12.3 Pediatric Cardiac Emergencies: SVT, Ventricular Arrhythmias & Myocarditis

Pediatric Cardiac Emergencies: SVT, Ventricular Arrhythmias & Myocarditis

Acute cardiac dysrhythmias and myocardial inflammatory crises in pediatric patients represent high-acuity, time-sensitive transport emergencies. Pediatric dysrhythmias present distinctly from adult presentations: primary structural coronary artery disease is rare, whereas reentrant circuits, channelopathies, myocarditis, and systemic metabolic insults dominate. Critical care transport clinicians must rapidly decipher complex electrocardiographic rhythms, execute precise pharmacological and electrical cardioversion protocols, and avoid common clinical pitfalls that precipitate cardiovascular collapse.

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Pediatric Supraventricular Tachycardia (SVT)

Supraventricular Tachycardia is the most prevalent pathological tachyarrhythmia in children, with an incidence of approximately 1 in 250 to 1,000 children. Over 90% of pediatric SVT cases are caused by reentry mechanisms involving the atrioventricular (AV) node:

• Atrioventricular Reentrant Tachycardia (AVRT): The most common mechanism in infants and young children (representing ~70% of pediatric cases). An accessory pathway (such as the bundle of Kent in Wolff-Parkinson-White [WPW] syndrome) connects atrial and ventricular myocardium, bypassing the AV node. In orthodromic AVRT, electrical conduction travels antegrade down the normal AV node-His bundle axis and retrograde up the accessory pathway, producing a narrow QRS complex.
• Atrioventricular Nodal Reentrant Tachycardia (AVNRT): More common in older children and adolescents (~20%–30% of cases). Dual electrical pathways within the AV node itself—a fast pathway with a long refractory period and a slow pathway with a short refractory period—create a localized micro-reentrant loop.
• Ectopic Atrial Tachycardia (EAT): An automatic focus within the atrial myocardium fires independently of the sinoatrial node, often exhibiting a warm-up and cool-down pattern.

Electrocardiographic Criteria of SVT

1. Narrow QRS Complex: QRS duration is typically normal ($<0.09\text{ seconds}$ in children $<4\text{ years}$; $<0.12\text{ seconds}$ in older children). Wide-complex SVT occurs rarely in children due to bundle branch block aberrancy or antidromic conduction.
2. Extreme Heart Rates: Rate is characteristically $>220\text{ bpm}$ in infants and $>180\text{ bpm}$ in older children (frequently 240 to 300 bpm in neonates).
3. Abnormal or Absent P Waves: P waves are typically absent, buried within the preceding T wave or QRS complex, or visible as retrograde inverted P waves in inferior leads (II, III, aVF) immediately following the QRS.
4. Fixed, Rigid R-R Intervals: A cardinal diagnostic hallmark is absolute lack of beat-to-beat variability. The rate does not fluctuate when the child cries, moves, or sleeps.

Clinical Presentation by Age

• Infants: Inability to communicate symptoms results in delayed medical presentation. Infants frequently tolerate SVT for 24 to 48 hours before presentation, developing secondary congestive heart failure. Physical findings include lethargy, poor feeding, tachypnea, vomiting, pale or ashen skin, cool extremities, and prominent hepatomegaly.
• Older Children: Sudden paroxysmal sensation of "racing heart," chest fluttering, lightheadedness, shortness of breath, dizziness, or presyncope.

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Discriminating SVT from Sinus Tachycardia

Misdiagnosing sinus tachycardia as SVT—or vice versa—leads to catastrophic therapeutic errors. Administering adenosine or cardioverting a child with sinus tachycardia does not correct the rhythm and delays treatment of underlying hypovolemia or sepsis, while failing to convert SVT causes intractable cardiogenic shock.

Diagnostic Comparison Table

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Transport Resuscitation Algorithm for Pediatric SVT

Transport clinicians must immediately categorize SVT as hemodynamically stable or unstable:

• Unstable SVT: Signs of decompensated shock, including altered mental status, marked lethargy, hypotension (systolic BP below age-specific 5th percentile), poor peripheral perfusion (capillary refill $>3\text{ seconds}$, thready pulses), or acute pulmonary edema.
• Stable SVT: Alert mental status, preserved peripheral perfusion, normal capillary refill ($<2\text{ seconds}$), and normal age-specific blood pressure.

