Americans are doing a little better on heart disease, but it's still the Number One killer. It still accounts for about half of all deaths in this country. The American Heart Association estimates that more than a million Americans may have heart attacks this year. About half a million people will die of them. Another half a million may succumb to strokes or other related cardiovascular illnesses. That is something like three times those who die from cancer.

There have been some major medical breakthroughs in the treatment of heart attacks in the past few years, some even within the past few months, such as the discovery that combining two classes of heart drugs -- beta blockers and calcium channel blockers -- may be a particularly efficacious way to protect heart attack survivors from having another attack. Physicians and researchers have a wealth of new knowledge about what causes heart attacks, what happens when they occur and what causes them to recur. These new diagnostic and surgical techniques and new drugs and combinations of drugs are revolutionizing treatment. But prevention is still the best weapon, and, tragically, perhaps the least utilized.

Prevention depends as much (or more) on the individual as it does on the doctor.

And although there are certain immutable things tending to predispose some individuals to heart attack or stroke, there are other things that can significantly reduce what the medical community now calls the risk factors.

Dr. Edward B. Diethrich, cardiologist, heart surgeon, medical director of the Arizona Heart Institute, and just-named heart consultant to the President's Council on Physical Fitness, has devised a test called "The Cardiovascular Risk Factor Analysis." It is based on the kind of testing he and his staff do at the institute, but is specifically designed for people before they become patients.

The bare bones test is reprinted here. An expanded version, with more details about how you can make it work for you, is included in Diethrich's book, The Arizona Heart Institute's Heart Test (Cornerstone/Simon&Schuster, $6.95. Proceeds to heart research.) The good news, says Diethrich, is that even if your score is in the danger zone (36 or more) "you can actually do something about reducing your risk of heart attack and stroke."

A little more than a year ago, ABC's 20/20 did a segment on Diethrich's work and, as it were, administered the heart test to the TV viewing public. To the astonishment of Diethrich -- and ABC -- there were 250,000 responses from people who took the test. More than just a phenomenon, it gave Diethrich's research arm, the International Heart Foundation, a hefty sample on which to base a test of the test itself.

Diethrich advertised for volunteers, won some financing and the help of the computer department of the University of Arizona. The subsequent follow up disclosed that those responding viewers who had scored 36 or more were either under a doctor's care, had already undergone angiograms and bypass surgery -- or were dead.

The risk factors that you have no control over, obviously, are your age, sex, and family and personal histories -- one through four on the chart. Number five, diabetes, can be lessened as a factor, although not eliminated.

But the rest of the factors are pretty much up to the individual. Not easy, mind you, but look at it this way:

Take Al. He is 56 years old. His mother had a heart attack. He had a mild stroke when he was 50. His score is already 24.

Say he smokes two packs a day and is probably 30 pounds overweight. That brings him up to 36 (high risk) already and that doesn't even count his blood pressure which is bound to be high -- perhaps because he is overweight, perhaps because he is also an A-type personality in a stressful job with no stress management training. (Add another 10 or 15 points.)

Al is no dope, so when he reads this, he stops smoking at once, goes on a low-fat, low calorie diet, joins a health club and takes a course in relaxation techniques like biofeedback. He also goes to his doctor and lets it all hang out.

His doctor is no dope either and is up on the latest techniques. Al's doctor does a series of blood tests which measure the cholesterol level and the level of high density lipoproteins (HDLs) in his blood. (At the moment, cardiologists tend to see HDLs as a protection against heart attacks, but some studies seem to link them to increased cancer. "The answers aren't all in yet," says Diethrich.)

Al's blood pressure is up and his doctor puts him on a mild drug for hypertension -- probably, at first, a diuretic. He encourages Al's exercise and no-smoke routines and recommends he limit his salt intake and cut down on excess alcohol. Al is doing fine, but he breaks a foot in a cross-country ski mishap and so much for the exercise. Still, he isn't smoking . . . but he isn't losing weight either.

At his next visit, his cholesterol (the stuff that clogs arteries) is still high and his doctor sees other trouble signs, such as breathlessness and occasional, transient chest pain. So Al is sent on to a specialist for a stress test combined with new nuclear medical techniques. These can actually permit cardiologists to observe the distribution of blood in the heart vessels while the patient is exercising. Another such test measures how well the chambers of the heart called the ventricles are contracting.

Combining the stress test with nuclear medicine has weeded out many potential candidates for angiography, now an almost last-resort technique in which a dye is injected into the arteries and any blockage shows up on X-ray movies that follow the course of the dye within the arterial system. Angiography is not without its risks, although it is essential when other tests are positive or equivocal.

Let's say that Al's tests are okay. By this time he has had, as they say, the fear of God put into him, gets himself into a healthful life style and dies at the age of 92 after a fight with his 5th wife.

But his ambitious, hard driving colleague Esme, has a heart attack. That is, the blood supply to a part of her heart is blocked. Esme is lucky. She is a good candidate for a technique in which a balloon-tipped catheter is inserted into the artery and blown up, dilating the blood vessel and thereby permitting blood to flow into the heart. Had more than one artery been blocked, or had it been the left main artery, she might have been a candidate for bypass surgery. (This is still the best option in certain cases, says Diethrich, despite recent indications that it has too often been performed on patients who might have done quite as well on drug therapy.) Although women, in general, are less at risk, more and more of them are having heart problems, other than the romantic kind, recent statistics show.

It wasn't just chance, by the way, that (fictional) Al's five wives helped in his ultimate longevity. New studies have indicated that loneliness may be an important factor in the onset of fatal heart attacks or stroke. Al also was a tropical fish collector and, although he never realized it, watching fish has been shown to lower blood pressure. Pets do too.

And finally, new tests are suggesting that once again the old wives had it right: Al's fight with his wife may indeed have triggered that final attack. Sudden stresses, like fright or fights, death or divorce can upset the heart's electrical system enough to set off an attack in susceptible persons.

Take the test. If your score is over 30, see your doctor. If it's over 36, and you are not yet under a doctor's care, better call him today. If it's over 20, the suggestions in Diethrich's book may help. In any case, don't panic. If Al could do it, couldn't you?

For more information write Arizona Heart Institute, P.O. Box 10,000, Phoenix, Ariz. 85064. Or call your local affiliate of the American Heart Association. In the Washington area, phone 337-6400.