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(7) High Blood Pressure

Measuring blood pressure - Types of devices

The initial starting point when high blood pressure is being assessed is for the doctor or nurse to ensure that an accurate and reliable reading is obtained. BP measurement is an area where technology is changing all the time. With the advent of electronic monitors it is now possible for you to measure your own blood pressure. 

Types of BP measuring devices

I have been called in for a BP reading. How will this be done?

Blood pressure can be measured in several ways, by means of an electronic, mercury or aneroid sphygmomanometer. ‘Sphygmomanometer’ is the technical term for an instrument used to measure BP (a manometer is an instrument for measuring the pressure of fluids; sphygmos is the Greek word for pulse). These instruments could equally well just be called ‘BP monitors’ but sphygmomanometer was the name chosen when they first came into use in the late 19th century and is still used today.

The sphygmomanometer in everyday use in doctors’ surgeries consists of a device for measuring BP connected to an inflatable cuff, which is wrapped around the upper arm. The differences between the three types of instrument relate to the pressure measuring devices they use.

How does the mercury sphygmomanometer work?

The inflation/deflation mechanism is connected by rubber tubing to a bladder which envelops the arm. The cuff is inflated by squeezing a bulb by hand and deflation by means of a release valve. The pump and control valve are connected to the inflatable bladder and hence to the sphygmomanometer by rubber tubing. The doctor or nurse listens to the artery wall with a stethoscope, detecting sounds produced as blood passes through as the bladder deflates. The corresponding mercury level is then read off; this reading corresponds to the systolic BP. When these sounds disappear, the reading corresponds to the diastolic BP.

What is the best sphygmomanometer for measuring BP?

The gold standard for BP measurement is the mercury sphygmomanometer. This instrument has been used in all the clinical trials that have assessed the effectiveness of drug treatment as described. However, mercury is now being phased out in several European Union countries and has been totally replaced in Sweden and the Netherlands. Unfortunately, in some European countries, including the UK and Ireland, the move to ban mercury in hospitals and clinics has not been received with enthusiasm, as there are no alternative accurate measuring devices in common use.

There are many electronic sphygmomanometers, but many of these devices are frequently inaccurate and unreliable. The British Hypertension Society has a website and this gives details of the accuracy of newer electronic sphygmomanometers against the ‘gold standard’ of a mercury sphygmomanometer. Prior to purchasing an electronic device, it is well to ensure that it has been validated by the British Hypertension Society.

What differences are there between the gold standard device and the other types, and why aren’t they used so much?

• Aneroid sphygmomanometers. These devices balance BP against pressure in a thin metal capsule containing air (‘aneroid’ comes from the Greek and means‘ without using fluids’). Aneroid sphygmomanometers register BP through a bellows and lever system, which is mechanically more complex and intricate than the mercury reservoir and column. The problem with the aneroid sphygmomanometer is that it frequently becomes inaccurate over time. The reason for this is that the jolts of everyday use affect the instruments. Checks have shown that a third or more of aneroid sphygmomanometers may be out by up to 4 mmHg or more when calibrated against a mercury sphygmomanometer. Up to 10% may be 10 mmHg or more out. For these reasons aneroid sphygmomanometers are no longer recommended for clinical use.

• Electronic sphygmomanometers. Automated sphygmomanometers can eliminate several important sources of error and are easy to use, particularly by people measuring their own BP at home.

Unfortunately their futuristic appearance is no guarantee of accuracy. Because they depend on either a microphone or a pressure transducer sewn into the cuff, it is always difficult and, in most cases impossible, to get outsize cuffs for people with large circumference arms, resulting in serious measurement errors.

Unlike traditional mercury machines, anything going wrong may not be obvious, so systematically incorrect and misleading readings can be taken time and time again. Like other measuring instruments, sphygmomanometers of all kinds should be compelled by law to conform to some common minimum standard of accuracy, otherwise different models will compete not in terms of accuracy, but appearance and ease of use by consumers.

Two published standards are now available, one from the British Hypertension Society and the other from the American Association for Advancement of Medical Instrumentation (AAAMI), but as yet nothing has been done to enforce them.

This may soon change, as a draft British Standard specification (BS EN1060) is now being circulated for expert appraisal.

Electronic machines are constantly improving, and once their accuracy is beyond doubt, they will certainly replace mercury machines.

I have seen some instruments available for BP measurement in the pharmacy. What kind of devices are available for self-measurement?

The automated devices available for self-measurement all use the same oscillometric technique. There are three categories available: devices that measure BP on the finger, the wrist and the upper arm:

• Finger devices. These devices measure BP at the fingertip and are not recommended because of the inaccuracies caused by measurement distortion when the smaller blood vessels in your hand constrict from factors such as temperature and position of your hand.

• Wrist devices. These devices are more accurate than fingertip measuring devices but many specialists have reservations about the correct use of these devices, particularly with regard to correct placement of the cuff on the wrist at heart level.

• Upper arm devices. Some of these devices can be recommended. However, the recommendations that apply to BP measurement in general by means of mercury sphygmomanometry also apply to these automated devices.

Appropriate cuff sizes should be used. When you take your BP, you should make sure that you have been resting for at least 5 minutes and that two separate readings are taken at least 30 seconds apart.

 

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