Key Facts

Innocent murmurs are extremely common in childhood and result from normal turbulent blood flow through a structurally normal heart. Distinguishing innocent from pathological murmurs is a key clinical skill in paediatrics. The vast majority require no investigation or follow-up.

Epidemiology

Innocent murmurs are detectable in up to 50-80% of children at some point. They are most commonly identified between ages 3-7 years. Referral for cardiac murmur accounts for a significant proportion of paediatric cardiology outpatient workload, with 50-70% of referred murmurs ultimately deemed innocent.

Aetiology

Innocent murmurs arise from normal turbulent flow and are not associated with any structural abnormality. Factors increasing their detection include:

• High cardiac output states: fever, anaemia, hyperthyroidism, exercise
• Thin body habitus (increased transmission)
• Anxiety, tachycardia

Pathophysiology

Turbulent blood flow generates audible vibrations even in a normal heart. The narrow outflow tracts and relatively higher flow velocities in children compared to adults explain the higher prevalence. As the heart grows and flow velocities normalise, most innocent murmurs disappear by adolescence. Still's murmur may relate to vibration of normal left ventricular false tendons (fibromuscular bands).

Types of Innocent Murmur

• Still's murmur: Musical/vibratory, grade 1-2/6, lower left sternal edge, age 2-7 years, softer when standing
• Pulmonary flow murmur: Soft ejection systolic, upper left sternal edge, louder when supine, common in adolescents
• Venous hum: Continuous low-pitched murmur below clavicles (usually right), abolished by lying flat or gentle jugular compression
• Carotid bruit: Systolic murmur over carotid arteries, radiates to neck
• Peripheral pulmonary stenosis (neonatal): Soft systolic murmur radiating to axillae/back, resolves by 6 months

Features Suggesting Innocence

• Asymptomatic child, thriving well
• Soft (grade ≤2/6), systolic only
• Varies with posture (softer on standing)
• No radiation, no thrill
• Normal heart sounds (S1, normal splitting of S2)

Red Flags

• Diastolic or continuous murmur (except venous hum)
• Loud (≥3/6) or harsh quality with thrill
• Fixed split S2 (ASD) or single S2 (TGA, pulmonary atresia)
• Associated symptoms: cyanosis, breathlessness, poor feeding, syncope
• Abnormal ECG or chest X-ray
• Family history of sudden cardiac death or inherited cardiomyopathy

Bedside

• Thorough clinical examination: Assess murmur character, grade, timing, variation with posture, heart sounds
• Growth and development assessment: Ensure normal growth centiles
• Pulse oximetry: Should be normal (>95%)
• Blood pressure: Should be normal for age

Bloods

• Not routinely required for typical innocent murmurs
• FBC: If anaemia suspected as cause of flow murmur
• TFTs: If hyperthyroidism suspected

Imaging

• Echocardiography: Only if features are atypical or concerning — diastolic component, ≥grade 3, thrill, abnormal S2, symptoms, or family history of inherited cardiac disease
• ECG: If concerned about arrhythmia, hypertrophy, or inherited condition
• CXR: Not routinely indicated; only if clinical concern about heart failure

Special Tests

• Not required for confident diagnosis of innocent murmur

Non-pharmacological

• Reassurance: Explain to parents that the murmur is normal and not indicative of heart disease
• No activity restriction: Child can participate fully in all activities including sports
• No follow-up required: For confident diagnosis of innocent murmur
• No endocarditis prophylaxis needed

Pharmacological

• No treatment required
• If underlying cause (anaemia, fever) treated, murmur typically becomes less prominent

Surgical/Interventional

• Not applicable

Referral Criteria

• Murmur with any atypical or pathological features — paediatric cardiology
• Parental anxiety despite reassurance — low threshold for echocardiography
• Neonatal murmur persisting beyond 6 weeks
• Murmur with associated symptoms (syncope, chest pain, dyspnoea on exertion)
• Family history of inherited cardiac disease or sudden cardiac death

• Innocent murmurs are entirely benign and carry no cardiovascular risk
• Most innocent murmurs disappear by adolescence as the heart grows
• Still's murmur may persist into early adulthood in some individuals
• Venous hum is lifelong in some but remains inconsequential
• No increased risk of cardiovascular disease, endocarditis, or arrhythmia
• Children with innocent murmurs should be treated as entirely normal

Distinguishing Innocent from Pathological Murmurs

Types of Innocent Murmurs Summary