PDA vs Aortic Coarctation causing differential cyanosis

[fw5tape6kq]

There's is a question in USMLE Rx that essentially presents as follows.

A 2 week old girl born to uncomplicated vaginal delivery is noted to be irritable, diaphoretic, and tachypneic. On exam she is afebrile but exhibits a blueish discoloration to her lower extremities, while her trunk and arms appear pinkish. What is is the main cardiac anomaly causing these symptoms?

A. Coarctation of the aorta (the correct answer)
B. PDA (what I put, which is incorrect)
C. Teratology of Fallot
D. Transposition of the Great Vessels
E. Truncus Arteriosus

So, given the fact that the patient is a girl (Turner syndrome) and that her upper extremities appear "pinkish," (via blood congestion) I can understand how one might be correctly surmise that the defect is aortic coarctation (preductal). But I still don't understand why PDA is wrong. Is the PDA not necessary to create the differential cyanosis in this case? Since this girl is only 2 weeks old, has her right to left shunt completely revered?

I find this questions especially odd because the explanation for A in RX indicated the SHE HAS a PDA and that the flow from the right to left heart via the PDA creates cyanosis. Yet, the answer is coarctation of the aorta. So is it simply the reduced blood flow to the lower extremities causing the differential cyanosis, or does the coarctation somehow exacerbate the PDA?

Perhaps this is me getting this answer wrong by simply assuming differential cyanosis=PDA; differential BP in upper vs. lower extremities = aortic coarctation

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[tasar1898]

Isolated PDA creates differential cyanosis as part of "its" own Eisenmerger syndrome years down the line. 2 wks aint enough for anthing like that so the answer is postductal coarctation (i.e. After branch of left subclavian a.) .In addition a postductal coarctation wont allow closure of DA due to the extra gradients , so you get the PDA.

This is from memory so anyone correct me if it looks wrong..

 

[fw5tape6kq]

Isolated PDA creates differential cyanosis as part of "its" own Eisenmerger syndrome years down the line. 2 wks aint enough for anthing like that so the answer is postductal coarctation (i.e. After branch of left subclavian a.) .In addition a postductal coarctation wont allow closure of DA due to the extra gradients , so you get the PDA.

This is from memory so anyone correct me if it looks wrong..

Thank you. That makes sense. I guess the PDA flow becomes left to right very shortly after birth as the pulmonary resistance rapidly decreases. I assume that the lower extremity cyanosis develops from overall decreased blood flow, and not from blood with a lower oxygen saturation then?

 

[tasar1898]

Yep , the inciting event is the reduced flow due to hugely increased R in the aorta.. Keep in mind though that due to less flow , blood will have more time for gas exchange in the distal capillary + reduced flow can't ''wash'' it out --> accumulation of venous blood with low O2 Sats peripherically

 

[acceptmeplease]

One thing to help keep this straight is to remember that it's right to left shunts that cause cyanosis, not left to right.

Upper extremities would be pink if coarctation is beyond left subclavian...

One thing that kind of bothers me about this question is that I've read that internal thoracic arteries may be larger to compensate for the coarctation, so the lower extremities may not in fact be cyanotic if distal collateral circulation is present..I think Pathoma mentions notches in the ribs on X-rays or something like that.

 

[Transposony]

One thing that kind of bothers me about this question is that I've read that internal thoracic arteries may be larger to compensate for the coarctation, so the lower extremities may not in fact be cyanotic if distal collateral circulation is present..I think Pathoma mentions notches in the ribs on X-rays or something like that.

That only happens in adult type coarctation since collateral circulation takes time to develop.
The important concept to remember about collateral circulation in coarctation is that the flow of blood in intercostal arteries is reversed.
Normal Post ---> Ant but in coarctation it is Ant ---> Post.