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: The ductus arteriosus connects the pulmonary artery to the aorta distal to the origin of the right subclavian artery (and the left common carotid). Blood reaching the right arm leaves the aorta proximal to the ductus, so it is pre-ductal.
Step 2: Pre-ductal oxygen saturation is therefore measured in the right upper limb (right hand/arm), which receives blood before any mixing through the ductus.
Step 3: Post-ductal saturation is measured in either lower limb, which receives blood after the ductus. A pre-ductal (right arm) saturation at least 3 percent higher than the post-ductal (lower limb) value indicates right-to-left ductal shunting.
Step 4: Hence the right upper limb is the site for pre-ductal SpO2, making option (c) correct. The left arm can be ambiguous (the left subclavian arises close to the ductus), so the right arm is the standard pre-ductal site.