Revision 18 for 'Cardiovascular shunts'

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Cardiovascular shunts

Cardiovascular (cardiac) shunts are abnormal connections between the pulmonary and systemic circulations. Most commonly they are the result of congenital heart disease.

Pathology

Blood can either be shunted from the systemic circulation to pulmonary circulation (i.e. 'left-to-right shunt') or between the pulmonary circulation and systemic circulation (i.e. 'right-to-left shunt') 1-4. Rarely, the shunted blood returns to the same cardiac chamber without traversing a capillary bed, termed a 'circular shunt' 5-7.

Left-to-right shunt

In a left-to-right shunt oxygenated blood flows directly from the systemic circulation to the pulmonary circulation, which results in decreased tissue oxygenation through reduced cardiac output 1-4. Causes include 1-4:

Right-to-left shunt

In a right-to-left shunt deoxygenated blood flows directly from the pulmonary circulation to the systemic circulation, decreasing tissue oxygenation by reducing the oxygen content of systemic arterial blood 1-4. Causes include 1-4:

A useful mnemonic to remember some of the cardiac causes of a right-to-left shunt can be found here.

Circular shunt

In most left-to-right or right-to-left cardiovascular shunts, shunted blood returns to the same chamber after traversing a capillary bed (either pulmonary or peripheral), if this does not occur then the term 'circular shunt' can be employed 5. Such shunts are generally present in complex congenital heart defects 5-7.

Examples that have been described in the literature include:

Radiographic features

Echocardiography

In simple right-to-left or left-to-right shunts, the pulmonary and systemic flow trend in opposite directions, with an increase flow through one circulation necessarily at the expense of the other. Application of this principle underlies the use of the shunt fraction calculation, a ratio between pulmonary (Qp) and systemic (Qs) flow.

The volume of fluid moving through a tube (over a given duration) may be calculated by multiplying the cross-sectional area by the velocity at which the fluid is moving; thus, one may use the cross-sectional area (CSA) of the left and right ventricular outflow tracts and respective summated average velocities during systole to determine the flow through the right and left circulations. Outflow tract (both the RVOT and LVOT) diameters are obtained in the parasternal short and long axis, respectively. Pulsed wave Doppler through the outflow tracts yields a spectral envelope which one may trace (in most common echocardiographic packages) and yield a velocity time integral (VTI). The flow ratio is then calculated:

Qp/Qs = (CSA RVOT) x (RVOT VTI) / (CSA LVOT) x (LVOT VTI) 

  • shunt fraction (Qp/Qs) > 1
    • left-to-right shunt present
    • pulmonary (right-sided) flow exceeds systemic (left-sided) flow, as oxygenated blood from the left heart joins the systemic venous return destined for the pulmonary circulation
    • hemodynamic significance typically evident as right atrial enlargement and right ventricular dysfunction when the shunt fraction exceeds 1.5
  • shunt fraction (Qp/Qs) < 1
    • indicates a right-to-left shunt, with some of the systemic venous return traversing directly to the left-sided circulation
  • shunt fraction (Qp/Qs) = 1
    • indicates that no shunt exists, or that an equal amount of right-left and left-right shunting coexists

See also

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