Diastolic ysfunction refers to an abnormality in the heart's (i.e. left ventricle's) filling during diastole. Diastole is that phase of the cardiac cycle when the heart (i.e. ventricle) is not contracting but is actually relaxed and filling with blood that is being returned to it, either from the body (into right ventricle) or from the lungs (into left ventricle).