Massachusetts General Hospital launched an effort to cut medical costs by reducing hospital stays and emergency room visits by the frail elderly. The initiative underscores just how hard it will be for the medical industry to make good on its promise to President Obama to pare healthcare spending by $2 trillion over the next decade. Under the Mass. General plan, the Medicare program would save millions of dollars by reducing hospitalizations of several thousand elders by 15% to 20% while improving their care. But to reap those savings the hospital has had to invest heavily up front, expanding the staffs of its primary care practices to provide time-consuming, hands-on help for patients struggling with a multitude of health problems.