Which statement about pulmonary shunt magnitudes is correct?

Which statement about pulmonary shunt magnitudes is correct?

• A. A shunt greater than 30% is life threatening
• B. A shunt greater than 5% is life threatening
• C. A shunt greater than 15% is life threatening
• D. A shunt greater than 50% is life threatening

Answer: (A)

Shunt magnitude and its impact on arterial oxygenation. A pulmonary shunt is blood that bypasses ventilated alveoli, so it cannot be oxygenated. Normal physiologic shunt is about 2-5%, and oxygen therapy can usually compensate for small increases. As the shunt rises, more blood bypasses gas exchange, so PaO2 drops and the oxygen content of arterial blood falls despite supplemental O2. When shunt fraction reaches around 30% or higher, the resulting hypoxemia becomes severe and refractory to oxygen therapy, making it life threatening. Lower values, such as around 5% or 15%, can cause hypoxemia but are not typically described as life threatening. Therefore, the statement that a shunt greater than 30% is life threatening best reflects the threshold at which hypoxemia becomes critically dangerous.

Shunt magnitude and its impact on arterial oxygenation. A pulmonary shunt is blood that bypasses ventilated alveoli, so it cannot be oxygenated. Normal physiologic shunt is about 2-5%, and oxygen therapy can usually compensate for small increases. As the shunt rises, more blood bypasses gas exchange, so PaO2 drops and the oxygen content of arterial blood falls despite supplemental O2. When shunt fraction reaches around 30% or higher, the resulting hypoxemia becomes severe and refractory to oxygen therapy, making it life threatening. Lower values, such as around 5% or 15%, can cause hypoxemia but are not typically described as life threatening. Therefore, the statement that a shunt greater than 30% is life threatening best reflects the threshold at which hypoxemia becomes critically dangerous.