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Measuring blood pressure (Blood pressure measurement)

Blood pressure measurement is a quick and easy, inexpensive and safe examination. It provides information about the pressure in blood vessels and thus provides valuable information about the function of the human cardiovascular system.

What is blood pressure?

The blood vessels form a flexible, widely branched system that supplies our body with oxygen and nutrients. The heart pumps the blood into the arteries, which transport it to the individual organs, and the blood flows back to the heart via the veins. In order for the blood to reach even the most remote corners of our body, a certain pressure must prevail in the circulation.

In medicine, two blood pressure values ​​are usually given - for example 120/80 mmHg, i.e. "120 to 80 millimeters of mercury". The first number - the upper or systolic value - corresponds to the pressure during the tension phase of the heart (systole). The second number - the lower or diastolic value - corresponds to the pressure in the relaxation phase (diastole).

Millimeter of mercury (mmHg) is a historical unit of measurement that is still used in medicine. The name comes from the fact that in earlier times a glass tube filled with mercury was used to measure pressure. The prevailing pressure shifted the mercury column by a certain number of millimeters, which could be read off on a scale.

How does blood pressure measurement work?

The blood pressure in the arterial vascular system is usually determined. There are also a number of special procedures that can be used to determine venous blood pressure, for example. However, these procedures play a subordinate role in everyday medical practice.

Arterial blood pressure is usually measured using an inflatable cuff that is placed around the patient's upper arm. This method was developed by the Italian doctor Riva-Rocci at the end of the 19th century. His initials RR are still used today by medical professionals as an abbreviation for blood pressure. For example, the entry "RR 120/80" in a patient file means that the patient has a blood pressure of 120/80 mmHg.

To measure the blood pressure, the cuff is placed about two finger widths above the crook of the elbow on the upper arm and inflated. This compresses the artery on the upper arm (brachial artery) more and more. Your doctor will inflate the cuff to such an extent that the pressure generated is safely above the expected systolic blood pressure value. The humerus artery is then completely compressed and no more blood can pass through.

As a result, the air is slowly released from the cuff - which reduces the pressure on the upper arm. If the cuff pressure falls below the systolic blood pressure, the heart can pump blood through the compressed artery during the contract phase. Because blood can only flow through the vessel during the pressure peaks, a characteristic knocking sound (Korotkow sound) is created with each heartbeat. This sound can be heard using a stethoscope placed in the crook of the elbow. Your doctor will read on the scale on the blood pressure meter at which pressure the knocking sound first occurs. This value corresponds to the upper, systolic blood pressure.

If the cuff pressure falls below the diastolic blood pressure when the blood pressure continues, the blood can flow unhindered through the upper arm artery and the knocking noise disappears. The value that can be read off the sphygmomanometer scale at this point is the lower, diastolic blood pressure value (auscultatory method).

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What other methods of measuring blood pressure are there?

In noisy environments, it is often impossible to hear the Korotkov sound. In these cases, doctors feel the pulse in the radial artery when the pressure is released from the cuff placed around the upper arm. The blood pressure at which the pulse can be felt for the first time corresponds to the upper, systolic value. The diastolic blood pressure value cannot be determined in this way (palpatory method).

Blood pressure is also determined with automatic measuring devices by placing an inflatable cuff around the upper arm or wrist. The systolic and diastolic values ​​are determined by automatic blood pressure monitors, however, in that the device records vibrations that are generated when the cuff pressure is released from the arterial wall (oscillatory method).

In addition to the externally performed (non-invasive) blood pressure measurement methods described above, there are invasive methods in which a pressure sensor is inserted directly into a vessel. In this way, the blood pressure in any vessel can be continuously monitored. These methods are mainly used in intensive care medicine, but hardly play a role in everyday medical practice.

To create a daily blood pressure profile, your doctor will take a long-term measurement with a special blood pressure monitor. The blood pressure cuff usually remains on the arm for 24 hours and measures the blood pressure every 15 to 30 minutes. The saved values ​​can be better interpreted if you note the time of day when you worked particularly hard or excited.

How high should the blood pressure be?

Blood pressure is not a stable variable; it fluctuates depending on the performance that the cardiovascular system has to provide. In healthy people, for example, blood pressure is lower during sleep than during the day and lower during rest than during physical exertion.

Only when the blood pressure values ​​at rest are repeatedly above 135/85 mmHg or above, one speaks of high blood pressure (arterial hypertension) by definition. So, to diagnose high blood pressure, it takes repeated measurements. According to the currently valid recommendations of the Austrian Society for Hypertensiology, at least 30 measurements are required: If seven or more of these 30 values ​​exceed the limit of 135/85 mmHg, hypertension is present. In the guidelines of other specialist societies, the limit to high blood pressure is only set at 140/90 mmHg

If the resting blood pressure values ​​are repeatedly below 95/65 mmHg, one speaks of low blood pressure (arterial hypotension). Tall, slim people, young people and pregnant women in particular are often affected. Permanently low blood pressure is usually harmless, and in most cases a cause cannot be found. Arterial hypotension can also be the result of diseases of the heart, nervous or endocrine systems. Other common triggers are infections or lack of fluids.

What should be considered when measuring blood pressure?

Since a visit to the doctor is a stressful situation for many people, the blood pressure values ​​measured in the ordination or ambulance are often higher than in everyday life. This so-called "white coat syndrome" leads to an increase in blood pressure values. Try to be as relaxed and rested as possible when measuring your blood pressure.

The measurement should not be carried out immediately upon arrival, but rather after ten minutes at the earliest. Also tell your doctor whether and for how long you have been taking antihypertensive medication before the measurement.

What should you watch out for with self-measurements?

If you have permanent blood pressure problems, you should get your own blood pressure monitor for relaxed self-measurement at home. Get advice from qualified personnel when purchasing a device. Use a cuff that is adapted to the circumference of your upper arm or wrist (important!).

Please note the following when measuring yourself: Place the cuff on the designated place on the upper arm or wrist. Since blood pressure is also influenced by gravity, measurements should always be taken in the same body position in order to obtain comparable values. Usually this is done while sitting. The legs should not be crossed, the back should be leaned and a few minutes of relaxation (important!) Should follow. Also make sure that the measuring point is always at heart level. This is automatically the case for devices with an upper arm cuff. With measuring devices for the wrist, it is advisable to place the forearm on a stable surface at about heart level.

If you have arterial hypertension that has been treated with medication, you should monitor your blood pressure with regular self-measurements. If the blood pressure values ​​change, you should not adjust the dose of the medication you are taking on your own. The best thing to do is to write down the measured values ​​in a blood pressure diary and take it with you to your next doctor's visit. It is advisable to compare the values ​​determined in the self-measurement from time to time with the manual measurement by a doctor. In this way, any device measurement errors can be detected.

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Dr. med. Matthias Thalhammer
Medical review:
Dr. Ludwig Kaspar
Editorial editing:
Philip Pfleger, Claudia Natschläger (2017)

Status of medical information:

Klinke, Pape, Silbernagl: Physiology. Georg Thieme Verlag 2005.
Levick, J.R. An Introduction to Cardiovascular Physiology. Butterworths 1991.
Eckert, S .: 100 Years of Blood Pressure Measurement According to Riva-Rocci and Korotkoff: Review and Outlook. Journal for Hypertension 10 (3) 2006, pp. 7-13.
O’Brien, E: Blood pressure measurement, part IV - automated sphygmomanometry: self blood pressure measurement. British Medical Journal (322) 2001, pp. 1167-70.