Dr. Tien Vu was fixing up a child's cut when the first victim was rolled into the emergency room. A bullet had ripped into his thigh. The emergency room at Children's Hospital Colorado, where Vu has worked for nearly a decade, mostly tends to kids' broken bones and stubborn fevers, though the staff has handled its share of ailing adults too. But a gunshot wound was unusual. Friday morning's massacre tested the limits of the area's mass-casualty response. Doctors said their approach to such catastrophes had been transformed by lessons from the Sept. 11 attacks and shooting rampages such as the one at Columbine High School, only 20 miles away.