Keith Orr thought he would surprise his doctor when he came for a checkup.

His doctor had told him to have a weight-loss operation to reduce the amount of food his stomach could hold, worried because Mr. Orr, at 6 feet 2 inches, weighed 278 pounds. He also had a blood sugar level so high he was on the verge of diabetes and a strong family history of early death from heart attacks. And Mr. Orr, who is 44, had already had a heart attack in 1998 when he was 35.

But Mr. Orr had a secret plan. He had been quietly dieting and exercising for four months and lost 45 pounds. He envisioned himself proudly telling his doctor what he had done, sure his tests would show a huge drop in his blood sugar and cholesterol levels. He planned to confess that he had also stopped taking all of his prescription drugs for heart disease.

After all, he reasoned, with his improved diet and exercise, he no longer needed the drugs. And, anyway, he had never taken his medications regularly, so stopping altogether would not make much difference, he decided.

But the surprise was not what Mr. Orr had anticipated. On Feb. 6, one week before the appointment with his doctor, Mr. Orr was working out at a gym near his home in Boston when he felt a tightness in his chest. It was the start of a massive heart attack, with the sort of blockage in an artery that doctors call the widow-maker.

He survived, miraculously, with little or no damage to his heart. But his story illustrates the reasons that heart disease still kills more Americans than any other disease, as it has for nearly a century.

Medical research has revealed enough about the causes and prevention of heart attacks that they could be nearly eliminated. Yet nearly 16 million Americans are living with coronary heart disease, and nearly half a million die from it each year.

It’s not that prevention doesn’t work, and it’s not that once someone has a heart attack there is little to be done. In fact, said Dr. Elizabeth Nabel, director of the National Heart, Lung and Blood Institute at the National Institutes of Health, age-adjusted death rates for heart disease dropped precipitously in the past few decades, and prevention and better treatment are major reasons why.

But the concern, Dr. Nabel and others say, is that much more could be done. In many ways, scientists’ hard-won and increasingly detailed understanding of what causes heart disease and what to do for it often goes unknown or ignored.

Studies reveal, for example, that people have only about an hour to get their arteries open during a heart attack if they are to avoid permanent heart damage. Yet, recent surveys find, fewer than 10 percent get to a hospital that fast, sometimes because they are reluctant to acknowledge what is happening. And most who reach the hospital quickly do not receive the optimal treatment — many American hospitals are not fully equipped to provide it but are reluctant to give up heart patients because they are so profitable.

And new studies reveal that even though drugs can protect people who already had a heart attack from having another, many patients get the wrong doses and most, Mr. Orr included, stop taking the drugs in a matter of months. They should take the drugs for the rest of their lives.

“We’ve done pretty well,” Dr. Nabel said. “But we could be doing much better. I’ve heard some people refer to it as the rule of halves. Half the people who need to be treated are treated and half who are treated are adequately treated.”

The result, heart researchers say, is a huge disconnect between what is possible and what is actually happening.

Crucial Miscalculations

Keith Orr’s story has themes that resonate with every cardiologist. He did many things right, but also made some crucial miscalculations that were so common that nearly every patient makes them, cardiologists say. But not everyone comes out as well.

Mr. Orr anticipated a pleasant day on Feb. 6, starting with a workout at his gym, then lunch with a friend before he went to work at Smith & Wollensky, a steakhouse where he is a manager.

He arrived at the gym around noon and lifted weights, concentrating on the pectoral muscles of his chest. Then he moved on to an elliptical cross-trainer for cardiovascular exercise.

After half an hour on the elliptical, Mr. Orr felt a tightness in his chest. “I attributed it to the weight training,” he said, but stopped exercising, showered, dressed and walked to his car.

“I felt really bad, out of sorts,” he said. The pressure in his chest would ease off and then intensify, and now he was sweating profusely and was nauseated. When he arrived at the restaurant, he told his friend Darrin Friedman that he would have to beg off from lunch. “I feel like hell,” he told Mr. Friedman.

He went home and lay on his bed.

“I knew at that point that it was not a pulled muscle,” Mr. Orr said. “It’s a completely different feeling of pressure and discomfort. You feel as though something is genuinely wrong.”

It was 3:15. And the pain was no longer intermittent. It was constant.

Mr. Orr called Mr. Friedman and asked him to drive him to an emergency room. A few minutes later, the two set off for Brigham and Women’s Hospital, about a 10-minute drive.