A national advisory panel is preparing a new definition of high blood pressure suggesting that roughly half of adult Americans are at added risk of developing heart disease, stroke or kidney failure.

If adopted, the proposed definition would place a whole new group of individuals previously considered as having "normal" blood pressure in a "greater risk" category.

The change also could have major practical implications in increased medical treatment of high blood pressure at previous "borderline" levels, says Dr. Darwin Labarthe, a University of Texas professor who heads the group in charge of the new definition.

Labarthe's nine-member "working group"--sponsored by the National High Blood Pressure Coordinating Committee, a government-backed board with broad representation from public and private medical organizations--has agreed upon a change that would lower the numerical level of blood pressure considered to be significant in terms of risk.

The proposed expansion represents a "significant change" that would "substantially enlarge" the number of people who should be concerned about their blood pressure, says Graham W. Ward, a National Heart, Lung and Blood Institute official.

He estimates that as many as 15 million Americans would fall into a newly defined category of "intermediate risk" that would suggest a need for added medical surveillance. About 25 million more people would fall into a broader category of "higher" and potentially "reducible" risk in which some form of treatment, whether diet or drug, might be considered. This group previously was regarded as suffering from "borderline" hypertension or high blood pressure.

An additional 35 million individuals already are considered under previous definitions to be in clear need of treatment.

The group had hoped to present a final draft to the parent coordinating committee at its regular meeting last Friday, but ended up giving a preliminary oral report. Because of possible controversy and confusion over the changes, the working group is taking extra time on the wording of its recommendations before disseminating them to the major medical and consumer organizations on the coordinating committee.

"We've agreed on principles but not the precise language," says Ward, who heads the government's health education effort on high blood pressure. He expects the working group to finish a draft by the end of the year, but it could be spring before the document undergoes final scrutiny by the national coordinating committee.

The move to redefine high blood pressure, according to Ward and Labarthe, is based upon two developments. First, an accumulation of new knowledge over the last decade --particularly long clinical trials in this country and others involving large numbers of patients--has confirmed that there are benefits from treatment of high blood pressure at lower levels.

Second, there has been a longstanding gap between public health workers and practicing physicians as to how high blood pressure should be defined. The World Health Organization has developed one standard of "borderline" and "definite" hypertension, while health care providers have frequently talked in terms of "mild," "moderate" and "severe" hypertension.

"The two categories don't fit together," says Ward. One person's "moderate" is somebody else's "severe," and "mild" and "borderline" may also differ.

Blood pressure, the force created by the heart as it pumps blood through the body, is measured in terms of how high it can push a column of mercury in a glass tube. The reading is expressed as two numbers: the upper, or systolic, represents the pressure when the heart contracts, while the lower, or diastolic, is the pressure when the heart rests between beats.

A reading of 120 over 80--written as 120/80--has generally been considered an average blood pressure for adults. The WHO has defined "borderline" hypertension as 140/90 to 160/95 and "definite" as 160/95 and above.

Although both readings are considered important, for practical purposes the lower or diastolic number has increasing been used to categorize high blood pressure. A conventional definition in use in this country has labeled mild hypertension as a diastolic reading of 90 to 104, moderate 105 to 114 and severe over 115.

But the new definitions agreed upon thus far by the working group would place the cutoff point for "minimal" risk at a diastolic of 80, Ward said. The group considers this change warranted, he said, because of scientific data showing that there is a doubling of risk in the range between 80 and 90, an area previously considered normal.

This means, said Ward, that in this new "intermediate" category of risk a person would face, roughly speaking, a one-in-10 likelihood of death from any cause over the next eight years, as compared to a one-in-20 chance for a person at the lowest ranges of blood pressure.

The risk continues to rise as the blood pressure increases, with long-term high blood pressure affecting not only life expectancy but likelihood of heart attack, stroke and kidney disease.

Panel members, sensitive to possible dangers of labeling people, are quick to point out that no treatment is recommmended for those in the "intermediate" group. But readings at this level should signal the need for "systematic" followup, says Labarthe, to make sure there are no further increases.

Although the working group is not in charge of specific management recommendations, one member of the panel, Dr. Robert H. Moser of the American College of Physicians, says that he personally would suggest reduced salt intake for the group at "intermediate" risk, as well as exercise and weight reduction-measures that can help lower blood pressure.

For those with a confirmed diastolic reading of 90 or above, panel members say that recent studies suggest a benefit from some form of treatment, whether diet, exercise or drugs. This follows recommendations last year of the national coordinating coommittee.

Labarthe and Moser believe the new definition will be adopted and gain rapid acceptance in the medical community, but Ward expects that some controversy will doubtless accompany the change. A major educational campaign is anticipated so that people who learn that they are at greater risk from high blood pressure do not become unnecessarily alarmed.

Assuming the campaign is effective, Labarthe says, the new definition should simply be regarded as the latest in "health advice." A national campaign to control high blood pressure, conducted over the last decade, is believed to have contributed to a dramatic down-swing in the death rates from stroke and heart disease.