It is also very important you find out what type of heart murmer it is. There are so many reasons and some are more serious than others. A heart murmer is in effect a symptom of something else - usually some physical abnormality or illness. Once you know how serious or otherwise the heart murmer is, you can treat him accordingly and manage his exercise and diet properly. Our own dog had a heart murmer as a puppy, but with appropriate exercise and the right diet, he's now been heart-murmer-free for 2 years. Tony
Sorry your pup is not feeling well. My dachshund had a heart murmur also. He passed at 12 years old. This may sound odd but I think he may have fluid in his lungs. He may have congestive heart failure. When his chest is lower than the rest of his body, it allows fluids to clear from his lungs. I could be totally wrong, of course, but, I woud take this up with your vet. I hope he is well. Research side effects of common drugs given to dogs for congestive heart failure before going to the vet. This will help you understand the disease and meds and arm you with the right questions before getting to the vets.