If you were a cardiologist or an optimist, you might say the glass was half-full. But if you were an epidemiologist or pessimist, you would be more inclined to say the glass was half-empty.

The glass, in this case, is the death rate from coronary heart disease in the United States, which declined sharply in the last two decades of the 20th century. From 1980 through 2000, the age-adjusted death rate for coronary heart disease fell to 266.8 deaths per 100,000 men from 542.9 deaths, and to 134.4 deaths per 100,000 women from 263.3.

This change, which meant 341,745 fewer coronary deaths in 2000 than would otherwise have occurred (along with a continued decline in such deaths in the years since), catapulted cancer last year into the lead position as a killer of Americans under age 85, even though cancer death rates are also declining.

Nonetheless, the pessimists would say, far more people are still dying of heart disease than should be, often before reaching age 30, even though the tools are at hand to virtually eliminate this scourge. And there are now strong indications that as Americans continue to get fatter and fatter, the decline in heart disease deaths will soon grind to a halt and may even reverse itself.

Furthermore, the pessimists note, the direct and indirect costs of coronary heart disease are still breaking the health care bank, at $142.5 billion last year and rising.

Why the Death Rate Fell

In the June 7 issue of The New England Journal of Medicine, a team led by public health specialists from the national Centers for Disease Control and Preventionanalyzed the changes that have occurred in American medicine and habits to account for the impressive drop in deaths from coronary heart disease (the most common kind of heart disease, which results from blocked blood vessels feeding the heart).

About 47 percent of the decrease in coronary deaths among Americans aged 25 to 84, the researchers concluded, could be attributed to ''a revolution in the treatments for established coronary heart disease'': the use of medical or surgical therapies that help prevent or postpone deaths from heart disease in patients already afflicted. These are therapies administered to patients who have suffered and survived a heart attack, people with chest pains indicative of blocked coronary arteries and patients with heart failure.

The remedies range from cheap (bystander CPR to forestall death from a heart attack, for example, and daily aspirin to prevent clots that can precipitate an attack) to costly and sometimes hazardous (angioplasty, stents and coronary bypass surgery to open or circumvent blocked arteries). Other therapies include clot-dissolving drugs, drugs that lower blood pressure and cholesterol levels and rehabilitation programs to improve cardiac function.

All told, the researchers estimated, nearly 160,000 of the coronary deaths that were prevented or postponed in 2000 ''were attributable to medical therapies'' administered to patients already known to have had heart disease.

While patients and their loved ones are no doubt extremely grateful for the ability of modern medicine to keep people alive and often well when their hearts are on the verge of giving out, the therapeutic approach to curbing the coronary death rate is like shutting the barn door after the horse has escaped. A more economical, not to mention less terrifying, approach is to prevent the development of this life-threatening and costly disease.

According to the new analysis, about 44 percent of the decline in coronary mortality during the 20 years studied was due to improvements in risk factors for heart disease: reduced cholesterol levels, better control of high blood pressure, a decline in smoking and a small rise in physical activity.