Right-to-left shunting is suggested when the P02 measured from the right radial artery is greater than the P02 from the umbilical artery by more than 15 mmHg. Which statement below correctly reflects this finding?

• A. P02 from the right radial artery is more than 15 mmHg higher than P02 from the umbilical artery
• B. P02 from the umbilical artery is more than 15 mmHg higher than P02 from the right radial artery
• C. P02 from both sites are equal
• D. P02 from the right radial artery is lower by more than 15 mmHg than P02 from the umbilical artery

Answer: (A)

The main idea is that differences in oxygen tension between two sampling sites reflect abnormal mixing from a shunt in fetal circulation. When a right-to-left shunt is present, blood from the right side of the heart can bypass the lungs and enter the systemic circuit, changing the usual pattern of oxygen levels seen in different vessels. If the PO2 measured from the right radial artery (pre-ductal systemic arterial blood) is more than 15 mmHg higher than the PO2 from the umbilical artery (blood returning toward the placenta), that gradient indicates the presence of such a shunt. So the statement that the PO2 from the right radial artery is more than 15 mmHg higher than the PO2 from the umbilical artery best reflects this finding. The other options either reverse the direction of the difference, state no difference, or state an opposite relationship, which would not indicate a right-to-left shunt.