Chiang reported a nomogram for calculating the percentage of venous shunting (Qs/Qt). The author is from the National Defense Medical Centre in Taipei.
Patient preparation: breathing 100% oxygen for 30 minutes (to allow simplified calculation of the PAO2)
Parameters:
(1) barometric pressure in mm Hg
(2) water vapor pressure in the alveoli (PAH2O) in mm Hg (47 mm Hg)
(3) PaCO2 in mm Hg
(4) PaO2 in mm Hg
PAO2 =
= (barometric pressure) - (PAH2O) - PaCO2
difference between PAO2 and PaO2 =
= (PAO2) - (PaO2)
percentage of venous shunting =
= (difference between PAO2 and PaO2) / ((difference between PAO2 and PaO2) + 1670) * 100%
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