How is blood pressure measured?

In adults, blood pressure is considered to be normal under a systolic value of 140 mmHg and under a diastolic value of 90 mmHg. In high blood pressure, at least one of these numbers (systolic or diastolic) is high. The medical term for high blood pressure is hypertension.

Many people have constant high blood pressure. If you or your doctor think you have high blood pressure, it's important to have your blood pressure checked regularly. If it is too high multiple times, there are various options for lowering your blood pressure, like making changes to your lifestyle or using medication. That lowers the risk of complications like stroke or heart attack.

Your doctor will typically use the following instruments in combination to measure your blood pressure:

• a cuff that can be inflated with air,
• a pressure meter (manometer) for measuring the air pressure inside the cuff, and
• a stethoscope for listening to the sound the blood makes as it flows through the brachial artery (the major artery found in your upper arm).

To measure blood pressure, the cuff is placed around the bare and extended upper arm, and inflated until no blood can flow through the brachial artery. Then the air is slowly let out of the cuff.

As soon as blood starts flowing into the arm, it can be heard as a pounding sound through the stethoscope. The sound is produced by the rushing of the blood and the vibration of the vessel walls. The systolic pressure can be read from the meter once the first sounds are heard. The diastolic blood pressure is read once the pounding sound stops.

A single high reading does not necessarily mean that you have constantly high blood pressure. For example, seeing the doctor makes some people so nervous that their blood pressure goes up.

So a careful reading at the doctor's involves the following steps:

• You first sit calmly for a few minutes for the circulation of your blood to slow down and your blood pressure to reach its normal level.
• Then the blood pressure in your upper arm is measured three times.
• These measurements should be taken two minutes apart.

When taking your blood pressure for the first time, it makes sense to measure the blood pressure in both arms, because it's sometimes high on only one side. The higher measurements are taken as your blood pressure. After that it is enough to measure the blood pressure only in the arm that produced the higher reading.

If you have abnormal levels, experts recommend ambulatory blood pressure monitoring to confirm the diagnosis.

Ambulatory blood pressure monitoring involves wearing a blood pressure cuff for a period of 24 hours. It is connected to a small, portable measuring device that automatically measures your blood pressure every 15 to 30 minutes and records the readings. It is then possible to tell how much the readings vary throughout the day or whether they are also high at night, for instance.

During those 24 hours, you can do all of the usual things you would otherwise do over the course of the day. If you are especially active at certain times, you can make a note in a diary. Then the doctor has a better idea of how to interpret the recorded values when evaluating them. Readings over 135 to 85 mmHg are considered high in ambulatory blood pressure monitoring.

Alternatively, you can measure your blood pressure at home once in the morning and once in the evening for seven days and record the readings each time. You will need a suitable blood pressure meter. An average of the readings is taken after the week is up. An average of 135 over 85 mmHg is considered high in this case as well.

Electronic measuring devices are suitable for self-monitoring. They are usually put on your upper arm, but can also be fitted on your wrist. Then they only need to be activated with the push of a button. When taking blood pressure measurements on the wrist, it's important to keep the hand level with the heart. Otherwise it can affect the readings.

Digital meters can sometimes be inaccurate and produce unreliable readings anyway – especially in people with certain heart rhythm problems or arteries that have hardened due to arteriosclerosis. In Germany, monitoring devices for home use are tested for accuracy by the German Hypertension Society (in German: Deutsche Hochdruckliga). They publish a list of reliable units.

If you have been diagnosed with high blood pressure, statutory health insurers in Germany will also cover part of the costs for the blood pressure monitoring device. You will need a prescription from a doctor that you can turn in at a medical supply store.

Follow these steps to measure blood pressure in your upper arm:

• Sit upright with your legs relaxed and resting on the floor.
• Roll up your sleeve and put the cuff on your bare upper arm tight enough that you can just fit your index and middle finger between the cuff and your arm. The cuff should be placed about 2 to 3 centimeters (about an inch) above your elbow.
• Rest your arm on a table so that your upper arm is level with your heart.
• Take five minutes to sit calmly and relax.
• Start the measurement while remaining calmly seated. Breathe normally and don't speak.
• Measure the blood pressure twice in a row in your upper arm. You just need to wait about one or two minutes between the readings.
• It is best to measure your blood pressure in the morning and in the evening.
• Write down all of the readings.

In Germany and other countries, people with high blood pressure can attend patient education courses that teach a number of things, including how to measure your blood pressure. As part of specialized disease management programs (DMPs) for people who have narrow coronary arteries (coronary artery disease, CAD), statutory health insurers offer additional healthcare services. These include patient education about high blood pressure. Some doctor’s practices offer these courses.

There is no general recommendation for how often you should measure your blood pressure at home. Some people take their blood pressure on two days per week or in the first week of each month. It's best to talk to your doctor about which approach would be most suitable in your case.

More frequent measurements are often recommended if your treatment is changed – for instance if you start taking your medication at a new dose. If your blood pressure is stable over a longer time, then less frequent readings are enough.

An upper blood pressure reading of over 180 mmHg and a lower reading of over 110 mmHg is called a hypertensive crisis. That can lead to problems like redness in the face, headaches or dizziness, and those kinds of symptoms will usually be caused by other things.

There is usually no reason to worry if the levels are only temporarily that high. Experts recommend resting and then measuring your blood pressure again 30 minutes later.

If any signs of an emergency develop, like a heart attack or stroke, you should lie down and call for emergency services or have someone call for you (112 in Germany and many other countries, 911 in the U.S.). These signs include:

• Breathing problems
• Chest tightness
• Paralysis
• Vision or speech problems
• Seizures
• Confusion

It is important to seek a doctor immediately if you’re pregnant and have a hypertensive crisis. That could be a sign of eclampsia, which can be dangerous for both you and the unborn child.