What is blood pressure and how can I measure it?

The heart is responsible for supplying the organs and tissues of the body with blood. To do this it pumps blood into the large vessels of the circulatory system with every beat. The blood that is pumped into these vessels puts pressure on the walls of the vessels. When the heart muscle contracts during each heartbeat, the pressure rises as well. When measuring blood pressure, there are two different values:

• Systolic blood pressure is taken while the heart muscle is contracting and pumping oxygen-rich blood into the blood vessels.
• Diastolic blood pressure is taken while the muscle is relaxing and refilling with blood.

Blood pressure is measured in units of “millimeters of mercury”; written mm Hg for short. Blood pressure measurements are always given in pairs, with the upper (systolic) value first, followed by the lower (diastolic) value. So someone who has a reading of 132/88 mm Hg (often spoken “132 over 88”) has a systolic blood pressure of 132 mm Hg and a diastolic blood pressure of 88 mm Hg.

Example for a blood pressure reading: 132/88 mm Hg

132	systolic blood pressure value
88	diastolic blood pressure value
mm Hg	blood pressure unit

What is normal blood pressure, and when is blood pressure considered to be high?

Blood pressure is always measured on several days and when you are at rest. If several of these measurements are too high, you are said to have high blood pressure, even if only one of the two – either the systolic or the diastolic value – is high. The medical term for high blood pressure is hypertension. Normal blood pressure in adults is below 140 mm Hg for systolic and below 90 mm Hg for diastolic.

When taking a blood pressure reading for the first time, it makes sense to measure the blood pressure in both arms, because it is sometimes high on only one side. The values that are higher are always the ones used for assessing blood pressure. After that it is enough to measure the blood pressure only in the arm that produced the higher reading.

A person has hypertension if the systolic value is over 140 mm Hg, the diastolic value is over 90 mm Hg, or if both are too high. High blood pressure itself usually goes unnoticed. Only if it is extremely high can it result in symptoms like dizziness or trouble seeing. If hypertension remains undetected for a long time and is not treated, the risk of cardiovascular disease such as heart attacks, strokes and heart and kidney failure increases. So if you or your doctor think you have high blood pressure it is important to have your blood pressure checked regularly. If the readings are repeatedly abnormal, there are several different ways of lowering your blood pressure and thereby decreasing the risk of complications.

Values for normal and high blood pressure

normal blood pressure	systolic under 140 mm Hg and diastolic under 90 mm Hg
high blood pressure	systolic over 140 mm Hg and/or diastolic over 90 mm Hg

How is blood pressure measured?

It is important to measure blood pressure more than once, because it fluctuates over the course of the day. Physical exertion, stress, pain or extreme heat or cold can affect blood pressure, for example. But if blood pressure changes due to any of these things, it is only temporarily high and will then fall back down to normal range quickly. So, if blood pressure is measured just once and found to be high, it does not automatically mean that it is also permanently too high. A blood pressure reading taken at the doctor’s office can also be misleading: a visit to the doctor makes some people so nervous that their blood pressure rises.

All of this means that to get reliable results it is important to always take readings on different days and while the person is at rest. This means sitting back and relaxing on a chair and waiting about three minutes before taking a measurement so that the circulatory system comes to rest. The upper arm, which is commonly used to take the measurement, should lie on a table at about the same level as the heart while the reading is being done.

There are basically two different instruments for measuring blood pressure: a sphygmomanometer, which is used for measuring blood pressure manually, and a digital blood pressure meter, which can take measurements automatically.

A sphygmomanometer has three parts:

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

The scale of the pressure meter ranges from 0 to 300 mm Hg. The pressure meter has a rubber bulb for inflating the cuff and a button for letting the air out.

To measure blood pressure, the cuff is placed around the unclothed and extended upper arm and then inflated until no blood can flow through the brachial artery. For this to happen, the pressure in the cuff must be greater than the blood pressure in the brachial artery. Then the air is slowly let out of the cuff. As soon as the air pressure in the cuff falls below the systolic blood pressure in the brachial artery, blood will start to flow through the arm once again. This can be heard by using the stethoscope close to the elbow, because the inrushing blood makes a pounding sound. Right when you start to hear this pounding for the first time you can read your systolic blood pressure off of the pressure meter. The pounding stops when the air pressure in the cuff falls below the diastolic blood pressure in the brachial artery. Right when the pounding stops, you can read the diastolic blood pressure off of the pressure meter.

Digital blood pressure meters are often used on the wrist, but they can also be placed on the finger or upper arm and only have to be activated by pressing a button. They read the blood pressure automatically from variations in the volume of the blood present in the arteries. When measuring on the wrist it is important to keep the hand level with the heart. Otherwise you can get skewed results. Digital meters can also sometimes be inaccurate and produce unreliable readings – especially in people with certain heart rhythm problems or arteries hardened due to arteriosclerosis. Your doctor can help you choose the right type of blood pressure meter.

What is an ambulatory blood pressure monitoring?

In some cases your doctor will also take your ambulatory blood pressure. For ambulatory blood pressure monitoring, you will wear a blood pressure cuff for 24 hours. It will automatically measure your blood pressure at fixed intervals. These readings are then recorded by a small portable meter. Ambulatory blood pressure monitoring is used, for example, to see whether blood pressure levels vary greatly over the course of the day and night or whether they are constantly elevated. While these measurements are being recorded you can do all of the usual things you would otherwise do over the course of the day, while also filling out a diary. You can write down any times when you were especially active in the diary. Then the doctor has a better idea of how to interpret the recorded values when evaluating them.

More on this topic here.


Author: Institute for Quality and Efficiency in Health Care (IQWiG)

© IQWiG (Institute for Quality and Efficiency in Health Care) Last update of this website: 05/22/2012