USMLE Forums banner
1 - 12 of 12 Posts

· Registered
487 Posts
Discussion Starter · #1 ·
2 yr old infant with uncomplicated COA appears to be in good health. growth and development are normal. the constriction is located just distal to the subclavian arteries. pulses in both arms feel unusually brisk ;pulses in the ankle feel weaker then normal. which of the following is most likely decreased in this patient?
A. blood flow in upper body
B. blood flow in lower body
C. blood pressure in upper body
D. vascular resistance in upper body
E. vascular resistance in lower body

hmm....2 yr old infant!!!!
infant should be less then 1 yr na!!!:p

· Registered
373 Posts
Thanks @khushboo for sharing these questions !!!!!!

I would chose D. vascular resistance in upper body for Q1

& Option B. for Q2.

· Registered
560 Posts
Can anyone explain how the first answer is E. And why the others are wrongg...??? :eek:
You have to think through the pathophys of what's going on when you have COA. So basically they tell you you have coarctation distal to the subclavian. This means that blood goes into the brachiocephalic trunk, the left common carotid and the right subclavian just fine. However, the descending aorta is narrowed due to the coarctation; this leads to 2 things: 1) there is a high pressure build up behind the coarctation and hence in the carotids and subclavians (rules out C), and 2) low pressure below the coarctation, hence lower pressure in lower limbs.
The stem also says the infant appears healthy; this means that the coarctation is fully compensated (if it wasn't, there would be stuff wrong with his legs since they wouldn't be getting enough blood). Since it's compensated, this means that blood flow to all his limbs and organs are normal (rules out A & B)
So how is the blood flow equal when the pressures are different? Remember the equation:
Pressure = Flow x Resistance (P=QR)
(easy to remember because they're in alphabetical order!)
From this you can see that the resistances must be different. In upper limb, the pressure is increased, so if flow is constant that must mean the resistance is increased (Opposite of answer D)
in lower limb, the pressure is decreased, so if flow is constant that must mean the resistance is decreased (correct answer E) This makes sense too if you think about it: you need to get blood to the legs, but the pressure is so low that you can't get enough. What do you do? You dilate the arteries and arterioles to get more blood in, hence the vascular resistance decreased.
1 - 12 of 12 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.