What is the normal range for the shunt fraction Qs/Qt?

What is the normal range for the shunt fraction Qs/Qt?

• A. 1-2%
• B. 6-8%
• C. 3-5%
• D. 2-4%

Answer: (C)

The shunt fraction, Qs/Qt, represents the portion of pulmonary blood flow that passes through the lungs without taking part in gas exchange. In healthy lungs there’s a small amount of perfusion that bypasses fully ventilated alveoli due to normal anatomic and physiologic factors, so the physiologic shunt is about 3 to 5 percent of the total cardiac output. This small, baseline shunt reflects normal ventilation–perfusion heterogeneity rather than disease. Values outside this range suggest abnormal gas exchange: higher shunt fractions indicate increased perfusion through poorly ventilated or non-ventilated regions, while much lower values are not typical of normal physiology. So, the normal range is 3–5%.

The shunt fraction, Qs/Qt, represents the portion of pulmonary blood flow that passes through the lungs without taking part in gas exchange. In healthy lungs there’s a small amount of perfusion that bypasses fully ventilated alveoli due to normal anatomic and physiologic factors, so the physiologic shunt is about 3 to 5 percent of the total cardiac output. This small, baseline shunt reflects normal ventilation–perfusion heterogeneity rather than disease. Values outside this range suggest abnormal gas exchange: higher shunt fractions indicate increased perfusion through poorly ventilated or non-ventilated regions, while much lower values are not typical of normal physiology. So, the normal range is 3–5%.