I had a heart pig, my old boar Boris (my avatar)! He was perfectly fine for the majority of his life, but started looking a bit skinny for no particular reason, still had a good appetite and such. He started breathing funny so I took him to my local vet who diagnosed him with a URI, and gave him some antibiotics but obviously these did nothing as his problem wasn't a URI, it was a heart problem.
He had a fair amount of conscious x-rays taken (vets didn't want to put him under due to the breathing problems), and then an ECG at a specialist vets, who confirmed that he had Dilated Cardiomyopathy.
He was put on a diuretic, and two heart meds, Vetmedin and Fortekor, which he had twice daily for all. He did manage to have a good few months after his diagnosis where he was happy, just skinny looking. His breathing did improve on the meds, but he had a heart attack and sadly passed away in September 2016. The vet was surprised that Boris had managed to last that long, as his heart was the largest he'd ever seen in a guinea pig, he had a particularly severe problem unfortunately.
He did show a few signs during his earlier years that I'd never added up before, like making noises when eating (assumed they were happy munching noises).
Hope this helps x
My old girl Cookie used to hoot a lot, the vet thought asthma and gave her a ventolin inhaler which didn't help, she later went for surgery to have a mammary tumour removed and they did an xray of her heart whilst she was under GA and found it enlarged, sadly Cookie's heart failed when they brought her off GA and she passed away, most likely because of this heart problem we didn't know about. Since then I've been very interested in heart problems with Guinea Pigs. I have two that regularly hoot (one is very overweight) and another one that has a clicking chest all the time with heavy breathing, she also can't wheek, so 3/8 of my pigs I suspect a heart problem, all of them are having conscious heart/lung xrays soon so I want to prepare myself and find out as much as possible about it. Hopefully I'm worrying about nothing, URI's have been ruled out so we're looking at the heart before putting it down to allergies.
