What is a flow heart murmur?

Flow Heart Murmur

A flow heart murmur is a benign midsystolic murmur caused by increased blood flow across normal heart valves due to conditions like anemia, pregnancy, or thyrotoxicosis, rather than structural heart abnormalities. 1

Characteristics of Flow Heart Murmurs

Flow murmurs (also called innocent or functional murmurs) typically have the following characteristics:

• Timing: Midsystolic (start after S1 and end before S2)
• Configuration: Crescendo-decrescendo (diamond-shaped)
• Intensity: Usually grade 1-2/6 (mild)
• Quality: Soft, not harsh
• Location: Most commonly heard at the aortic or pulmonic outflow tracts
• Radiation: Limited or no radiation to other areas
• Response to position: Often changes with position

Causes of Flow Heart Murmurs

Flow murmurs occur in structurally normal hearts due to:

1. Increased cardiac output states:

   • Pregnancy
   • Thyrotoxicosis
   • Anemia (particularly iron-deficiency anemia) 2
   • Fever
   • Exercise
   • Arteriovenous fistula

2. Ejection of blood into a dilated vessel beyond the valve 1

3. Increased transmission of sound through a thin chest wall (common in children and thin adults) 1

Differentiating Flow Murmurs from Pathologic Murmurs

Red Flags Suggesting Pathologic Murmur

• Holosystolic (panfocal) timing 3
• Diastolic timing 4
• Grade 3/6 or higher intensity 5
• Harsh quality
• Abnormal S2
• Maximum intensity at upper left sternal border
• Systolic click
• Increased intensity when standing 3, 5

Benign Features of Flow Murmurs

• Midsystolic timing
• Grade 1-2/6 intensity
• Soft quality
• Normal S2
• Position-dependent 4
• No associated symptoms

Clinical Significance

Flow murmurs are extremely common, present in:

• Up to 80% of school children
• 52% of adults 4

When a flow murmur is identified with no other abnormal findings on history or physical examination, no further workup is necessary 4, 5. However, echocardiography should be considered if:

• There are symptoms suggesting underlying cardiac disease
• Physical examination reveals other abnormal findings
• The murmur has characteristics suggesting pathology
• The diagnosis remains uncertain 5

Special Consideration: Anemia and Flow Murmurs

Iron-deficiency anemia frequently causes systolic flow murmurs that resolve with treatment. In one study, 29 out of 30 children with anemia-related murmurs had complete resolution of their murmurs after iron therapy 2. This highlights the importance of checking hemoglobin levels in patients with new flow murmurs.

Common Types of Flow Murmurs

• Still's murmur: Vibratory parasternal-precordial murmur common in children
• Pulmonary ejection murmur: Heard best at the left upper sternal border
• Venous hum: Continuous murmur heard in the neck
• Mammary souffle: Heard over the breasts in pregnant or lactating women 6

Flow murmurs represent the most common reason for pediatric cardiac referrals but are benign findings that should not cause undue concern 6.