A condition in which pulmonary hypertension is associated with a septal defect, so that blood flows from the right to the left side of the heart or from the pulmonary artery to the aorta. This allows blue blood, poor in oxygen, to bypass the lungs and enter the general circulation. This reduces the oxygen content of the arterial blood in the aorta and its branches, resulting in a patient with a dusky blue appearance (‘cyanosis) and an increased number of red blood cells (polycythemia). There is no curative treatment at this stage, but the patient may be helped by the control of heart failure and polycythemia. The condition may be prevented by appropriate treatment of the septal defect before irreversible pulmonary hypertension develops.