Continuous Murmur

Continuous Murmur


A continuous murmur is a type of heart murmur that persists throughout the cardiac cycle, meaning it is heard during both systole and diastole. Continuous murmurs are typically caused by abnormal blood flow between areas of high and low pressure during both phases of the heartbeat. 

Pathophysiology of Continuous Murmur

A continuous murmur occurs when there is persistent, abnormal blood flow between high-pressure and low-pressure areas throughout both systole and diastole. This sustained blood flow creates a murmur that is audible during the entire cardiac cycle. The pathophysiology can be broken down into the following key elements:

1. Abnormal Blood Flow:

  • A continuous murmur arises when there is a connection between two structures that have a significant pressure gradient during both systolic and diastolic phases.
  • Systole: Blood flows from a high-pressure area (e.g., systemic circulation or left heart structures) to a low-pressure area (e.g., pulmonary circulation or venous structures).
  • Diastole: The pressure gradient persists, leading to continued abnormal flow even when the heart is relaxed.

2. Pressure Gradient Across Systole and Diastole:

Continuous murmurs are most often caused by conditions that create a persistent pressure gradient between a high-pressure arterial system and a low-pressure venous or pulmonary system during the entire cardiac cycle. Some common conditions include:

a) Patent Ductus Arteriosus (PDA):

  • The fetal ductus arteriosus, a vessel connecting the aorta and pulmonary artery, remains open after birth.
  • This leads to continuous shunting of blood from the high-pressure aorta (systemic circulation) to the lower-pressure pulmonary artery (pulmonary circulation).
  • The pressure difference persists during both systole and diastole, leading to the characteristic “machinery-like” continuous murmur.

b) Arteriovenous Fistula (AVF):

  • An abnormal connection between an artery and a vein causes blood to flow continuously from the high-pressure arterial system into the low-pressure venous system.
  • As there is no capillary bed to dissipate the pressure, the flow continues unabated throughout the cardiac cycle.

c) Aortopulmonary Window:

  • A congenital defect between the ascending aorta and pulmonary artery, similar to PDA, creates continuous flow of blood from the aorta to the pulmonary artery during both phases of the cardiac cycle.

Characteristics:

  • Timing: Present during both systole and diastole.
  • Quality: Often described as a “machinery-like” or “to-and-fro” murmur.
  • Location: Varies depending on the cause. For example:
    • PDA: Best heard at the left infraclavicular area.
    • Arteriovenous fistula: Depends on the location of the fistula.
    • Venous hum: Best heard above the clavicles.

Causes

1. Congenital Causes:

  • Patent Ductus Arteriosus (PDA): Persistence of the fetal ductus arteriosus connecting the aorta and pulmonary artery.
  • Aortopulmonary Window: A congenital defect between the ascending aorta and the pulmonary artery.
  • Coronary Arteriovenous Fistula: Abnormal connection between coronary arteries and cardiac chambers or great vessels.
  • Pulmonary Arteriovenous Malformations (AVMs): Abnormal connections between pulmonary arteries and veins, bypassing the capillary system.

2. Acquired Causes:

  • Arteriovenous Fistula: An abnormal connection between an artery and a vein, which may be congenital or acquired (e.g., from trauma, surgery, or dialysis access).
  • Ruptured Sinus of Valsalva Aneurysm: Rupture of an aneurysm in the aortic sinus into a cardiac chamber, usually the right atrium or right ventricle.
  • Systemic-Pulmonary Shunt: Post-surgical shunts (e.g., Blalock-Taussig shunt used in congenital heart disease) can create continuous murmurs.
  • Aortic Stenosis with Collateral Circulation: When coarctation or severe aortic stenosis results in collateral vessels, a continuous murmur may be produced.

3. Vascular Causes:

  • Cervical Venous Hum: Benign continuous murmur, often heard in children, caused by blood flow in the jugular veins.
  • Mammary Souffle: A benign continuous murmur heard over the breast, often in pregnant or lactating women due to increased blood flow in the mammary arteries.

4. Infective or Inflammatory Causes:

  • Infective Endocarditis with Fistula Formation: Infection may lead to the formation of fistulas between chambers or vessels.

5. Miscellaneous:

  • Severe Aortic Regurgitation with Aortic Root Dilation: Sometimes causes continuous flow through a dilated aortic root.
  • Bronchial Arteriovenous Fistulas: Rare cause of continuous murmur due to abnormal connections between bronchial arteries and veins.
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