The two valves most often affected are the mitral and aortic valves on
either side of the main pumping chamber (left ventricle) on the high pressure
side of the heart. The corresponding valves on the low pressure side of the
heart concerned with pumping blood to the lungs need attention less
frequently.
When the mitral valve is affected by disease so that it becomes
narrowed or leaky, it is sometimes possible to repair or reconstruct it. This
is preferred, as the patient keeps his or her own valve; but conditions have to
be absolutely right (i.e., the valve must not be too badly damaged by
disease) for a successful outcome. For this reason, if there is any doubt
about the valve continuing to function properly in the long term, it will be
replaced by a new one.
With the aortic valve, repair is, for all practical purposes, never
possible. This valve always has to be replaced if diseased severely enough
to need surgery.