My Mom 68 Years Old Was Suffering From Mitral Stenosis.

My mom 68 years old was suffering from mitral stenosis. Her mitral valve was replaced with a tissue valve in last Fall. She was perfectly fine for 5 months after the valve replacement. From the beginning of May she got sick again. It is found that her heart is only 25% functioning whereas it was 65% after the surgery. What is the possible cause of going down the heart function? What is the survival rate of people with 25-30% heart fundtion?