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First of all, the practical stuff--While you are waiting, here is what I would suggest (and your own vet may already have done this): Chest X-ray to see if and how much fluid is accumulating in the lungs; a ACE inhibitor such as enalapril (Enacard); and a diuretic such as furosemide (Lasix or Salix). If your dog is doing poorly before the cardiology consult can be arranged, I would take him to an emergency facility where they have a round-the-clock ICU and can put him in oxygen if needed.
As far as what could have caused the worsening of the murmur--I would doubt that the new diet would have aggravated the heart disease unless his kidneys were not functioning well and could not handle the increase in protein. Most valve problems (on of the most common cause of heart murmurs in dogs) do get worse with age. The valve problem is generally from a leakage as the valve closes. With age, the valves may become stiffer and more irregular on the edge, making them close less completely with time. Often, when there is a sudden, dramatic worsening of heart disease in a dog, it means that one of the supporting structures (cordae tendinae) inside the heart has snapped. This makes it harder for the heart to beat effectively. His echocardiogram (cardiac ultrasound) will show this. It will also show how thick the valves and the chambers are and how effectively the heart is beating. I hope he does well. Let me know if you have any more questions.
The strange thing is, he's not exhibiting ANY symptoms. None. He's eating normally (even more than usual - he seems to really like the new food!), playing with his brother, and doing all of his normal things. His gums are pink, he's not breathing heavily or showing any signs of exertion or exercise intolerance. Even the vet confirmed this, so it's not just my wishful thinking. :) The only thing different than before was that the murmur had progressed - everything else about him was the same upon examination.
He had x-rays on Friday with no fluid present in the lungs, around the heart, or in his abdomen. He also had blood work not long ago, and the kidneys and liver function and everything else was within normal ranges. He's not, however, on the ACE inhibitor that you mentioned, so I'll ask the vet about that - if they haven't called me by this afternoon to let me know that they've arranged for the ultrasound, I'll be calling them anyway, as I want to be aggressive about getting the test done so we can find out what's going on. They were on a cooked-meat and vegetables diet before this, so I don't think he's getting any more protein than he was before - the vet said it should be an easy transition to the raw food diet and so far, it has been.
If this happens to be something like bacterial endocarditis (I have a friend who had it), is time of the essence in getting this diagnosed and treated before it gets much worse? Frankly, I'm frightened for him - both of them are the picture of health otherwise. The supplements and new diet seem to have cleared up both their sporadic diarrhea and dry itchy skin, which are the only problems they've had in their whole lives apart from some mild age-related arthritis in their hips.