If scientists achieved the impossible goal of eliminating all heart disease, the victory would add just three years to the average American's life span, a study concludes.

That conclusion is one of several from a study that asks such provocative questions as: What if all Americans got their cholesterol levels below 200? What if everyone stopped smoking?Heart disease, which kills 500,000 Americans annually, is the country's leading cause of death. The researchers said that while the gain in longevity from eliminating heart disease may seem surprisingly small, their finding reflects the difficulty of pushing back the boundaries of old age.

Even if people escape the No. 1 killer, a host of other ailments are likely to quickly take its place as people reach their 80s and beyond.

"If you wipe out heart disease, people don't live forever," said Dr. Lee Goldman, a co-author of the study. "It is the leading killer, but there are other things people die from," such as cancer, pneumonia and strokes.

Similar analyses of cancer have concluded that life expectancy would increase about two years if that disease were conquered.

The average life span in the United States rose from 47 in 1900 to 75 today.

The latest study was based on a computer program developed by Goldman of Brigham and Women's Hospital in Boston and Dr. Milton C. Weinstein of Harvard School of Public Health.