Paediatric Murmurs and Arrhythmias: Innocent vs Pathological Murmurs, SVT, and Resuscitation Protocols

Paediatric Murmurs and Arrhythmias: Innocent vs Pathological Murmurs, SVT, and Resuscitation Protocols

Cardiac murmurs and rhythm disturbances are among the most frequent reasons for paediatric consultation in primary care and emergency departments. Up to 50% to 80% of healthy children have an audible heart murmur at some stage during childhood, of which more than 90% represent benign, innocent flow phenomena. Conversely, rhythm disturbances such as supraventricular tachycardia (SVT) can rapidly deteriorate into cardiogenic shock if unrecognized. Paediatricians must possess the clinical acumen to confidently distinguish innocent murmurs from structural lesions and execute acute antiarrhythmic protocols with precision.

---

Clinical Assessment of Murmurs in Children

A cardiac murmur is generated by turbulent blood flow. In children, normal cardiovascular anatomy paired with thin chest walls, high cardiac output, hyperdynamic circulation, and compliant great vessels creates innocent vibrations. Distinguishing these from structural heart defects relies on structured physical examination.

The 7 "S" Framework for Innocent Murmurs

The classic 7 "S" features describe typical innocent systolic murmurs; continuous venous hums and radiating neonatal branch-flow murmurs have distinct benign patterns:

1. Soft: Intensity is typically Grade 1 or 2 out of 6 on the Levine scale. Some innocent murmurs reach grade 3; a thrill requires evaluation.
2. Systolic Only: Typical innocent flow murmurs are early or mid-systolic. An isolated diastolic murmur is pathological; the benign cervical venous hum is continuous.
3. Short: The murmur occupies only a portion of the systolic phase; it is never pansystolic (holosystolic).
4. Sensitive to Position: Significant changes in acoustic intensity occur with postural alterations (e.g., Still's murmur softens or vanishes when sitting upright; venous hum abolishes supine).
5. Sweet / Musical / Vibratory: Distinct acoustic quality characterized as musical, twanging-string, vibratory, buzzing, or soft blowing, rather than coarse, harsh, or raspy.
6. Sternal Border / Localized: Typical Still's murmurs are localized to the left sternal border; normal neonatal branch-flow murmurs radiate to the axillae and back.
7. Sound Normal Second Heart Sound ($S_2$): Normal physiological splitting of $S_2$ during inspiration that fuses on expiration; no ejection clicks, fourth heart sounds, or friction rubs.

Bonus Clinical Requirement: Symptom-free child—normal feeding, age-appropriate exercise tolerance, normal growth percentiles, and normal, symmetrical peripheral pulses.

---

Common Innocent Murmur Phenotypes

Clinical Deep Dive into Benign Phenotypes

• Still's Murmur: First described by Dr. George Frederic Still in 1909, this is the most common innocent murmur of childhood. Believed to arise from the vibration of left ventricular false tendons (fibromuscular bands) or innocent resonance of normal aortic valve leaflets during peak ventricular ejection. Accentuated in high-output states such as pyrexia, anemia, or anxiety.
• Peripheral Pulmonary Artery Stenosis (PPS) of Infancy: In the fetus, branch pulmonary arteries receive minimal blood flow, leaving them small and branching at acute angles from the main pulmonary trunk. At birth, increased pulmonary blood flow creates transient physiological turbulence at the branch bifurcations. The murmur radiates prominently to bilateral axillae and the lung fields posterior to the scapulae. As the vessels grow and become more compliant, the murmur resolves spontaneously by 6 to 12 months of age.
• Cervical Venous Hum: A continuous benign murmur generated by gravitational turbulence as blood flows from the internal jugular veins into the superior vena cava. Uniquely, it is louder in diastole than in systole because venous filling accelerates during ventricular diastole. Its benign nature is confirmed instantly at the bedside: having the child lie supine immediately abolishes the murmur, as does gentle digital compression of the right internal jugular vein or turning the child's head.

---

Red Flags for Cardiology Assessment

These findings raise suspicion of structural disease and warrant paediatric cardiology assessment, with echocardiography as indicated. Interpret them alongside the recognized innocent patterns and the child's symptoms:

• Diastolic Murmurs: Any early, mid, or late diastolic murmur is inherently pathological (e.g., aortic regurgitation, pulmonary regurgitation, mitral stenosis).
• Pansystolic (Holosystolic) Murmurs: Suggests ventricular septal defect, mitral regurgitation, or tricuspid regurgitation.
• Loud Murmur Intensity: Grade $\ge 3/6$ murmur, or the presence of a palpable precordial thrill (defining Grade 4 to 6/6).
• Abnormal Second Heart Sound ($S_2$): A widely split and fixed $S_2$ (atrial septal defect), a single loud $S_2$ (transposition of great arteries, pulmonary hypertension), or an accentuated pulmonary component.
• Extra Systolic Sounds: Presence of a systolic ejection click (indicative of a bicuspid aortic valve, aortic stenosis, or pulmonary valve stenosis).
• Radiation: Significant transmission of the murmur to the axilla (mitral pathology), neck/carotids (aortic stenosis), or back (coarctation of the aorta).
• Abnormal Vascular Examination: Weak, delayed, or impalpable femoral pulses (radio-femoral delay in coarctation) or bounding peripheral pulses with wide pulse pressure (patent ductus arteriosus, aortic regurgitation).
• Symptom Flags: Exertional chest pain, exertional syncope or presyncope, feeding diaphoresis, faltering growth, unexplained respiratory distress, cyanosis, or a family history of sudden unexplained cardiac death in young relatives, hypertrophic cardiomyopathy, or hereditary channelopathies (e.g., Long QT syndrome).

