Heart failure is defined as a condition in which the heart is unable to pump enough blood to meet the demands of the body. Patients with heart failure may experience breathlessness especially when lying flat, as well as ankle swelling, known as peripheral oedema. Heart failure is the end result of many diseases affecting the heart, but is most commonly associated with ischaemic heart disease, valvular heart disease, or high blood pressure. Less common causes include various cardiomyopathies. Heart failure is frequently associated with weakness of the heart muscle in the ventricles (systolic heart failure), but can also be seen in patients with heart muscle that is strong but stiff (diastolic heart failure). The condition may affect the left ventricle (causing predominantly breathlessness), the right ventricle (causing predominantly swelling of the legs and an elevated jugular venous pressure), or both ventricles. Patients with heart failure are at higher risk of developing dangerous heart rhythm disturbances or arrhythmias.