Risks, burdens and benefits The risk of using pressure and flow wires: The wires will be advanced approximately 1cm outside the guide catheter that is used in routine clinical practice. In the pulmonary artery the wire will be positioned approximately 15cm more proximal than where the balloon is inflated during conventional pulmonary capillary wedge pressure measurement. In a recently published study of >7000 patients undergoing right heart catheterisation, there was only one incidence of pulmonary artery dissection. Damage to the arteries will be minimised by using an experienced interventional cardiologist to perform the procedures. The investigators have extensive knowledge in making these measurements resulting in minimal risk to the patient. To date no patient has had an adverse event secondary to the use of the pressure and flow wires in either the pulmonary arteries, aorta or coronary arteries. A pilot safety and feasibility study in 23 patients undertaken at the Royal Brompton Hospital in both normal subjects and those with pulmonary hypertension of various aetiologies showed that measurements of pressure and flow using Combiwires in the pulmonary arteries are possible in patients studied and there were no complications.