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Leading heart organizations have lowered the threshold that defines high blood pressure—meaning nearly half of all American adults now have this dangerous condition.

As a result, millions of people who didn’t think their blood pressure was high enough to be a problem are being told to reduce it right away.

Why guidelines were changed

New guidelines, published by the American Heart Association and the American College of Cardiology in the journal Hypertension, are based on a review of the latest evidence from hundreds of studies. The findings link blood pressure that’s modestly higher than normal to a doubled risk of heart attack, heart failure, stroke, and kidney failure.

“If you already have a doubling of risk, you need to know about it,” said lead author of the guidelines Paul K. Whelton, MB, MD, MSc, in a news release. “It’s a yellow light that you need to be lowering your blood pressure.”

How the numbers changed

Blood pressure is written as two numbers. The top or first number measures systolic blood pressure—the pressure on artery walls as the heart is pumping. The bottom or second number shows diastolic blood pressure—reduced pressure when the heart is resting between beats.

The new guidelines say that hypertension (high blood pressure) is a reading of 130 mm HG or higher for the systolic number, or 80 and above for the diastolic number. The previous thresholds were 140 and 90.

Normal, healthy blood pressure is anything below 120/80 mm Hg. That hasn’t changed.

People between the normal and hypertension ranges, who have a systolic blood pressure from 120 to 129 and normal diastolic blood pressure, are now classified as “elevated.” The old category called “prehypertension” has been eliminated.

Under the old guidelines, 72 million adults in the United States—about one in three—had high blood pressure. The new guidelines boost that figure to a staggering 103 million Americans—almost one in two adults.

Many of the newly added people are under age 45.

How dangerous is high blood pressure?

Hypertension is the second greatest cause of preventable heart and stroke deaths. Only smoking ranks higher. Because it often has no noticeable symptoms, high blood pressure is often called “the silent killer.”

According to the Centers for Disease Control and Prevention, only about half of U.S. adults with high blood pressure have it under control—leaving the others at risk for possibly deadly complications. That’s tragic, because high blood pressure usually responds well to treatments.

Treating high blood pressure

Millions of Americans take pills to help reduce high blood pressure, but the new guidelines say most people newly classified with high blood pressure won’t have to do that. If your blood pressure has an upper reading in the 130s or a lower number in the 80s, lifestyle changes can bring it down enough in many cases. Those measures include:

• Getting regular exercise
• Eating a healthy diet
• Losing excess weight
• Limiting sodium intake
• Quitting smoking

People in this group who already have heart disease or are at high risk for it should also take blood pressure medicine, according to the new guidelines.

The goal is to get high blood pressure below 130/80 mm Hg—still a little higher than normal, but low enough to avoid the risks that researchers found.

Know your numbers. Get your blood pressure checked at least every two years or as recommended by your doctor. If you’re over 50 or your doctor says you’re at risk for hypertension, more frequent checks are likely a good idea.

Talk with your doctor about where you fit under the new guidelines and whether new or different treatments may be needed. The sooner high blood pressure is controlled, the better—so take steps now to make this “silent killer” disappear.

> BONUS TIP: White-coat hypertension
> Some people who think they have high blood pressure really don’t. They may feel anxious or stressed in the doctor’s office where blood pressure is taken, causing an abnormally high reading. The problem is called “white-coat hypertension.” If you’re told you have high blood pressure, ask your doctor about checking it again a few minutes later, or at another appointment. Also, consider buying a home blood pressure monitor to check it yourself in more relaxed surroundings.

Do you have high blood pressure? How has it responded to treatments? Share your experience with our community by commenting below.