(In atrial fibrillation, this additional 20% filling volume is lost and the patient may experience systolic heart failure symptoms).[27] Complete left ventricular filling is essential to maintain maximum cardiac output. Left ventricular filling is dependent upon ventricular relaxation and compliance, mitral valve area, atrio-ventricular gradient, atrial contraction and end-systolic volume. Diastole has four phases: isovolumetric relaxation, rapid filling, diastasis and atrial contraction.