Narrowing of the main valve which controls blood leaving the heart leads to a marked increase in death. To overcome this narrowing the heart muscle thickens abnormally and contracts more vigorously. However, in doing so, blood flow patterns to the heart muscle change through mechanisms which are incompletely understood. New technology allows such heart valve blockages to be fixed using balloons and metal stents inserted through tubes placed in arteries in the leg. Using sensors placed in these tubes it is possible to make detailed measurements of coronary flow and pressure, and apply new mathematical techniques to allow a better understanding of the detrimental effects that aortic stenosis has on coronary flow, before and after valve surgery.