Innocent Murmurs (Flow Murmurs)

Innocent murmurs are also known as flow murmurs. They are very common in children. They are caused by fast blood flow through various areas of the heart during systole.

Innocent murmurs have typical features, all beginning with S:

  • Soft
  • Short
  • Systolic
  • Symptomless
  • Situation dependent, particularly if the murmur gets quieter with standing or only appears when the child is unwell or feverish

Clear innocent murmurs with no concerning features may not require any investigations. Features that would prompt further investigations and referral to a paediatric cardiologist would be:

  • Murmur louder than 2/6
  • Diastolic murmurs
  • Louder on standing
  • Other symptoms such as failure to thrive, feeding difficulty, cyanosis or shortness of breath



The key investigations to establish the cause of a murmur and rule out abnormalities in a child are:

  • ECG
  • Chest Xray
  • Echocardiography


Pan-Systolic Murmurs

The differentials of a pan-systolic murmur and where they are heard loudest are:

  • Mitral regurgitation heard at the mitral area (fifth intercostal space, mid-clavicular line)
  • Tricuspid regurgitation heard at the tricuspid area (fifth intercostal space, left sternal border)
  • Ventricular septal defect heard at the left lower sternal border


Ejection-Systolic Murmurs

The differentials of an ejection-systolic murmur and where they are heard loudest are:

  • Aortic stenosis heard at the aortic area (second intercostal space, right sternal border)
  • Pulmonary stenosis heard at the pulmonary area (second intercostal space, left sternal border)
  • Hypertrophic obstructive cardiomyopathy heart at the fourth intercostal space on the left sternal border


Splitting of the Second Heart Sound

During inspiration the chest wall and diaphragm pull the lungs open. This also pulls the heart open. This is called negative intra-thoracic pressure. This causes the right side of the heart to fill faster as it pulls in blood from the venous system. The increased volume in the right ventricle causes it to take longer for the right ventricle to empty during systole, causing a delay in the pulmonary valve closing. When the pulmonary valve closes slightly later than the aortic valve, this causes the second heart sound to be “split”.


Atrial Septal Defect

Atrial septal defects cause a mid-systolic, crescendo-decrescendo murmur loudest at the upper left sternal border, with a fixed split second heart sound. Splitting of the second heart sound can be normal with inspiration, however a “fixed split” second heart sound means the split does not change with inspiration and expiration. This occurs in an atrial septal defect because blood is flowing from the left atrium into the right atrium across the atrial septal defect, increasing the volume of blood that the right ventricle has to empty before the pulmonary valve can close. This doesn’t vary with respiration.


Patent Ductus Arteriosus

A small patent ductus arteriosus may not cause any abnormal heart sounds. More significant PDAs cause a normal first heart sound with a continuous crescendo-decrescendo “machinery” murmur that may continue during the second heart sound, making the second heart sound difficult to hear.


Tetralogy of Fallot

The murmur in tetralogy of Fallot arises from pulmonary stenosis, giving an ejection systolic murmur loudest at the pulmonary area (second intercostal space, left sternal border).


Cyanotic Heart Disease

Cyanosis occurs when deoxygenated blood enters the systemic circulation. Cyanotic heart disease occurs when blood is able to bypass the pulmonary circulation and the lungs. This occurs across a right-to-left shunt. A right-to-left shunt describes any defect that allows blood to flow from the right side of the heart (the deoxygenated blood returning from the body) to the left side of the heart (the blood exiting the heart into the systemic circulation) without travelling through the lungs to get oxygenated.

Heart defects that can cause a right-to-left shunt, and therefore cyanotic heart disease, are:

  • Ventricular septal defect (VSD)
  • Atrial septal defect (ASD)
  • Patent ductus arteriosus (PDA)
  • Transposition of the great arteries

Patients with a VSD, ASD or PDA are usually not cyanotic. This is because the pressure in the left side of the heart is much greater than the right side, and blood will flow from the area of high pressure to the area of low pressure. This prevents a right-to-left shunt. If the pulmonary pressure increases beyond the systemic pressure blood will start to flow from right-to-left across the defect, causing cyanosis. This is called Eisenmenger syndrome.

Patients with transposition of the great arteries will always have cyanosis because the right side of the heart pumps blood directly into the aorta and systemic circulation.


Last updated July 2019
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