Physicians know the basics of blood pressure, but patients often don’t, and that can lead them to seek answers to their questions on the internet, where advice can be dicey.

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So instead of leaving patients’ understanding of this crucial health issue to chance, physicians should have some baseline guidance at the ready, including why BP is so important to keep an eye on, what normal blood pressure is, what qualifies as high blood pressure, and the best ways to keep a weather eye on one’s numbers.

It starts with simply understanding what blood pressure is and why it matters.

“Blood delivers oxygen to our organs and also takes waste away from them,” said Kate Kirley, MD, a family physician and director of chronic disease prevention at the AMA. “If blood pressure gets too high, it can cause damage to our circulatory system—it can hurt our brains, our hearts, our kidneys, pretty much all of our organs.”

Think of an ordinary doctor’s office visit: The patient is seated in a work-up area and has their blood pressure measured. If it’s normal, the nurse or medical assistant doing the measuring might not say anything at all. So how do patients know their BP numbers are normal? And what does “normal” mean anyway?

In this case, normal is a goal—under 120/80 mm Hg. Any numbers over  that threshold are considered elevated or high. If a person has multiple elevated BP measurements at different points in time, they often have high blood pressure, which is also known as hypertension.

“The easy way of understanding these numbers is that the top one—what we call systolic—refers to the pressure  in the arteries when the heart pumps blood,” Dr. Kirley said. “The bottom number—diastolic—refers to the pressure in the arteries between beats when the heart is relaxed and refilling. There’s obviously more pressure when the heart is pumping than when it is relaxed.”

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Each is measured in millimeters of mercury, commonly abbreviated mm Hg. This sounds very technical but is actually easy to understand.

“Older blood pressure devices use a mercury column. If you exert pressure on the mercury, it rises,” Dr. Kirley said. “People may be familiar with this if they have seen an older BP device and they have watched that mercury column rising and falling when their blood pressure is measured. Those devices are still the gold standard, although we don’t use them often today because of safety issues with mercury.”

Target: BP™, a national initiative co-led by the AMA and American Heart Association, offers guidance for patients to understand their blood pressure numbers.

Patients might find it interesting to know that just having their blood pressure measured can affect their numbers. In one case, the stress of being in a doctor’s office can cause their blood pressure to go up—a phenomenon called white-coat hypertension.

But the opposite can also be true. It’s less common, but some patients with high blood pressure can appear to have normal blood pressure when it is measured in a doctor’s office. This is called masked hypertension.

While white-coat hypertension and masked hypertension are different scenarios with different treatments, they are both signs that a person’s blood pressure needs to be checked on a regular basis.

"Whenever we’re concerned about a patient’s blood pressure, we want to know their blood pressure in the office and out of the office so we can really understand them,” Dr. Kirley said. “Your BP is not normal unless your numbers are normal in both settings.”

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Patients should know the blood pressure monitors found in pharmacies, grocery stores and worksites  aren’t always proven to be accurate, so they should be careful about trusting them. If patients want to track their numbers outside of their doctors’ visits, their physician may recommend they look for devices that have been validated for accuracy. Their doctors can reference the U.S. Blood Pressure Validated Device Listing (VDL™), which lists BP measurement devices that have been validated in the U.S.

In addition, for patients looking to track their blood pressure at home, Target: BP has an easy-to-use set of tools and resources to measure accurately.

No matter what, patients should reach out to their doctors for guidance on which BP measurement device to purchase, as even the size of the cuff can spell the difference between trustworthy numbers and inaccurate ones.

“Your doctor’s office should measure your arm to figure out which size is appropriate for you,” Dr. Kirley said. “That fit is all-important.”

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