Hello Tracey, I'm Dr. Deb and I'll do my best to help you today.First, let me extend my sincerest condolences to you and your family on the loss of your beloved cat, Zac. I know this must have been absolutely heartbreaking for you and is still difficult for you to process.Without an autopsy, we can't entirely know for sure what caused Zac's sudden death but I can pretty much assure you that the lump he had was not in any way responsible. More than likely, it was a small abscess which was resolving on it's own.I agree with your vet that Hypertrophic Cardiomyopathy could have caused his sudden passing even though he hasn't shown any previous symptoms of heart issues.
Cats are quite adept at hiding and masking disease until this condition has progressed and it catches up with them. They can seem fine one day and yet be gone the next; it's always shocking when this happens. Owners feel that they must have missed something and if they had only had their cats seen sooner, then a different outcome would have happened. This may be the case with some heart conditions but since he died so suddenly, it is unlikely that you would have been able to save him or effectively treat him with drugs.
However, there are a few other possible explanations for his sudden passing:1.He could have thrown a clot or embolism. 2. He could have had the equivalent of a heart attack unrelated to cardiomyopathy 3. He could have had an aneurysm (rupture of a blood vessel). I hope I can reassure you that you are in no way responsible for his death. You didn't miss anything and could not have prevented this from happening...of this I am absolutely certain.Again, my sincerest condolences on your loss. Deb