How is right-to-left shunt determined by blood gases in pulmonary function testing?

Updated: May 14, 2020
  • Author: Kevin McCarthy, RPFT; Chief Editor: Nader Kamangar, MD, FACP, FCCP, FCCM  more...
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Determination of right-to-left shunt by blood gases: The fraction of the cardiac output that bypasses normal circulatory pathways can be estimated by obtaining an ABG sample after 20 minutes of breathing 100% oxygen from a large reservoir bag. This period of oxygen breathing should wash out all of the nitrogen from all ventilated alveoli. This makes the measured oxygen gradient (A-a PO2) independent of V/Q inequalities. While the true shunt requires a measurement of a mixed venous oxygen level, this often is not practical, and an estimated arterial-to-mixed venous oxygen content difference of 5 volume% often is assumed, as shown.

Qs/Qt = (0.0031 X [A-a]PO2)/(0.0031 X [A-a]PO2 + 5)

Qs/Qt is the calculated right-to-left shunt fraction, 0.0031 is the solubility coefficient of oxygen in blood, (A-a) PO2 is the gradient of alveolar to arterial oxygen partial pressure after 20 minutes of breathing 100% oxygen, and 5 is the assumed difference in resting arterial-to-mixed venous oxygen content. States of low and high cardiac output may invalidate the assumed arteriovenous oxygen (A-VO2) difference and cause significant error in the calculated shunt fraction.

Causes of increased right-to-left shunting may be intrapulmonary, such as pulmonary arteriovenous malformations, dilated capillaries in hepatopulmonary syndrome, lobar collapse or consolidation, or extrapulmonary conditions (eg, right-to-left intracardiac shunts, bronchial artery-to-pulmonary vein connections.

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