---

Paediatric Supraventricular Tachycardia (SVT)

Supraventricular tachycardia is the most common pathological tachyarrhythmia in paediatric patients, with an incidence of approximately 1 in 250 to 1 in 1,000 children.

Electrophysiological Mechanisms

• Atrioventricular Re-entrant Tachycardia (AVRT): Accounts for 70% to 80% of paediatric SVT, predominating heavily in infants and young children. An anatomical accessory bypass tract connects the atrium and ventricle outside the AV node. In orthodromic AVRT, conduction travels antegradely down the normal AV node-His-Purkinje system (yielding a narrow QRS complex) and retrogradely up the accessory pathway.
  • When the accessory pathway conducts antegradely during baseline sinus rhythm, the 12-lead ECG demonstrates the Wolff-Parkinson-White (WPW) pre-excitation pattern: short PR interval (<0.08–0.10 s), widened QRS, and a slurred initial upstroke called a delta wave.
  • In concealed accessory pathways, pre-excitation is invisible during sinus rhythm.
• Atrioventricular Nodal Re-entrant Tachycardia (AVNRT): Accounts for 15% to 20% of cases, increasing in frequency in older children and adolescents. The re-entrant circuit is confined entirely within dual functional pathways (slow and fast) inside the AV node itself.

Clinical Features by Age Group

• Infants (<1 year): Tachycardia rates typically range from 220 to 280 beats/min (rarely <220 bpm). Infants cannot articulate palpitations. Presentation is subtle and insidious: poor feeding, irritability, pale or mottled skin, tachypnea, and vomiting. Prolonged undetected SVT can cause tachycardia-induced ventricular dysfunction and reduced cardiac output, progressing to cardiogenic shock, hepatomegaly, pulmonary congestion, and lactic acidosis.
• Older Children (>1–2 years): Heart rates generally range from 180 to 220 beats/min. Children report a sudden sensation of rapid fluttering in their chest or neck, lightheadedness, chest tightness, or mild shortness of breath. Symptoms start and stop abruptly ("paroxysmal").

ECG Distinction: SVT vs Sinus Tachycardia

---

Acute Termination Protocol for Paediatric SVT

Treatment depends on hemodynamic stability. The algorithm below uses AHA 2025 PALS energy and initial adenosine doses. ERC 2025 cardioversion starts at 1 J/kg, doubling subsequent shocks to 2 and then 4 J/kg.

                                [Paediatric SVT Confirmed on ECG]
                                                |
                                Assess Hemodynamic Stability
                                                |
                 +------------------------------+------------------------------+
                 |                                                             |
      [Hemodynamically Stable]                                     [Hemodynamically Unstable]
                 |                                                 (Decompensated Shock, SBP Low)
        1. Vagal Maneuvers                                                     |
     (Ice to face 10-15s infant;                                  Synchronized DC Cardioversion
      Valsalva in older child)                                      Initial: 0.5 to 1.0 J/kg
                 |                                                             |
      [If SVT Persists]                                            [If SVT Still Persists]
                 |                                                             |
    2. Rapid IV Push Adenosine                                    Escalate to 2.0 J/kg
      Initial: 0.1 mg/kg (max 6 mg)                               Sedate if conscious, do not delay!
      Flush: 5-10 mL 0.9% saline
                 |                                                             |
      [If Still No Conversion]
                 |
    Double Dose: 0.2 mg/kg (max 12 mg)

Management of Hemodynamically Stable SVT

A stable child is alert, oriented, with normal blood pressure for age, crisp peripheral pulses, capillary refill time $\le 2$ seconds, and clear lung fields.

Step 1: Vagal Maneuvers

Vagal stimulation increases parasympathetic tone, slowing AV nodal conduction velocity and prolonging its refractory period, terminating AV-nodal-dependent re-entrant circuits.

• Infants and Young Children: The Diving Reflex
  • Apply a plastic bag or glove filled with an ice-water slurry directly over the midface (forehead, eyes, and bridge of nose) for 10 to 15 seconds.
  • The cold stimulus activates ophthalmic trigeminal afferents, triggering the evolutionary mammalian diving reflex, producing profound vagal outflow and transient AV block.
  • Safety Precautions: Never occlude the mouth or nostrils. Never use dry ice or chemical ice packs (risk of chemical/cold burns). Carotid sinus massage and eyeball/orbital pressure are strictly contraindicated in children due to the high risk of retinal detachment and globe injury.
• Older Children:
  • Valsalva Maneuver: Instruct the cooperative child to blow forcefully into a 10 mL or 20 mL syringe for 15 seconds with enough pressure to move the plunger, followed immediately by lying flat with passive leg elevation (modified Valsalva technique).
  • Alternative maneuvers include headstands, blowing through an occluded straw, or ice-water facial immersion.

