CHICAGO (Reuters) - First heart attacks are less likely to kill people in the United States than they used to be, helped by better prevention efforts and better treatments, US researchers said on Monday.

"The severity of heart attacks is decreasing," said Dr. Merle Myerson of Columbia University in New York, whose study appears in the journal Circulation.

"That is one reason among many that deaths from coronary heart disease are declining," Myerson said.

Government figures released last month found heart disease deaths in the United States have fallen 30 percent in the past decade, helped by better control of cholesterol and blood pressure, declining smoking rates and better medical treatments.

Better control of risk factors can not only prevent a heart attack, but it reduce the severity of a first heart attack. "People come into the hospital and their heart attack is not that bad. It may involve a smaller portion of their heart," Myerson said.

Her research updates a study known as the Atherosclerosis Risk in Communities, an ongoing study that collects data from a cross-section of Americans.

Previous data from this study from 1987 to 1994 found a decline in many measures of heart attack severity. Myerson and colleagues included data through 2002 on more than 10,000 first-time heart attack patients aged 35 to 74.

They checked a number of markers for heart attack severity, including enzyme levels in the blood that might indicate heart muscle damage and changes in blood flow and blood pressure.

Overall, they found improvements across the board.

"This study shows everyone has a decrease of severity of their heart attacks," Myerson said. "Men and women, blacks and whites — all showed the same trends," she said.

The team looked very carefully at the trends for differences by race, and in one category, a measure on an electrocardiogram known as a Q-wave, was higher in blacks.

"When we see that particular pattern, it generally indicates more of the heart muscle was affected by the heart attack," Myerson said.

In most other measures, however, blacks fared as well as whites, suggesting a general trend of improvement, she said.

One area that did not improve during the study was the overall percentage of patients who arrived at the hospital within two hours of their first symptoms. That remained at about 33 percent.

Myerson said the findings suggest patients are taking better care of their heart risks, and doctors and hospital staff are giving heart patients better treatment once they arrive.