I have never heard that portosystemic liver shunt (also called portal caval shunt) could cause a heart murmur - I'll have to check that in my veterinary textbooks when I go home tonight. There are many symptoms for PSS - general failure to thrive, low weight, lethargy, seizures, GI problems, excessive drinking and urinating – but that doesn’t seem to fit your description of your puppy. I had a shih tzu with congenital PSS who had repair surgery when she was 1, and she lived to be 15 with no problems as a result of the PSS and subsequent surgery.
This link is a veterinary paper which mentions that heart murmurs have been found is some cats with PSS:
Ventricular septal defect (VSD, hole in the heart) is a birth defect; here is one of my favorite sites which describes it well:
“Dogs can suffer from the classic 'hole in the heart' - otherwise known as a ventricular septal defect (VSD) - seen in babies. The term describes a malformation of the heart where there is a hole between the left and right ventricles. The effect is not dissimilar to that caused by patent ductus arteriosus: a short circuit leading to a loss in the heart's ability to pump blood.”
Again, the symptoms don’t really seem to fit because your puppy is very active.
“A dog with a 'hole in the heart', or VSD, may appear overly quiet and inactive. During any period of excitement, it may experience shortness of breath and or collapse, and its gums may appear tinged blue, due to a lack of oxygen. The symptoms of VSD will vary according to how large the defect is.”
Actually, PDA is a congenital heart defect that is more common in Cavaliers than VSD.
“Patent ductus arteriosus (PDA) occurs when a special blood vessel, used to bypass the pup's lungs in the womb, fails to seal after birth. This compromises the circulation of blood through the heart. PDA is the most commonly diagnosed congenital heart defect in dogs. It occurs in many breeds and is seen more often in females.” (From the above web site.)
As far as MVD (acquired valvular disease) – a ten month old with a grade IV/V heart murmur (indicating advanced MVD) would be very unusual since this is an acquired disease (takes time to develop and progress) versus a congenital disease (present at birth). I was surprised by Bev’s Oz who was diagnosed so young with MVD, but he also had a heart block and only a grade 1 murmur. This would be even more shocking. At the 1998 heart symposium in Atlanta, Dr. Beardow stated that the youngest Cavaliers he had seen with early MVD and low grade murmurs were about a year old.
Please keep us posted as you learn more, and I'm glad that you are able to see a specialist and have an ultrasound.