Pediatric Supraventricular Tachycardia (SVT)
(Infant >220 bpm, Child >180 bpm; Narrow QRS, No Variability)
       │
       ▼
Assess Hemodynamic Stability
       │
  ┌────┴──────────────────────────────────────────────────────┐
  ▼                                                           ▼
STABLE SVT                                                  UNSTABLE SVT
(Alert, Normal BP, Intact Perfusion)                        (Hypotension, Altered Mentation, Shock)
  │                                                           │
1. Obtain 12-Lead ECG                                        1. Immediate Synchronized Cardioversion
  │                                                             • Energy: 0.5 to 1.0 J/kg
2. Vagal Maneuvers                                              • Verify "SYNC" mode active on R-wave
  • Infants: Ice-water slurry to mid-face (15-20 sec)           • Sedate if conscious, but do not delay
  • Children: Valsalva / Blow into 10-mL syringe              │
  │                                                          2. Refractory: Escalate to 2.0 J/kg
3. Rapid IV Adenosine via Proximal Line
  • Initial dose: 0.1 mg/kg rapid push (max 6 mg)
  • Immediate 5 - 10 mL saline flush via stopcock
  │
4. Second Adenosine Dose (if unconverted in 1-2 min)
  • Second dose: 0.2 mg/kg rapid push (max 12 mg)
  • Immediate 5 - 10 mL saline flush

Stable SVT Protocol

1. Continuous 12-Lead ECG: Document baseline rhythm before intervention and run a continuous rhythm strip during all procedures.
2. Vagal Maneuvers:
   • Infants & Toddlers (The Diving Reflex): Place a slurry of crushed ice and water inside a sealed plastic glove or bag. Apply it directly over the forehead, bridge of the nose, and mid-face for 15 to 20 seconds. This stimulates trigeminal sensory afferents, triggering the mammalian diving reflex and intense vagal efferent discharge to the AV node. Rules: Never occlude the mouth or nostrils; never apply pressure to the ocular globes (pressure on the eyes risks permanent retinal detachment and is strictly prohibited).
   • Older Cooperative Children: Valsalva maneuver. Instruct the child to blow forcefully into the tip of a 10-mL syringe for 15 seconds to move the plunger, or bear down as if having a bowel movement. The modified Valsalva maneuver (blowing into a syringe followed by immediate passive leg elevation to $45^\circ$) significantly enhances conversion efficacy.
3. Intravenous Adenosine Pharmacotherapy:
   • Mechanism: Binds to cardiac $A_1$ adenosine receptors, activating potassium efflux channels and inhibiting calcium influx, inducing transient profound hyperpolarization and complete conduction block across the AV node.
   • Dosing:
     • Initial Dose: 0.1 mg/kg rapid IV push (maximum initial dose 6 mg).
     • Second Dose: 0.2 mg/kg rapid IV push (maximum second dose 12 mg), administered if the initial dose fails within 1 to 2 minutes.
   • Transport Administration Technique: Adenosine has an elimination half-life of $<10\text{ seconds}$ in circulating blood due to immediate uptake by erythrocytes and vascular endothelial cells and deamination by adenosine deaminase.
     • Proximal Line Access: Deliver only through a large proximal venous access site (antecubital fossa or central line). Avoid foot or hand veins, where the drug degrades before reaching coronary circulation.
     • Two-Syringe / Stopcock Method: Use a three-way stopcock connected directly to the IV catheter. Inject the adenosine bolus rapidly over 1 second, immediately turn the stopcock, and slam an aggressive 5 to 10 mL Normal Saline flush to propel the drug to the central circulation.
     • ECG Monitoring: Record a continuous rhythm strip during injection. Prepare the patient and family for transient flushing, chest tightness, and a brief 2 to 5 second period of complete asystole before normal sinus rhythm emerges.

