Breathlessness

My mother is 47 yers old and she had her valve replaced 4 years ago. She has been suffering from shortness of breath for the last ten years and it is even worse now, after the surgery. She often has arrhythmias and her heart rate gets under 50 beats per minute. Nevertheless, the main problem is her difficulty with breathing. Can you please propose a possible cause and treatment? Her lungs are healthy, and the doctors said so is her heart, except for a minimal leak from the artificial valve which seems to be harmless. Her ergometry results were excellent two years ago, and this year she couldn't even get through with the whole exercise.