Sudden cat death : possible heart disease ?

[Norfolklass22]

Just looking to find others with same experience and see if others think if was heart disease
Cat had been fine that morning having cuddles and purring. In evening laboured breathing. Within few hours he was panting with mouth open struggling to breath. Bit of drool.

Put in cat carrier to go vets and he freaked out trying to escape, after few minutes of fighting it just went silent. He had passed.
Vet took small look and thibgs heart disease. They said a lot of fluid came out I
Of Dennis so think heart disease.

Has anyone else experienced this?
Do others think heart disease? There were no signs before