Unstable SVT Protocol

• Synchronized Electrical Cardioversion:
  • Initial Energy: 0.5 to 1.0 J/kg.
  • Subsequent Energy: Escalate to 2.0 J/kg if the initial shock is ineffective.
  • Synchronization Requirement: The defibrillator must be placed in "SYNC" mode, verifying that synchronization markers flag every R-wave. This ensures electrical delivery coincides with ventricular depolarization, avoiding discharge during the vulnerable repolarization phase (T-wave peak) that triggers ventricular fibrillation (the R-on-T phenomenon).
  • Sedation: If the child is conscious, administer rapid IV sedation/analgesia (e.g., Ketamine 1 mg/kg or Fentanyl 1 mcg/kg), but never delay electrical cardioversion if the child is in decompensated shock.
• Refractory SVT: For SVT with cardiopulmonary compromise that does not respond to vagal maneuvers, adenosine, and synchronized cardioversion, the 2025 AHA/AAP guidelines say it may be reasonable to consider IV procainamide, amiodarone, or sotalol when expert consultation is not available. Transport teams should involve pediatric cardiology through medical control.

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Pediatric Ventricular Arrhythmias & Resuscitation

Ventricular arrhythmias originate below the bifurcation of the bundle of His and produce widened QRS complexes ($>0.09\text{ seconds}$ in infants/young children; $>0.12\text{ seconds}$ in older children).

Ventricular Tachycardia (VT) with a Pulse

• Stable VT: Rare in pediatrics. Obtain 12-lead ECG, consult pediatric cardiology, and consider Amiodarone (5 mg/kg IV over 20 to 60 minutes) or Procainamide (15 mg/kg IV over 30 to 60 minutes). Avoid adenosine unless regular monomorphic rhythm is suspected to be SVT with aberrancy.
• Unstable VT with a Pulse: Immediate synchronized electrical cardioversion at 0.5 to 1.0 J/kg, escalating to 2.0 J/kg.

Pulseless Arrest: Ventricular Fibrillation (VF) & Pulseless VT (pVT)

Shockable pulseless arrest mandates immediate high-quality CPR and early unsynchronized defibrillation:

1. Immediate CPR: Deliver continuous compressions at 100 to 120/min with a compression-to-ventilation ratio of 15:2 with two rescuers. Compress at least one-third the AP chest diameter (~1.5 inches in infants, ~2 inches in children), ensuring complete chest recoil.
2. Unsynchronized Defibrillation Sequence:
   • First Shock: 2 J/kg unsynchronized. Immediately resume CPR for 2 minutes without pausing to check rhythm or pulse.
   • Second Shock: 4 J/kg unsynchronized (delivered if shockable rhythm persists after 2 minutes of CPR).
   • Subsequent Shocks: $\ge 4\text{ J/kg}$ up to a maximum of 10 J/kg or adult dose (200 J biphasic).
3. Resuscitation Pharmacotherapy:
   • Epinephrine: Administer 0.01 mg/kg IV/IO ($0.1\text{ mL/kg}$ of $0.1\text{ mg/mL}$ [1:10,000]) every 3 to 5 minutes, beginning after the second defibrillation.
   • Antiarrhythmic Therapy (Refractory VF/pVT): Administer after the third defibrillation:
     • Amiodarone: 5 mg/kg IV/IO rapid bolus (may repeat up to two times for refractory VF/pVT; maximum single dose 300 mg), OR
     • Lidocaine: 1 mg/kg IV/IO loading bolus, followed by continuous maintenance infusion of 20 to 50 mcg/kg/min.

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Pediatric Viral Myocarditis: The Concealed Transport Crisis

Acute myocarditis is an inflammatory infection of the myocardium resulting in myocyte necrosis, ventricular dilation, and acute heart failure. Viral pathogens represent the primary etiology: Coxsackievirus B, Enterovirus, Adenovirus, Parvovirus B19, Human Herpesvirus 6 (HHV-6), and SARS-CoV-2 (including Multisystem Inflammatory Syndrome in Children [MIS-C]).

Biphasic Clinical Course

• Prodrome: A non-specific viral illness characterized by fever, malaise, rhinorrhea, vomiting, or diarrhea 1 to 2 weeks prior to cardiac presentation.
• Myocardial Failure: The child presents with insidious, progressive cardiac decompensation. The hallmark physical sign is unexplained resting tachycardia that is completely out of proportion to fever, crying, or mild agitation.
• Physical Signs: Tachypnea, grunting, subcostal retractions, S3/S4 gallop, hepatomegaly, cool pale extremities, and weak peripheral pulses.

