Definition of a Physiologic (Innocent) Murmur
A physiologic murmur is a heart murmur that occurs in patients with structurally normal hearts, caused by increased blood flow velocity through normal cardiac structures, and has no pathological significance. 1
Core Characteristics
Physiologic murmurs are benign findings that do not represent cardiac disease and require no treatment. 1 These murmurs arise from three main mechanisms in the absence of structural abnormality:
- High blood flow rate through normal orifices (e.g., pregnancy, thyrotoxicosis, anemia, arteriovenous fistula) 1
- Ejection of blood into a dilated vessel beyond a normal valve 1
- Increased transmission of sound through a thin chest wall 1
Auscultatory Features
Physiologic murmurs have specific characteristics that distinguish them from pathologic murmurs:
- Timing: Midsystolic (systolic ejection pattern), occurring between S1 and S2 1, 2
- Intensity: Grade 1-2 out of 6 2, 3
- Configuration: Crescendo-decrescendo (diamond-shaped) 1, 3
- Location: Most commonly heard at the left sternal border, originating from aortic or pulmonic outflow tracts 1, 2
- Radiation: No significant radiation to distant sites 2
- Quality: Vibratory or musical (as in Still's murmur), not harsh 4
- Pitch: Medium-pitched 4
Dynamic Maneuvers
Physiologic murmurs demonstrate characteristic responses to positional changes and maneuvers:
- Standing: The murmur typically diminishes or disappears 1, 4
- Valsalva maneuver: Does not increase in intensity (unlike hypertrophic cardiomyopathy) 2
- Normal S2 splitting and intensity: Preserved physiologic splitting without abnormalities 2
Clinical Context
Most systolic heart murmurs do not signify cardiac disease, particularly in children and young adults. 1 The prevalence is remarkably high:
- Present in up to 80% of school children 3
- Found in 52% of adults 3
- Most innocent murmurs in children and young adults are midsystolic and originate from the aortic or pulmonic outflow tracts 1
Critical Distinctions from Pathologic Murmurs
The following features indicate a murmur is NOT physiologic and requires echocardiographic evaluation:
- Any diastolic or continuous murmur (virtually always pathologic) 1, 2
- Holosystolic or late systolic murmurs 2
- Grade 3 or higher intensity 2
- Harsh quality 5
- Abnormal S2 (fixed splitting, paradoxical splitting, or abnormal intensity) 5
- Increases with Valsalva or standing 2
- Associated symptoms (syncope, angina, heart failure, thromboembolism) 2
Common Pitfalls
In older patients with systemic hypertension, grade 1-2 midsystolic murmurs may be related to sclerotic aortic valve leaflets or flow into tortuous, noncompliant great vessels, representing age-related changes rather than true innocent murmurs. 2
Echocardiography may detect trivial or physiological valvular regurgitation in many normal patients who have no audible heart murmur at all, which should not be confused with pathologic regurgitation. 2
Iron-deficiency anemia commonly causes systolic murmurs that resolve after iron therapy, representing a functional rather than structural cause. 6