Over the last 30 years, American hospitals have become a showcase of medical progress, saving lives that not long ago would have been lost. "Rapid response teams," drilled in precision teamwork and the latest techniques of critical care, have become commonplace. Cardiac and respiratory monitors, once found only in intensive care units, are now standard equipment on most wards and even in many patient rooms. CAT scanners and M.R.I. machines, once rare, have become de rigueur, with some hospitals boasting duplicates and even triplicates. But up to one-fifth of patients treated with these new medical advances and then deemed well enough to leave the hospital end up being re-admitted within 30 days of their discharge, at considerable cost.