September 25, 2018
Checking blood pressure regularly

Measuring Your Blood Pressure at Home

by Tim Gower

If you have high blood pressure, checking it regularly at home might help you better control the condition, even if your doctor takes your blood pressure at most appointments. A British study published in March 2018 by the journal Lancet confirms previous findings that out-of-office measurements can give you additional insight into the state of your cardiovascular health.

Compared with office measurements, home monitors (see inset) provide a better picture of your average blood pressure level, predict your risk of heart attacks and strokes more accurately, and provide instant feedback as to whether your treatment regimen is working. It’s also beneficial for people with chronic kidney disease and diabetes who need to closely monitor their readings to control high blood pressure (hypertension).

Problems with clinic BP readings

Fifteen to 30 percent of people whose blood pressure is elevated when tested in a doctor’s office or other healthcare setting have normal readings when they measure their own blood pressure at home. This phenomenon is known as white-coat hypertension, which is most common in older adults. Anxiety about visiting the doctor is presumed to be the cause of artificially elevated measurements. Additional home monitoring may reveal that those patients may not need antihypertensive medications, which can cause side effects, or that they can get by with lower doses. However, people with white-coat hypertension are at increased risk for sustained high blood pressure so they still need to regularly monitor their blood pressure.

Other factors can also affect the accuracy of in-office blood pressure measurements. Having a doctor or nurse check your blood pressure is routine, but you may be surprised to learn that current guidelines list 19 requirements for obtaining an accurate measurement. They include having patients sit for five minutes before a reading with their back supported and feet flat on the ground—perching on an exam table with legs dangling is unacceptable. At least two measurements should be taken during an office visit and averaged to estimate blood pressure.

Some research suggests there can be a significant disparity between blood pressure readings taken under ideal circumstances and those that aren’t. A study published in February 2017 in the Journal of the American Heart Association bears this out: Researchers obtained in-office blood pressure measurements from 275 patients with chronic kidney disease and normal blood pressure. Each patient was retested three times after sitting alone in a quiet room for five minutes. After patients sat quietly, their readings averaged 12 to 13mmHg (millimeters of mercury) lower compared to when their readings were taken with no prior quiet time.

How to Buy a Home Blood Pressure Monitor

Home blood pressure monitors provide a better snapshot of your average blood pressure level than office measurements do. They are available for purchase in drugstores or online, and some insurance plans may cover the cost.

Benefits of home data

Participants in the Lancet clinical trial included nearly 1,200 men and women (average age, 67) with high blood pressure of 140/90mmHg or greater despite antihypertensive drug therapy. Study subjects were randomly divided into three groups. One group checked their blood pressure at home using an automated electronic monitor twice in the morning and twice in the evening the first week of each month. Patients recorded their measurements on paper for the week and mailed them to their doctors.

The second group followed a similar routine but used a telemonitor system to transmit readings electronically to their doctors. One telemonitoring benefit was that it prompted patients to alert their doc­tors if their readings were above their target level. The doctors for both groups used self-measurements to decide whether to adjust patients’ antihypertensive drug dose.

A third group served as the control and received standard management of blood pressure based on in-office blood pressure readings.

After one year, participants in both self-monitoring groups had reduced their systolic blood pressure (the top number) by 3 to 4mmHg. By contrast, patients whose blood pressure was measured only during office visits still had readings slightly higher than 140mmHg. A 3- or 4-point difference in systolic blood pressure may not seem like much, but it’s enough to reduce the risk of stroke by 20 percent and coronary heart disease by 10 percent.

One likely reason for the difference is that the patients who self-monitored and regularly communicated their readings to their doctors were prescribed more medications to reduce blood pressure than patients who were checked in-office.

Another possible reason for the differences: Previous studies have found that patients who self-monitor were more likely to adhere to their treatment as a result of being involved in their own healthcare.

Most participants were white, which may limit how much results can be generalized to other populations. Age appeared to make no difference in how effectively patients could self-check blood pressure.

Do it right

If you use a home blood pressure monitor, follow these basic rules to help ensure that you obtain accurate readings:

  • Don’t smoke, consume caffeine, or exercise in the 30 minutes before checking your blood pressure.
  • Don’t measure your blood pressure when you have a full bladder.
  • Try to measure your blood pressure shortly before taking a dose of your anti­hypertensive medications.
  • Sit on a chair with a straight back (not a sofa) quietly for at least five minutes with your feet flat on the ground. Don’t cross your legs.
  • Relax, breathe normally, and don’t talk.
  • Place an arm cuff on your bare arm just above the elbow. Make sure the cuff is snug and won’t slip and that it’s not squeez­ing too tightly. A cuff that’s too large or too small can cause inaccurate readings.
  • Support the arm wearing the cuff on a flat surface, such as a table, at roughly the height of your heart.
  • Measure your blood pressure at the same time(s) each day.
  • Don’t measure when you’re under stress or anxious, which can cause an artificially high reading, or if you suspect your blood pressure is elevated.

Ask your doctor what your target blood pressure level should be. American College of Cardiology (ACC) and American Heart Association (AHA) guidelines define hypertension as a systolic blood pressure (the top number) of 130mmHg or higher, and a diastolic blood pressure (the bottom number) of 80 mm Hg or higher.

Ask how often and when to check your blood pressure. The AHA recommends taking measurements every day for one week before your next doctor’s visit. Take two or three readings one minute apart. The AHA also suggests checking your blood pressure beginning two weeks after a change in treatment.

If your device digitally stores data (most do), bring it to your next office visit; otherwise, jot down your results on paper. If your blood pressure suddenly rises to 180/120 or higher, test your measurement again in five minutes. If it’s still high, contact your doctor right away; you could be undergoing a hypertensive crisis, which could lead to a stroke if not treated. If excessive levels are accompanied by symptoms such as chest pain, trouble breathing, back pain, numbness, vision changes, or difficulty speaking, call 911 immediately.

This article first appeared in the September 2018 issue of UC Berkeley Health After 50.

Also see New Blood Pressure Guidelines: The Numbers Game.