Diagnostic Hallmarks

• Cardiac Biomarkers: Elevated high-sensitivity Cardiac Troponin I or T (marking active myocyte destruction) and elevated NT-proBNP or BNP (reflecting severe ventricular wall stretch).
• 12-Lead ECG: Demonstrates low QRS voltage in limb leads, generalized ST-segment depressions or elevations, deep T-wave inversions, prolonged QT intervals, and high susceptibility to premature ventricular contractions, ventricular tachycardia, or high-grade AV block.
• Radiograph: Marked cardiomegaly with pulmonary venous congestion.

Transport Pitfalls & Management Rules

1. Severe Arrhythmogenic Vulnerability: The inflamed myocardium is extraordinarily irritable. Minor mechanical or noxious stimulation (vigorous suctioning, cold exposure, agitated crying, or rough patient movement) can precipitate refractory ventricular tachycardia, ventricular fibrillation, or complete heart block. Transport crews must minimize stimulation, apply defibrillator pads prophylactically before departure, and maintain emergency antiarrhythmics at the bedside.
2. Strict Fluid Restriction: The failing myocardium cannot handle volume expansion. Large fluid boluses cause catastrophic alveolar flooding. If preload assessment is mandatory, give only an ultra-cautious 5 mL/kg fluid challenge over 30 to 45 minutes.
3. Targeted Inotropic Support:
   • Milrinone: The primary inodilator of choice ($0.25\text{ to }0.75\text{ mcg/kg/min}$). It decreases left and right ventricular afterload, enhances myocardial contractility, and promotes lusitropic relaxation without escalating myocardial oxygen demand.
   • Low-Dose Epinephrine: ($0.02\text{ to }0.05\text{ mcg/kg/min}$) or Dobutamine if systemic arterial hypotension requires inotropic pressure support.

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Summary Comparison of Pediatric Cardiac Emergencies

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Realistic Transport Scenario: Critical Infant SVT Conversion in Transit

A neonatal and pediatric critical care transport team arrives at an outlying emergency department to transfer a 3-week-old female infant (weight 3.6 kg). The parents reported poor feeding, listlessness, and rapid breathing over the preceding 24 hours. Monitor evaluation reveals a rapid, regular, narrow-complex tachycardia at a fixed rate of 288 bpm with unidentifiable P waves. Vital signs: blood pressure 76/50 mmHg (mean arterial pressure 58 mmHg; term infant normal systolic threshold is $\ge 60\text{ mmHg}$), respiratory rate 62 breaths/min, SpO2 95% on room air, and capillary refill 2.5 seconds. Abdominal palpation reveals the liver edge 2.5 cm below the right costal margin, reflecting early hepatic venous congestion from prolonged SVT.

Recognizing stable SVT with preserved blood pressure, the transport team attempts vagal maneuvers. A small latex glove filled with ice-water slurry is gently applied across the infant's forehead and bridge of the nose for 15 seconds, ensuring the mouth and nostrils remain completely unobstructed. The heart rate momentarily drops to 275 bpm but instantly resumes at 288 bpm. The flight nurse confirms a patent 24-gauge IV catheter in the right antecubital fossa and connects a 3-way stopcock. A dose of 0.36 mg Adenosine (0.1 mg/kg) is loaded into port 1, and a 5-mL normal saline flush is loaded into port 2. The adenosine is injected via rapid push over 1 second, followed immediately by the saline flush.

The cardiac monitor displays brief slowing to 260 bpm, but normal sinus rhythm fails to establish. Two minutes later, the team prepares the second dose of Adenosine at 0.72 mg (0.2 mg/kg). The dose is delivered via identical rapid-push stopcock technique with a 5-mL flush. The continuous rhythm strip captures a 2.5-second pause of AV nodal block, followed immediately by normal sinus rhythm at 144 bpm with crisp, upright P waves preceding each narrow QRS complex. Over the next 5 minutes, the infant's peripheral color turns pink, capillary refill shortens to 1.5 seconds, and blood pressure normalizes to 84/52 mmHg. The infant remains in stable sinus rhythm throughout the 40-minute interfacility helicopter flight to the regional pediatric cardiac intensive care unit.

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Clinical Pearls for Pediatric Cardiac Crises