Agree it depends on what the problem was...it is not uncommon for us to see patients require diuretics after their surgeries. Much of the answer to your question requires additional information: what's the timeframe you are speaking of that she is still having heart failure? If immediately post-op, I would not be surprised.
It depends on the magnitude of pressure overload, the pathological stage of cardiac remodeling at the time of surgery , surgical time and CPB utilization/time and some other factors like what has been mentioned in comments above
The surgery was two years ago, and the outflow obstruction was due to accessory mitral valve. The pressure before surgery was 130 mmHg and after surgery it became 20 mmHg.