QUESTIONS

1. What is the function of the ductus arteriosus in the embryo?
    
2. What breeds are associated with patent ductus arteriosus?
    
3. One feature of the PDA is a "water-hammer" pulse.  Explain how one gets a water hammer pulse in this disease.
    
4. The other main feature of the PDA is a "machinery" or "continuous murmur."  Explain how it comes to be that you hear a murmur in both systole & diastole.
    
5. How is it possible to get a Right to Left shunt with a PDA?  Why is the caudal half of the body cyanotic in this event?
    
6. What percentage of PDA pups will die before age 1?
    
7. So correction of the L>R PDA is possible surgically.  It is recommended that after exposing the PDA, safety sutures of umbilical tape are placed around the aorta, brachycephalic & L subclavian arteries but only for dogs >7kg.  Why?
    
8. So you tie off the aorta & then the pulmonary artery.  You watch for the Branham reflex.  What is the Branham reflex?
    
9. It is expected that the patient will be Polyuric for 6-12 hours after ductus surgery. Why is this?
    
10. Surgery is no benefit if the shunt has turned to R>L. Why not?
    
    
    
    
     

ANSWERS

1. In the embryo, the lungs are not functional (no air to breathe in there anyway) so the ductus shunts blood right to left from pulmonary artery to aorta.  At birth the direction of circulation changes as the lungs fill w/air & the right side suddenly becomes the low pressure side.  The ductus is supposed to close within 72 hours of birth.
   
   
   
   
    
2. Breeds that seem predisposed to PDA are: the miniature poodle (a genetic basis here), keeshond, cocker, German Shepherd Dog, Pomeranian, collie, & shelty.  Most affected dogs are female.
   
   
   
   
    
3. Water hammer pulses result when there is a big difference between systolic & diastolic pressure.  A communication between the aorta & pulmonary artery is analogous to someone sticking a straw in the aorta & sucking blood out.   This means that there is tons more blood than normal going through the pulmonary circulation & returning to the left heart. In other words, there's a major volume overload on the left side.
   
   So let's look at systolic pressure.  Let's go sit right there in the aorta and watch everything in slo mo. Systole happens.  This is a volume overload so more blood than normal comes in.  This means systolic pressure is higher than normal.  Now diastole.  SLURP! the right side  sucks the blood off.  Now there is less blood than normal. Diastolic pressure is lower than normal.
   
   This = a big diff between systolic &diastolic pressure. VOILA!  A water-hammer pulse.
   
   
   
   
    
4. To get a murmur in both systole & diastole, you have to get turbulence in both systole & diastole. (I really don't get it.  I can see why shunting would cause turbulence in diastole but unless the left heart enlargement & consequent mitral regurg is accounting for the systolic part, I don't see where the systolic murmur comes from.  And I really don't see why there is no murmur if the shunt reverses R>L.  It seems there should be turbulence somewhere.)
   
   
   
   
    
5. After 6-12 months of living w/a PDA (assuming the patient has survived)  blood vessel changes occur in the pulmonary vasculature.  Hypertrophy occurs in the vessel linings which progresses to fibrosis & obstructive change. Pretty soon there is so much resistance in the pulmonary vasculature that the aorta is sucking blood out of the pulmonary artery instead of the other way around.
   
   The machinery murmur disappears when this occurs & caudal cyanosis appears.  (I thought you got caudal cyanosis because a major vessel to the front half of the body -not sure of its name-brachycephalic trunk?-comes off the aorta before the un-oxygenated blood from the pulmonary artery shunts over.  Is this even slightly correct?)
   
   
   
   
    
6. According to CVT, 50% die before age one.
   
   
   
   
    
7. The most common complication of surgery is tearing of the ductus.  If this happens quickly tie all the safety sutures to occlude the output of the L heart.  You have 2-3 minutes to fix the tear & untie the heart.  If the dog is <7kg, there is no way to tie off everything before the patient bleeds to death anyway so no point in safety sutures.
   
   
   
   
    
8. The Branham reflex is represented by a drop in heart rate & rise in aortic pressure as the ductus is ligated.  (Er, so if the heart rate starts to drop too fast, do you let up on the aortic ligation for a little while before tying down all the way?)
   
   
   
   
    
9. He isn't volume overloaded any more so he has to dump that fluid SOMEwhere.
   
   
   
   
    
10. Post-operatively pulmonary pressure tends to go up further making pulmonary hypertension worse :(
    
    (Should I be able to figure out why or is this something there is no explanation for?)