Hundreds of hospitals around the country are joining an ambitious project intended to give faster emergency room care to people having major heart attacks.

Fewer than a third of such patients now have their blocked arteries reopened within 90 minutes of arrival, as guidelines recommend. The risk of dying goes up 42 percent if care is delayed even half an hour longer.

''There's a very, very large opportunity here to improve patient care,'' said Dr. John Brush, a heart specialist from Norfolk, Va., who helped the American College of Cardiology design the project, which is to begin Monday at the American Heart Association conference in Chicago.

Major medical groups and government agencies, including the National Heart, Lung and Blood Institute, have endorsed the project. The institute's director, Dr. Elizabeth G. Nabel, called it the biggest heart care initiative since paramedics were trained in CPR in the early '90s.

It is directed at heart attacks caused by a total or near-total blockage of a major artery that prevents enough oxygen from reaching the heart tissue. About a third of the 865,000 heart attacks in the United States each year and 10 million worldwide are of this type.

The preferred remedy is angioplasty, in which doctors snake a tube up a blood vessel in the groin to the blockage and inflate a tiny balloon.

Guidelines call for a ''door-to-balloon'' time of 90 minutes, ''but we just haven't engineered our emergency rooms to cut out some of these steps that aren't needed,'' Nabel said.

The project's recommendations include the following (with amount of time saved in each instance):

Letting emergency room doctors activate the catheterization lab and prepare it for angioplasty instead of waiting for a cardiologist to decide what to do (8.2 minutes).

Having an operator page an angioplasty team instead of having the emergency room staff members hunt down doctors (13.8 minutes).

Keeping a cardiologist on site at all times (14.6 minutes).

Also at the meeting, Italian researchers said waltzing could be effective in cardiac rehabilitation.

Waltzing proved to be as effective as bicycle and treadmill training for improving exercise capacity in a study of 110 heart failure patients.