Question

Where do you look for pre-ductal oxygen saturation in a patent ductus arteriosus (PDA) in a 3-minute-old born infant?

• A. Fetal left upper limb
• B. Fetal left lower limb
• C. Fetal right upper limb
• D. Fetal right lower limb

Hint:

Pre-ductal blood goes to the right arm because it leaves the aorta before the duct.

Answer 1

The Correct Option is C

Step 1: Think about the aortic anatomy. The brachiocephalic trunk, which feeds the right arm, branches off the arch well before the ductus arteriosus joins the aorta.
Step 2: Because of this upstream position, the right hand always samples blood that has not yet mixed with deoxygenated pulmonary blood crossing the ductus. That makes it the true pre-ductal reading.
Step 3: A lower-limb probe (either leg) samples flow downstream of the ductal junction and gives the post-ductal value.
Step 4: A right-arm minus leg gap of $\ge 3\%$ confirms right-to-left shunting at the duct. So the correct site is the fetal right upper limb.
\[\boxed{\text{Fetal right upper limb}}\]