Cardiovascular (cardiac) shunts are abnormal connections between the pulmonary and systemic circulations. Most commonly they are the result of congenital heart disease.
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') . Rarely, the shunted blood returns to the same cardiac chamber without traversing a capillary bed, termed a 'circular 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 . Causes include :
- vascular pulmonary shunts
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 . Causes include :
- vascular pulmonary shunts
- parenchymal intrapulmonary shunts
A useful mnemonic to remember some of the cardiac causes of a right-to-left shunt can be found here.
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 . Such shunts are generally present in complex congenital heart defects .
Examples that have been described in the literature include:
- a ventricular septal defect, pulmonary stenosis, tricuspid regurgitation, and a patent foramen ovale : some blood moves from the left atrium to the left ventricle, shunts to the right ventricle, regurgitates into the right atrium, and shunts back to the left atrium without traversing a capillary bed
- a patent foramen ovale, pulmonary atresia, and a Gerbode defect : some blood shunts from the right atrium to the left atrium, to the left ventricle, and shunts back to the right atrium without traversing a capillary bed
- a communication between a pulmonary artery and the right atrium : some blood moves from the right atrium to the right ventricle, to the pulmonary artery, and communicates back to the right atrium without traversing a capillary bed
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 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
- extracardiac shunts
- pediatric cardiovascular procedures (many of which are mechanical shunts)