Hello and Welcome to ‘Ask A Doctor’ service.
I have reviewed your query and here is my advice.
Loss of appetite for the last couple of days along with
nausea and
fever in a 75 year old who has a
stroke 3 months ago indicates that there is a possibility of an infection somewhere in the body.
Since you have already taken the patient to the hospital and
oxygen started and waiting for an X-ray means other tests of blood an urine must also have been taken.
This can simply be a viral infection or a specific infection that will soon be found out on clinical examination by a Doctor, supported by investigations to get a proper cause.
Till then he must have been started on an antibiotics, symptomatic and supportive tests.
Heart problem usually does not cause such symptoms as you have asked about the possibility.
Hope I have answered your query.
Let me know if I can assist you further.