Blood pressure measurement: The Sphygmomanometer.

How is the blood pressure measured?

The blood pressure usually is measured with a small, portable instrument called a blood pressure cuff (sphygmomanometer). The blood pressure cuff consists of an air pump, a pressure gauge, and a rubber cuff. The instrument measures the blood pressure in units called millimeters of mercury (mm Hg).

The cuff is placed around the upper arm and inflated with an air pump to a pressure that blocks the flow of blood in the main artery (brachial artery) that travels through the arm. The arm is then extended at the side of the body at the level of the heart, and the pressure of the cuff on the arm and artery is gradually released. As the pressure in the cuff decreases, a health practitioner listens with a stethoscope over the artery at the front of the elbow. The pressure at which the practitioner first hears a pulsation from the artery is the systolic pressure (the top number). As the cuff pressure decreases further, the pressure at which the pulsation finally stops is the diastolic pressure (the bottom number).

The sphygmomanometer

The word comes from the Greek sphygmós (pulse), plus the scientific term manometer (pressure meter). The device was invented by Samuel Siegfried Karl Ritter von Basch in 1881.[1] Scipione Riva-Rocci introduced a more easily used version in 1896. Harvey Cushing discovered this device in 1901 and popularized it.

The sphygmomanometer is designed to monitor blood pressure by measuring the force of the blood in the heart where the pressure is greatest. This occurs during the contraction of the ventricles, when blood is pumped from the heart to the rest of the body (systolic pressure). The minimal force is also measured. This occurs during the period when the heart is relaxed between beats and pressure is lowest (diastolic pressure).

A sphygmomanometer is used to establish a baseline at a healthcare encounter and on admission to a hospital. Checking blood pressure is also performed to monitor the effectiveness of medication and other methods to control hypertension, and as a diagnostic aid to detect various diseases and abnormalities.

Description

A sphygmomanometer consists of a hand bulb pump, a unit that displays the blood pressure reading, and an inflatable cuff that is usually wrapped around a person’s upper arm. Care should be taken to ensure that the cuff size is appropriate for the person whose blood pressure is being taken. This improves the accuracy of the reading. Children and adults with smaller or larger than average-sized arms require special sized cuffs appropriate for their needs. A stethoscope is also used in conjunction with the sphygmomanometer to hear the blood pressure sounds. Some devices have the stethoscope already built in.

A sphygmomanometer can be used or encountered in a variety of settings:

  • home
  • hospital
  • primary care clinic or professional office
  • ambulance
  • dental office
  • pharmacy and other retail establishment

There are three types of equipment in common use for monitoring blood pressure.

  • A mercury-based unit has a manually inflatable cuff attached by tubing to the unit that is calibrated in millimeters of mercury. During blood pressure measurement , the unit must be kept upright on a flat surface and the gauge read at eye level. Breakage of the unit may cause dangerous mercury contamination and would require specialist removal for disposal. Due to the hazards of mercury, the use of mercury-based sphygmomanometers has declined sharply since 2000.
  • An aneroid unit is mercury free and consists of a cuff that can be applied with one hand for self-testing; a stethoscope that is built in or attached; and a valve that inflates and deflates automatically with the data displayed on an easy-to-read gauge that will function in any position. The unit is sensitive and if dropped may require recalibration.
  • An automatic unit is also mercury-free and is typically battery-operated. It has a cuff that can be applied with one hand for self-testing, and a valve that automatically inflates and deflates. Units with manual inflation are also available. The reading is displayed digitally and a stethoscope is not required. This is useful for persons who are hearing-impaired, for emergency situations when staff is limited, and for automatic input into instruments for storage or graphical display. A wrist monitor is also available for home testing. Some more expensive models also remember and print out recordings. The automatic units tend to be more portable than bulkier mercury devices.

Why do people monitor their blood pressure at home?

Some people are asked by their doctor or nurse to measure their blood pressure at home for a short period of time, usually two weeks, to find out what it is like away from the surgery or clinic. Sometimes your blood pressure is only raised when taken in a clinical or medical environment, such as your doctor’s surgery. This is called white coat hypertension and can be caused by feeling anxious, or by being in a busy or noisy environment. This can affect readings by as much as 30mmHg (systolic readings). 

