Diastolic heart failure or diastolic dysfunction refers to decline in performance of one or both ventricles of the heart during the time phase of diastole. Diastole is that phase of the cardiac cycle when the heart is not contracting to propel blood out systole to the body, brain and lungs but instead is relaxed and filling with incoming blood that is being returned from the body through the inferior vena cava IVC from the lungs through the pulmonary veins and from the brain through the superior vena cava SVC.