U.S. News & World Report says:

Three of the nation’s top academic medical systems – Dartmouth-Hitchcock Medical Center, Johns Hopkins Medicine and the University of Michigan – will  impose minimum-volume standards to bar their hospitals  from performing certain procedures “unless both the hospitals and their surgeons do them often enough to keep their skill level up.”

The move comes after  U.S. News  story showing that hospitals “that do small numbers of common procedures place patients at far greater risk than those that do lots of them.”