Blood pressure readings taken at home are often lower than those taken at a doctor’s surgery, whether a doctor or a nurse takes them. Readings taken away from the clinic are now seen to give a more accurate picture of your blood pressure.

To be sure that you have high blood pressure your doctor needs more than one reading, and home measurements can be a good way of finding out what your blood pressure is like when you are relaxed and going about your daily life. The other way of doing this is by having 24-hour (ambulatory) blood pressure monitoring done. 

Monitoring blood pressure over a period of time at home can also provide your doctor or nurse with more information about how well your treatment is working and how you respond to medications. Home blood pressure monitoring can also be useful for people who have high blood pressure and who are taking medications as it can mean that you are able to cut down on the number of visits to your surgery for checks. 

Many people enjoy measuring their own blood pressure and monitoring their condition because it helps them to feel in control of their high blood pressure and health, and involves them in making decisions about their treatment.

When and how often should I take readings?

When and how often you take readings will depend on your blood pressure. Some people may need to take a number of readings over a short period of time, others will need to take a reading once a week. Your doctor or nurse will be able to advise you. If you take readings less than once a week you may be likely to forget, so try and make your readings part of your routine. A few people find that there is a difference between readings taken on days when they work and days when they do not, so try taking readings during these times as well.

When you first start taking readings at home discount the first day’s measurements. They may not be accurate because you are unfamiliar with the monitor and could be anxious. It is useful to take readings at the same time of day so that you are consistent and are comparing ‘like with like’.

Which arm should I use to measure blood pressure?

There can be a difference between the readings on each arm. When you first use the monitor take a reading on both the left and right arm and then in future use whichever arm gives you the higher reading. Always use the same arm for readings. If you find that the difference between the readings on either arm is greater than 20mmHg systolic (top number) or 10mmHg diastolic (bottom number) talk to you doctor or nurse about your readings. 

How do I take a blood pressure reading?

· Wear a short sleeved T-shirt or loose fitting clothing so that the sleeve can be rolled up comfortably
· Before taking readings rest for five minutes. You should be sitting down, preferably at a desk or a table, in a quiet place, with your arm resting on a firm surface. It is very important that your arm is supported so that the cuff around the arm is at the same level as your heart. You may need to support your arm with books to make sure it is at the correct height. Ensure that the arm is totally relaxed and not tensed
· The deflated cuff needs to be tight, but not too tight. You should be able to insert two fingers between it and the skin
· If you are using a machine that needs you to set the systolic level for the cuff inflation, choose a setting that is at least 20mmHg above your last reading. For example, if your last systolic reading was 153, set the machine to inflate to 180.
· Start the machine – keep still and silent whilst the reading is taken. Moving and talking can affect the reading
· Take two or three readings each about two minutes apart. If you take two readings disregard the first and use the second, if you take three readings disregard the first and take an average of the second and third
· Some people find that when they take two or three readings the first is always highest and then the readings become lower each time they take them. If this is the case keep taking readings until they level out and then use this as your reading
· Record the reading either in the memory of the machine or on computer or paper. Make sure you keep accurate records – some people are tempted to round the numbers up or down, or to record a lower blood pressure. This can affect your treatment and therefore your long-term risk of stroke and heart attack. Also record any medications you have taken and when, and any other information such as whether you have just eaten or exercised.

What is my ideal level of blood pressure?

The aim of treatment for high blood pressure is to reduce it, when measured by your doctor or nurse, to below 140/85mmHg. When it is measured in a clinic, particularly by a doctor or nurse, it may be higher than when you are more relaxed at home. It is therefore important that with home measurements they should at least be below 140/85mmHg and preferably below 130/80mmHg. This can be hard to achieve for some people and your doctor or nurse may set you a different target.

What do I do if my blood pressure readings start to go up?

As blood pressure varies in everybody by quite a large amount, you may get unexpected ‘one off’ high readings, so always repeat the measurement again at another time. However, if you find that on repeated measurement your blood pressure over a period of time is higher than usual go and see your doctor or nurse, taking details of the measurements with you. The important readings are the averages over a period of time and not individual readings.

Is there anything that might affect my blood pressure readings?

There are several factors that can make your blood pressure rise temporarily; exercise, alcohol, caffeine, tobacco, a full bladder, talking, anxiety, excitement and pain are just a few. It is worth recording what you have been doing before a reading, as it will act as a guide for you and your doctor or nurse.

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