Some of the most commonly used drugs for treating high blood pressure may actually increase the chances of having a heart attack by raising the level of cholesterol in the blood.

For the hypertensive patient, this can mean a trade-off between two known contributors to heart disease: elevated levels of cholesterol, which can clog arteries, or high blood pressure, which can damage blood vessels, Dr. Richard R. Grimm Jr., assistant professor of preventive cardiology at the University of Minnesota, said at a conference on cholesterol and hypertension here last week.

Two groups of blood-pressure-lowering drugs, thiazide diuretics and beta blockers, cause a 6 to 8 percent increase in cholesterol.

"Now you may say big deal, 6 to 8 percent is not so large," said Grimm, who does not believe the increase dissipates over time. "But for every 1 percent that you lower total cholesterol, you lower risk of heart disease by 2 percent. So an 8 percent increase in cholesterol would be comparable to a 16 percent increase in risk."

Thiazide diuretics eliminate liquids from the body. These drugs, the most frequently prescribed group of medications for treatment of hypertension, are taken by some 30 million Americans.

Beta blockers are the second most commonly used group of drugs. These medications ease the heart's work load by decreasing blood pressure, heart rate and resistance of blood vessels to flowing blood.

According to Grimm, selective alpha blockers like Prazosin, a relatively new arrival to the field of antihypertensive drugs, do not increase cholesterol. Some studies, in fact, have shown that Prazosin decreases cholesterol by 5 to 8 percent while also lowering blood pressure.

Selective alpha blockers work directly on the blood vessels outside the heart and prevent them from constricting. Prazosin, manufactured by Pfizer Laboratories, which sponsored the conference, is the only alpha blocker currently available. Prazosin is not widely used, some doctors say, because it's difficult to determine correct doses and because the agent initially can cause a sudden drop in blood pressure that makes people feel faint.

Agents that have no adverse cholesterol effect are not in higher demand because many physicians are unaware of the cholesterol-raising consequences of the more popular drugs. "Maybe one fourth of physicians know about the cholesterol increase caused by diuretics and beta blockers," Grimm said in an interview.

Cholesterol may be one of the health buzzwords of the '80s, but most physicians do not monitor its levels in the blood -- despite its known link to heart disease. In one study, 80 percent of physicians thought it was important to treat elevated blood pressure but only 39 percent thought elevated blood cholesterol levels were significant.

"It's much more glamorous to treat heart attacks and perform transplants than it is to prevent heart attacks," said Dr. Antonio M. Gotto Jr., chairman of the internal medicine department at Baylor College of Medicine in Houston. "But it's important to put major emphasis on prevention because that's where you can accomplish the most in a long run."

Even when physicians do monitor cholesterol, however, some are unaware of recently lowered standards for what constitutes "normal" levels of cholesterol.

"The average cardiologist and physician do not treat cholesterol -- most get excited about 300 milligrams per 100 milliliters of blood serum or higher," said Dr. William Castelli, medical director of the Framingham Heart Study. The high risk level for an adult in his thirties, defined in December by the National Institutes of Health, is anything greater than 240 milligrams.

The study in Framingham, Mass., under way for the past 34 years, indicates that every fifth man and every 17th woman has a heart attack by age 60. "When people hear that, they say get the heck out of Framingham," said Castelli. "But these studies have been run everywhere and the rates are largely the same."

Coronary heart disease, in which the arteries become so clogged that the blood supply no longer reaches the heart muscle, kills 550,000 Americans each year. According to Castelli, many physicians and patients aren't aware of all the factors contributing to heart disease.

Physicians now believe the narrowing of the arteries due to fat deposits begins at an early age. So children's cholesterol levels ought to be checked periodically as soon as they are old enough to attend school, said Gotto. Annual monitoring, though advisable for adults over 30, is unnecessary for children.

Children should be taught to maintain a low-fat diet that helps control cholesterol levels. "People tend to develop eating habits early in life. It's not reasonable to expect someone to follow a pattern of eating for 20 years and then one day just change it -- although that's what we ask people to do," said Gotto, who noted that physicians are now advising a similar diet to protect patients from risks of heart disease, diabetes and cancer.

Such a diet, which can lower cholesterol by as much as 8 percent, includes avoiding salt, red meat and processed meat (a hot dog, for instance, is 81 percent fat) and increasing consumption of legumes, fresh produce and fish.

Fish may, in fact, play an increasingly important role in the American diet because some scientists now believe fish oil reduces cholesterol levels. A recent study published in The New England Journal of Medicine compared the diets of 852 men for more than 20 years. The results showed that men who ate more than one ounce of fish daily were half as likely to suffer heart attacks.

Gotto explained that shellfish such as oysters, clams and scallops, which do not eat other fish, contain the least cholesterol. "I always tell my patients," said Gotto, "if you can't be a vegetarian, eat a sea vegetarian."