Hello! After having a thorough review of your fathers problem, It appears that he have had a coexisting illnesses like malignancy, ( like bronchogenic carcinoma
) considering the history of a cardiac valve replacement
or may just be an initial reacting phenomenon to the artificial metallic valve replacement.
So, It would be better to have a consultation with your cardiologist to investigate into the cause.
Thanks & wishing a good health to your father!