Step 2: Pharmacological Conversion with Intravenous Adenosine

If vagal maneuvers fail, intravenous adenosine is the definitive pharmacological agent of choice.

• Mechanism of Action: Adenosine binds to myocardial $A_1$ purinergic receptors, activating inward-rectifying potassium channels and inhibiting adenylate cyclase. This causes transient, profound hyperpolarization of AV nodal myocytes and temporary complete AV block, abruptly interrupting the re-entry circuit. Because adenosine is rapidly scavenged by vascular endothelium and deaminated by erythrocytes, its elimination half-life is less than 10 seconds.
• Dosing Regimen:
  • Initial Bolus Dose: 0.1 mg/kg IV push (maximum first dose 6 mg).
  • Second Bolus Dose: If conversion does not occur within 1 to 2 minutes, double the dose to 0.2 mg/kg IV push (maximum second dose 12 mg).
  • A third dose of 0.3 mg/kg (maximum 18 mg) may be attempted in refractory circumstances under paediatric cardiology guidance.
• Two-Syringe Rapid-Push Administration Technique:
  • Establish vascular access in the most proximal large vein available (antecubital fossa or central vein preferred; avoid distal hand or foot veins).
  • Connect a three-way stopcock directly to the venous cannula.
  • Attach Syringe 1 containing the calculated adenosine dose to one port, and Syringe 2 containing 5 to 10 mL of 0.9% normal saline to the second port.
  • Deliver the adenosine bolus as an ultra-rapid push over 1 to 2 seconds, immediately turn the stopcock, and administer the saline flush with maximal velocity to project the drug bolus into the central circulation before it undergoes enzymatic degradation.
  • Maintain continuous 12-lead ECG recording throughout administration. Inform the family and patient that a transient sensation of chest tightness, flushing, or brief nausea will occur, coinciding with a brief period of asystolic pause or transient AV block before sinus rhythm restores.

Management of Hemodynamically Unstable SVT

Unstable SVT is defined by decompensated shock: hypotension, altered mental status, absent or weak peripheral pulses, cool mottled extremities with capillary refill $>3$ seconds, or acute pulmonary edema.

• Immediate Intervention: Synchronized Direct-Current (DC) Cardioversion
  • Initial Energy Dose: 0.5 to 1.0 J/kg delivered via a synchronized defibrillator.
  • Subsequent Energy Dose: If the rhythm fails to convert, escalate energy immediately to 2.0 J/kg.
  • Synchronizer Mode: Ensure the "SYNC" button is active and tracking the R-wave on the monitor to prevent an unsynchronized shock falling on the T-wave (which can induce ventricular fibrillation).
  • Sedation: Provide brief, rapid intravenous sedation/analgesia (e.g., ketamine or midazolam) if the patient is conscious, but never delay emergency cardioversion to secure venous access or prepare drugs if the child is in extremis.

---

Chest Pain, Syncope and Hypertension: Common Causes and Red Flags

Chest Pain

Chest pain in children is rarely cardiac: in a large cohort referred to paediatric cardiology, about 1% had a cardiac cause. Musculoskeletal pain (costochondritis, muscle strain, precordial catch) is the commonest identified cause, followed by respiratory (asthma, pneumonia, pneumothorax), gastrointestinal (reflux) and anxiety-related causes. Cardiac red flags are pain or syncope on exertion, an abnormal cardiovascular examination or ECG, a family history of cardiomyopathy or sudden death, and features of pericarditis or myocarditis such as positional pleuritic pain, a rub, fever or heart failure.

Syncope

• Reflex (vasovagal) syncope is most common: there is usually a trigger such as prolonged standing, heat, pain or emotion, a prodrome of light-headedness, warmth, pallor and nausea, and rapid recovery.
• Cardiac red flags are syncope during exertion or when supine, preceding palpitations, no prodrome, triggering by a startle, loud noise or swimming, a family history of sudden death at a young age or inherited arrhythmia, and an abnormal examination.
• Record a 12-lead ECG in every child with syncope, looking for long QT, pre-excitation, Brugada pattern or hypertrophy. Distinguish syncope from seizures and from breath-holding spells in toddlers.

Hypertension

• Secondary hypertension is more common in younger children. Renal parenchymal disease is the commonest secondary cause, for example glomerulonephritis, reflux nephropathy, congenital renal anomalies and polycystic kidney disease. Other causes include renovascular disease, coarctation of the aorta, endocrine causes such as catecholamine-secreting tumours, and drugs such as corticosteroids or stimulants. Neonatal hypertension can follow umbilical arterial catheter thrombosis.
• Primary hypertension is increasingly common in adolescents and is strongly linked to obesity and family history.

Sources: Saleeb et al. Pediatrics 2011 paediatric chest pain; 2018 ESC Guidelines for the diagnosis and management of syncope; 2016 ESH guidelines for high blood pressure in children and adolescents.