The mortality rate of stroke -- the third largest cause of death in the United States -- dropped a "dramatic 45 percent" in the past 16 years, largely due to improved detection and treatment of high blood pressure, the American Heart Association announced yesterday.

Calling the decline a "dramatic public health achievement," the AHA report, released at the association's annual meeting, said death rates for stroke could be lowered even more with continued aggressive treatment of high blood pressure and of other key risk factors for stroke, including:

* Heart disease, which triples the risk of stroke, and now can be treated with a variety of drugs, diet and surgical techniques.

* Transient ischemic attacks (TIAs), an interruption of blood to the brain lasting less than 24 hours. TIAs precede about 10 percent of strokes, and can be treated with low doses of aspirin or the drug dypyridamole -- also known by the trade name Persantine.

"We recommend that active programs be initiated to identify these risk factors in people and treat them vigorously," said Dr. Mark L. Dyken, incoming chairman of the AHA's Council on Stroke.

There is a high likelihood, the council said in its first report on stroke in more than a decade, that a "concerted effort to reduce these risk factors will decrease the incidence of stroke."

Foremost among the treatable risk factors is high blood pressure, which the AHA estimates afflicts some 37.3 million Americans.

The death rate for stroke has dropped 45 percent in 16 years, with most of the drop coming since 1972, when the federal government's National Heart, Lung and Blood Institute began the National High Blood Pressure Education Program. Prior to that program, which teaches physicians and the public about high blood pressure and the ways to treat it, stroke mortality decreased about 1 percent a year.

The decline "has affected all ages, both sexes, and blacks even more than whites," said Dr. William Kannel, professor of epidemiology at Boston University Medical Center.

Stroke is a type of cardiovascular disease that affects the blood vessels supplying oxygen and nutrients to the brain. During a stroke, blood flow to the brain, known as cerebrovascular blood flow, is interrupted either by a clot or by bleeding.

Physicians used to believe that "stroke was a cerebrovascular accident, inevitable because of age and genetic makeup," Kannel said. "But now, we realize that it is really a chain of events that take place in the body over a period of years."

Other important risk factors for stroke include elevated blood levels of cholesterol and triglycerides, cigarette smoking, alcohol consumption, oral contraceptive use, physical inactivity and obesity.

"We can make the general recommendation that people maintain lean body weight, get more exercise, eat a diet lower in saturated fats and sodium, stop smoking and use moderate levels of alcohol," Kannel said.

The AHA estimated that more than 164,000 die of stroke each year, and about 2 million survive, making stroke a leading cause of disability. Costs of treating patients run between $6.6 billion and $8.5 billion annually, the AHA said.