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High Blood Pressure (Hypertension)

How is Blood Pressure Measured?

Last updated on:
10/04/2012

Contributing Author: Guy Slowik FRCS

Blood pressure is measured in millimeters of mercury (mm Hg). A typical normal blood pressure is 120/80 mm Hg, or "120 over 80." The first number represents the pressure when the heart contracts and is called the systolic blood pressure. The second number represents the pressure when the heart relaxes and is called the diastolic blood pressure.

Blood pressure measurement is a painless and simple test. Blood pressure is one of the key identifiers of general health that will almost always be measured at the doctor's office. Reliable machines are available for you to measure your own blood pressure at home.

How To Information:

To help make sure your blood pressure measurement is accurate, avoid smoking and eating or drinking anything that contains caffeine for 30 minutes before the test. Rest for five minutes before the measurement is taken. Your health care provider will usually ask you to sit in a chair, with your bare arm comfortably supported at the level of your heart.

Blood Pressure Measurement In The Doctor's Office

Most people are familiar with having their blood pressure measured during routine visits to the doctor's office or other health care facility. But did you ever wonder exactly what your health care provider is doing? Here are the steps your health care provider should follow when taking your blood pressure measurement:

  • Your health care provider will probably use a blood pressure cuff and stethoscope to measure your blood pressure. The blood pressure cuff, also called a sphygmomanometer, consists of a cuff with an inflatable bladder, a rubber hand bulb with a valve used to inflate and deflate the cuff, and a pressure gauge.
  • First, the cuff is wrapped snugly around your upper arm. The cuff is then rapidly inflated until the pulse in the upper arm is no longer felt. At this point blood flow in the underlying blood vessel is cut off by pressure in the cuff. The health care provider will continue to inflate the cuff a bit beyond this point.
  • Next, a stethoscope is placed over the brachial artery at the elbow and the cuff is slowly deflated, while the health care provider listens for sounds produced by turbulent blood flow in the artery.
  • The health care provider listens through the stethoscope until he or she hears the heartbeats. At this point, cuff pressure matches pressure in the artery, and blood flow resumes. This is the systolic blood pressure (SBP).
  • The health care provider continues to slowly deflate the cuff until the sounds stop. This is the diastolic blood pressure (DBP).

Experts recommend that two or more readings (separated by two minutes) be taken to determine an average blood pressure for the visit. If the first two readings differ by more than 5 mm Hg, additional readings should be obtained and averaged. Your doctor should tell you if your blood pressure should be measured again and/or if you should consider treatment for high blood pressure.

Measuring Your Own Blood Pressure

Stress, caffeine, smoking, pain, and other factors can cause blood pressure to rise temporarily. For some people, just the act of having their blood pressure taken in a doctor's office, clinic, or hospital can cause a rise in blood pressure. At other times, their blood pressure is normal. These people have what is called white coat hypertension.

Because so many different things can cause a temporary rise in blood pressure, your doctor may ask you to measure your own blood pressure at home. Various home monitors are available that simplify the process of measuring your own blood pressure.

Home monitors are reasonably accurate. However, some types of monitors (for example, finger monitors) have been associated with inaccurate blood pressure readings. Be sure to check your home monitor periodically for accuracy. When you visit your doctor, bring your home monitor and ask the person who takes your blood pressure measurement to compare the reading they obtain with a mercury sphygmomanometer to the reading on your home device

Keeping Track Of Blood Pressure On The Go

On-the-go, or ambulatory, blood pressure monitoring may be useful for some people. These include people who suffer from white coat hypertension, those who are not responding to blood pressure medication, or those with other complicating factors.

A variety of reliable, easy-to-use, and accurate ambulatory monitors are available commercially. These monitors can be worn over the shoulder or around the waist and typically record readings every 15 to 30 minutes, 24 hours per day.

Nice To Know:

Blood pressure values obtained with ambulatory monitors are 5 to 10 percent lower than those obtained by other means.

 

 
 

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From Andrew Maynard - Chair of the University of Michigan Department of Environmental Health Sciences, with help from David Faulkner - 2013 Master of Public Health graduate.