Guenter.
The heart has a period when it ejects blood called systole and a period when it relaxes and refills with blood to pump again called diastole.
Abnormalities in diastole are known as disatolic dysfunction. Some degree of diastolic dysfunction occurs from the natural process of aging. Things that cause the heart to stiffen up or improperly relax, impain its ability to fill properly, increase pressures in the heart and can cause heart failure. The most common cause of this phenomenon is thickening of the heart form hypertension. Actually, pretty much all people with heart failure have some element of both systolic and diastolic dysfunction. Commonly, echocardiograms will report measurement of diastolic dysfunction.
Unfortunaltely, the diagnosis of diastolic heart failure is sometimes difficult to make in the setting of normal ventricular function. Given your EF is low, you probably have both systolic and diastolic dysfunction. The cause of your heart failure was more likely due to your mitral valve disease and your fibrillation.
If you continue to have symptoms, I would recommend seeing a heart failure specialist to help tease out treatment modalities.
good luck