Can anyone explain how the first answer is E. And why the others are wrongg...???
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.