The latest news from the nation’s ongoing war on heart disease is a classic good news-bad news story.

The number of people hospitalized or killed by serious heart attacks each year is down sharply, new studies show. The overall rate of hospitalization for heart disease, whether looking at Medicare or the overall population, is down, too. But the cost of treating people hospitalized with heart disease is increasing.

Experts attribute improving heart health to the decline in smoking, more people getting treated for high blood pressure and high cholesterol, and the greater attention many people now give to eating healthier foods and getting exercise. Prevention clearly pays off for those who pay attention.

The experts also credit better outcomes to Medicare for encouraging hospitals to follow best treatment practices. For instance, in 1992, only 60 percent of Medicare beneficiaries received aspirin within two days of their heart attacks. Today the rate is over 90 percent, which helps reduce the chance of a recurrence.

Yet according to the American Heart Association’s annual report, Medicare spending on seniors who were hospitalized with coronary heart disease rose from $26.3 billion or $7,883 per discharged patient in 1999 to $32.7 billion or $10,201 per discharge in 2006, even though the total number of coronary cases declined by nearly 120,000 over those seven years. That’s a 4.2 percent annual increase, or twice the rate of inflation.

What’s going on? A new study of Californians who belong to Kaiser Permanente in last week’s New England Journal of Medicine provides some clues.

The researchers, who looked at people over 30 and included Medicare beneficiaries in the Kaiser plan, found that the overall heart attack rate fell from 274 cases per year per 100,000 beneficiaries in 1999 to 208 cases per 100,000 in 2008. The entire decline was attributed to the sharply falling rate of serious heart attacks, which are caused by total coronary blockages and almost always lead to immediate heart surgery and longer hospital stays for those who survive.

These chest-grabbing, harbinger-of-doom events (so-called STEMI or ST-segment elevation myocardial infarctions) fell by more than half-from 47 percent of heart attack cases in 1999 to just 22.9 percent in 2008. However, there was no increase in the survival rate for people who suffered those more serious events. In other words, the plethora of new technology, such as sophisticated scanners, invasive procedures and other devices, available to emergency room physicians and cardiologists has had no measureable impact on one’s ability to survive a serious heart attack.

Milder heart attacks, on the other hand, actually rose slightly over the decade (they rose in the first half of the decade before starting to fall). The authors of the study offer two possible explanations. First, the use of heart-protective medications like statins, beta-blockers and aspirin, which increased sharply in the past decade, “may contribute to a lower severity of subsequent cardiac events.”

Another possibility is that the wider use of a sophisticated blood test to detect very mild heart attacks may have inflated the total. “Observed reductions in case fatality rates could be attributable to secular trends in ascertainment of myocardial infarction and decreased severity on presentation,” the authors noted. “The observation that mortality after (the more serious) ST-elevated myocardial infarction (which is less influenced by the use of highly sensitive biomarkers) did not decrease over time provides support for this hypothesis.”

Of course, one doesn’t have to redefine what constitutes a heart attack to deploy the wonders of invasive cardiology. There’s already exploding use of balloon angioplasty to clear partially clogged arteries and stents to prop them open without patients having suffered a heart attack. Many critics argue these patients could just as effectively be managed with medication and intensive lifestyle modification. This could also be true for the increasing number of patients whose heart attacks are classified as mild.

Yet artery-clearing procedures within 30 days of hospitalization for these mild heart attack patients rose from 33.4 percent of cases in 1999 to nearly 41.8 percent in 2008 at Kaiser. That probably helps to explain why total costs for treating hospitalized heart attack patients and the costs per episode continue to rise, even as the overall rate of heart attacks in the population is falling. While some might argue that these procedures help prevent more serious heart problems later, the data are unclear.

It may well be that cardiologists are defining down what constitutes a heart attack, and defining up what it takes to treat one.