Ärzte Zeitung online, 20.11.2008

How Blood Pressure is Measured Correctly

Measuring blood pressure correctly is anything but trivial. Hypertension specialists at the Medica have tips for both patients and doctors. Specialists complain that measuring blood pressure correctly is anything but trivial because a great many errors are made.

How Blood Pressure is Measured Correctly

Elderly patient at the doctor having his blood pressure checked. The cuff is correctly positioned at the same level as the heart. Photo: Lisa F. Young©www.fotolia.de

By Thomas Meißner

They see a need for learning not only on the part of patients, but also amongst medical personnel. And when it comes to the measuring devices themselves, the wheat must be separated from the chaff. "Come in, just sit down there and I'll take your blood pressure.

" Well, you can be certain that this will produce a blood pressure measurement 30 to 40 mm Hg higher than it really is, according to Professor Bernd Krönig, an internist and hypertensiologist in Trier, Germany. At the Medica, the world's largest medical trade fair with accompanying specialist congress, Prof. Krönig will be providing tips on how to measure blood pressure correctly. But measurements that are too low are also possible if the person handling the equipment does not know how to use it properly.

In fact it is not really difficult to take some one's blood pressure correctly, but you do have to follow a couple of rules. And these are rules that a great many hypertensive patients do not know about even though they take their own blood pressure, and which are frequently not observed by professionals, either. For example, the blood pressure should be measured before the patient takes the next dose of antihypertensive medication, e.g. mornings and evenings. This is the best way of assessing the long-term effects of individual antihypertensive medicines, which can vary from person to person.

In addition, the patient should rest for three to five minutes before the measurement is made. If the classical method is used, with a stethoscope, an inflatable cuff on the patient's upper arm, and a manometer, the practitioner also needs to avoid rushing; the cuff should be deflated relatively slowly, at a rate of about two to three mm Hg per second. If the patient's blood pressure is 160/90 mm Hg (with an amplitude of 70 mm Hg), this means that it takes 25 to 35 seconds to measure both the systolic and diastolic values. Medical personnel should make sure they take sufficient time. A survey of 500 men and women showed that fewer than three out of ten hypertensive patients knew that the inflatable cuff has to be positioned so that the point of measurement is level with the heart. Furthermore, the blood pressure should always be measured on the arm with the higher pressure.

Again, this was known to only one third of those questioned. If the cuff is too small, the systolic blood pressure (upper value) measured is too low and the diastolic blood pressure (lower value) is too high. First of all, the cuff must cover two thirds of the upper arm. Secondly, the cuff itself has to be the right length. As an increasing number of people are overweight, we more and more frequently see patients whose upper arms measure far more than 35 cm in circumference. The standard cuff (with a length of 22 to 34 cm) is not long enough for these patients and the cuff has to be replaced with a longer one.

Worn out hook-and-loop fastenings that come open during inflation also lead to unreliable values. Patients can leave the sleeves of their shirt or blouse down when their blood pressure is being taken as long as the garment is of thin fabric. A Canadian study involving 376 patients recently revealed that the values measured on a bare arm did not differ significantly from those measured on a sleeved arm (CMAJ 178, 2008, 585). However, Krönig recommends that patients should remove thick jumpers and jackets rather than trying to squeeze the cuff in between the elbow and a pushed-up sleeve.

For years now automatic blood pressure meters have been available, although in tests they often turned out to be of very variable quality. It should be noted that the principle used by these meters is quite different from that of the classical reference method with a stethoscope (see box). Automatic meters only determine a mean pressure by oscillometry. They then calculate the systolic and diastolic values. High-quality meters apply calculation algorithms that are adjusted so that the values produced correspond to those delivered by classical blood pressure measuring devices - at least for adults.

This means that in principle the different methods come up with comparable values. Now scientific studies are being carried out with such automatic meters. However, the methods of calculation have not been standardised as would be desirable for scientific reasons. Many electronic blood pressure meters available on the market regularly exhibit deviations of 10 mm Hg or more from the correct values.

The German Hypertension Society (DHL) therefore now awards a seal of quality to high-quality devices. A device qualifies after passing elaborate testing according to a European standard on over 90 subjects under standardised conditions. Discrepancies between the values obtained and those measured using the reference method must not exceed certain narrow limits. The devices with the seal are published on the DHL's Internet site (www.hochdruckliga.de). For older people meters are recommended that have clear displays, and possibly also use colour-coding (in red, yellow and green) to indicate a bad or good blood pressure.

For measuring blood pressure in children, experts such as the paediatrician and senior lecturer Dr. Michael Bald from Stuttgart advise against the automatic meters because most of them have not been tested for use on children. He added that hardly any scientific data exist for adjusting the results obtained with these meters to take into account the child's sex and size. Very often cuffs suitable for children are not available, either. Half a minute is needed to measure correctly.


Hypertension: a Modern Epidemic

In Germany there are at least 20 million people with hypertension (more than 140 over 90 mm Hg). Over many years hypertension can lead to strokes, heart attacks and renal failure. Only one in three of those affected is aware of their own high blood pressure. Only just under 16 percent of Germany's hypertension patients are keeping the disease under control by means of medication and other general measures.

Measuring with Riva-Rocci, Korotkoff & Co>

Classical blood pressure measurement

Indirect measurement of the blood pressure using an inflatable cuff on the upper arm and a sphygmomanometer was developed by the Italian doctor Scipione Riva-Rocci (1863-1937). When the cuff is inflated it compresses the brachial artery. Then the air is gradually released until a pulse can be felt again in the patient's wrist (radial artery), which means that the arterial pressure just overcomes the pressure in the cuff. This air pressure in the cuff corresponds to the systolic blood pressure. Later the method was combined with listening to the artery using a stethoscope placed on the inside of the elbow. This allows you to hear the sounds produced by the flow of the blood in time with the pulse (Korotkoff sounds, named after Nikolai S. Korotkoff, 1874-1920). The pressure in the cuff matches the systolic pressure when the first Korotkoff sound is heard, and the pressure when the Korotkoff sounds disappear completely (in adults) matches the diastolic blood pressure. Traditionally blood pressure is still given in millimetres of mercury (mm Hg), although use of measuring devices actually containing mercury has been forbidden for a long time now.

Automatic blood pressure measurement

Automatic devices measure blood pressure by oscillometry. This means that as the cuff gradually deflates the machine records not acoustic signals but oscillations of the cuff pressure caused by the re-appearance of the arterial pulse in the patient's arm. Initially these oscillations become stronger, and then they gradually die away until they cease altogether. The machine actually measures the mean blood pressure (time of the largest oscillation). Systolic and diastolic values are calculated by applying mathematical algorithms. As the blood pressure has a diurnal rhythm, blood pressure measurement over 24 hours using automatic machines has proved its value in diagnostic workup for hypertension. This also applies to treatment monitoring, because it is often not possible to predict whether an antihypertensive medicine taken in the morning will still have the desired effect late in the evening.

Direct blood pressure measurement

This can be done using a sensor located in the arterial bloodstream which registers the pressure wave emanating from the heart. This method is used, when necessary, in intensive care departments or during major operations. Its advantage is the continuous registration of the blood pressure and greater precision in comparison to indirect measurement.


In German

Donnerstag 20.11.08,
14.30 bis 17.30 Uhr, CCD-Süd, Raum 01
Leitung: Prof. Bernd Krönig, Trier und Prof. Manfred Anlauf, Bremerhaven

MEDICA 2008 in Düsseldorf (110)
DHL (290)
Thomas Meißner (593)

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