (C-fibers) fire inappropriately, triggering the Bezold-Jarisch reflex—a paradoxical central surge in vagal tone and sympathetic withdrawal that causes severe bradycardia and vasodilation.
• Clinical Presentation: Occurs while standing, in crowded or warm rooms, or following emotional distress/pain. Always accompanied by a clear prodrome (lasting $> 10\text{ to }30\text{ seconds}$) of lightheadedness, nausea, pallor, visual "graying out" or tunnel vision, diaphoresis, and warmth. Spontaneous, full recovery occurs within seconds to minutes of becoming recumbent.
• Management: Reassurance, aggressive oral fluid hydration ($2\text{ to }3\text{ liters/day}$), liberalized dietary sodium intake, avoidance of dehydration triggers, and physical counter-pressure maneuvers (leg crossing, isometric arm tensing). Refractory cases may be treated with fludrocortisone ($0.05\text{ to }0.1\text{ mg/day}$) or midodrine.

Cardiogenic Syncope (Red Flags & Diagnostic Workup)

Cardiogenic syncope accounts for only 2% to 5% of pediatric episodes but carries an exceptionally high mortality risk from underlying structural anomalies, cardiomyopathies, or channelopathies.

• Critical Clinical Red Flags:
  1. Exertional Syncope: Syncope occurring during (rather than immediately after) vigorous sports, running, or swimming.
  2. Syncope While Supine or Asleep: Highly suspicious for primary electrical channelopathies (e.g., Long QT syndrome type 3, Brugada syndrome).
  3. Absence of a Prodrome: Sudden, instantaneous "drop-attack" collapse resulting in facial or dental trauma.
  4. Preceding Palpitations or Anginal Chest Pain.
  5. Family History: Sudden unexplained death in a first- or second-degree relative under 40 years of age, sudden infant death syndrome (SIDS), unexplained motor vehicle collisions, or drowning in a competent swimmer.
• Underlying Etiologies:
  • Channelopathies: Congenital Long QT Syndrome (LQTS; $QTc > 460\text{ ms}$ in males, $> 470\text{ ms}$ in females, $> 500\text{ ms}$ high-risk), Catecholaminergic Polymorphic Ventricular Tachycardia (CPVT; triggered by exercise), Brugada Syndrome.
  • Structural / Outflow Obstruction: Hypertrophic Cardiomyopathy (HCM), anomalous origin of the left coronary artery from the pulmonary artery (ALCAPA) or right sinus of Valsalva, critical aortic stenosis, Arrhythmogenic Right Ventricular Cardiomyopathy (ARVC).
• Diagnostic Mandate: Every child evaluated for syncope must receive a thorough personal and family history, physical examination, and a 12-lead ECG. If any red flags are present, urgent pediatric cardiology referral, echocardiogram, Holter/event monitoring, and exercise stress testing are mandatory.

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Practice Pearls & BCPPS Exam Traps

• Exam Trap 1: Never give intravenous verapamil to an infant younger than 1 year for SVT. It causes irreversible electromechanical dissociation and death. Adenosine is the first-line pharmacologic drug of choice.
• Exam Trap 2: Do not push adenosine slowly through a small peripheral line. Its half-life is under 10 seconds; slow delivery guarantees therapeutic failure. Always use a proximal vein and immediate rapid saline flush.
• Board Rule: Syncope occurring during physical exertion is cardiogenic until proven otherwise. A 12-lead ECG is mandatory in every pediatric patient presenting with syncope to screen for long QT syndrome, WPW pre-excitation, and hypertrophic cardiomyopathy.