Half joking, the heart surgeon warns his audience that what they're about to see may shake them up -- "If you pass out, raise your hand." the laughter is nervous.

What follows is a film strip of surgery on a blocked artery. The sureon is removing what looks like a hard, white pencil from the delicate, thin, translucent casing it has completely filled. The "casing" is somebody's artery. The "pencil" -- it is about four inches long and solid as wood -- is plaque. "No way blood is going to flow through that," says Dr. Edward B. Diethrich.

It seems to Dr. Diethrich that there's something out of whack somewhere.

It worries him that Jerry Lewis can raise $38 million in a weekend telethon for a disease that claimed the lives of fewer than 1,700 in 1978, but the disease in which Dr. Diethrich specilizes will kill a million Americans this year, cost about $40 billion, and it's almost as though nobody notices.

Diethrich, founder and medical director of the Arizona Heart Institute, is concerned with the range of ailments that fall within the heading of "cardiovascular." Mainly heart attacks and strokes, mainly caused by atherosclerosis: the occlusive buildup on artery walls of material known as plaque. Sometimes called arteriosclerosis or hardening of the arteries. Depending on where the artery is, the result is usually either a heart attack or a stroke.

At 44, Diethrich is an internationally known surgeon and does about 80 operations a month on 22 operating days. A graduate of the University of Michigan and its medical school, he spent five years on the Michael DeBakey heart team in Houston before founding in 1971 the heart institute.

His frustration that Americans do not appreciate the seriousness of the problem leads him to spend as much time lecturing as he can spare from the institute.

He has put together a starting lecture-slide-move program which he presents to health professionals, corporate executives and other groups. He was in the area recently, doing his cardiovascular show-and-tell for a group of Marriott Corporation and IBM executives at Marriott headquarters in Bethesda.

Although surgery is his ultimate weapon, Dr. Diethrich is equally committed to prevention and has established the Arizona facility as one of the top cardiovascular screening centers in the world.

In brief, his message is this: Although no one really knows the cause of the disease, they do know something about what Diethrich calls "the mechanisms of action":

Heredity. You are at high risk if members of your family had the disease, probably because of your protein physiology, says Diethrich.

Some proteins, called high-density lipoproteins or HDL's, seem to "act like mine sweepers" through the arterial system, picking up the cholesterol and triglycerides which, when they are deposited on the artery walls, build up into plaque. If you have lots of HDLs in your genes, you are in better shape then your neighbor without. Some people have lots of low-density proteins. They don't help much.

Arterial trauma. This occurs when the delicate inner lining of the artery is injured.

High blood pressure can set up an actual turbulence in the blood at the points where arteries divide. Such injury seems to attract the deposition of the cholesterol and triglycerides. There are also, says Deithrich, "certain things that we do to ourselves, in our environment, that can alter the receptivity of this lining of the blood vessel . . . and one of the strongest things we've learned is carbon monoxide (which we get) from cigarette smoking, cigar smoking, automobile exhaust."

It increases the "permeability" of the delicate inner lining of the artery, "opening it up like pores of the skin," and therefore, providing little niches for the deadly elements to settle into.

Stress. Another mechanism is provided by hormone secretions, often set off in stressful situations.

These too can "alter and change the lining of the blood vessel and make it more receptive to atherosclerosis." Diethrich feels this may be the major role stress and the A-type (or AA or AAA) personality play in the disease.

The bottom line: This is a disease that "really is a silent killer," says Diethrich; in this country, 40 percent of those with heart attacks have never had any previous symptoms.

Diethrich's object is to identify people whose risk is high, even when there are no obvious symptoms, and put them on a life-style plan which will forestall the illness. For example:

Smoking: "Poison to the cardiovascular system. In addition to the carbon monoxide, nicotine is a vaso-constrictor"; when there's a build-up in a vessel, smoking adds a double-whammy.

Cholesterol: May be overproduced by the liver, but there is plenty of it around in high-fat foods like egg yolks (whites are fine), butter, red meats. The correlation between cholesterol and atherosclerosis seems clear, believes Diethrich.

Triglycerides: Come from things high in carbohydrates, like pasta, vegetable oil, alcoholic liquor, sugar-you know, the fattening things, the (sigh) good things . . .

Weight: See above and add exercise to maintain a lean profile.

Finally, Diethrich recommends regular physicals and insists they include an electrocardiogram during exercise, a so-called stress test for men and women without symptoms.

"If you don't have it," he insists, "you might as well not have paid for your annual physical. It's a waste of time."

There is no simple way -- yet -- to prevent the disease, although research continues. "You can take Vitamin E. Vitamin B, Vitamin C, lecithin, all those things," says Diethrich, "and put them together and there is still no scientific evidence that . . . atherosclerosis is prevented by any of these over any of the other things that are sold across the counter and get a lot of publicity."