physiologic shunt =
= (shunt fraction) / (cardiac output) =
= ((oxygen content of pulmonary end-capillary blood) - (oxygen content of arterial blood)) / ((oxygen content of pulmonary end-capillary blood) - (oxygen content of mixed venous blood))
where:
• Oxygen content in volume per cent = (1.39 * (oxygen saturation of blood) * (hemoglobin in g/dL)) + (0.0031 * (PO2))
• Oxygen content is expressed in mL/dL (vol%)
• The oxygen content of pulmonary end-capillary blood cannot be directly measured; since the oxygen saturation in pulmonary capillary blood is assumed to be 100%, the oxygen content can be estimated as: (1.39 * (hemoglobin in g/dL)) + (0.0031 * (PAO2))
• Mixed venous blood is measured from the pulmonary artery.
If the arterial blood is fully saturated (PaO2 > 100 mm Hg) and cardiac output is assumed to be normal, this can be simplified to:
physiologic shunt =
= ((2.0 - (0.003 * (PaO2))) / (7.0 - (0.003 * (PaO2))))
Interpretation:
• normal shunt is < 7%
A quick test for shunting is to administer 100% oxygen to a patient and then measure PaO2.
(1) normal response: PaO2 > 550 mm Hg
(2) If PaO2 > 500 mm Hg, then suspect a nonshunt causes of hypoxemia (decreased ventilation, V/Q imbalance, diffusion abnormality).
(3) If PaO2 < 500 mm Hg, then suspect shunting.
|
PaO2 (mm Hg)
|
approximate shunting
|
|
260
|
20%
|
|
160
|
25%
|
|
100
|
30%
|
|
60
|
40%
|
|
50
|
50%
|
|
35
|
60%
|
