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Photo of an electronic blood pressure monitor, or digital sphygmomanometer, wrapped around someone's wrist.

Blood pressure monitors

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by Chris Woodford. Last updated: December 20, 2015.

Have you ever stood on a highway bridge watching the trucks all rushing beneath you, some carrying goods to towns and cities, others running back again empty? Our bodies are a bit like that too. If you could stand "above" your own body and peer inside, you'd see millions of blood vessels packed with cells carrying food and oxygen to your internal organs, all of them dashing around like trucks on the road. To live active lives and stay healthy, we need blood to be pumping round our bodies under pressure, day and night. Sometimes, though, that doesn't happen. If you have a poor diet or don't exercise, or if you live a particularly stressful life, your blood pressure may be outside the normal range—and that can make you susceptible to serious medical conditions such as heart attacks and strokes. One way to check your health is to measure your blood pressure regularly. Once, that used to mean a trip to the physician, but now simple, affordable electronic monitors are available that can measure your blood pressure in moments. Let's take a closer look at how they work.

Photo: Measuring your blood pressure with an electronic blood pressure monitor (also called a digital sphygmomanometer). This one is made by IBP Healthcare, but many other makes are available. The grey cuff fastens tightly around your wrist and then inflates and deflates to measure your blood pressure.

Why blood is under pressure

Animation of a smilng, beating, heart

Imagine if you were like the Tin Man in The Wizard of Oz and you didn't have a heart. You might think your blood would just sit in your legs all day, never going anywhere, with your head and upper body entirely starved of oxygen. In fact, things would be even worse than that. Since your blood wouldn't circulate without a heart, there'd be no way to get food and oxygen to all the parts of your body. There'd be nothing to supply your brain or your muscles so you wouldn't be able to think or move!

Having a heart ensures blood can reach every part of your body and provide the energy and oxygen it needs. To supply blood effectively, the heart has to pump quite fast—especially when you're exercising hard. The pumping of your heart makes blood flow under surprisingly high pressure, pushing against the walls of blood vessels much like the air inside a bicycle tire. You'll discover just how much pressure if you're ever unlucky enough to cut through a major blood vessel: the blood will literally spurt from your body!

Photo: Blood is amazing: it's the vital fluid that keeps you alive. But it has to move efficiently around your body to do it's job properly, pumped under pressure by your ever-helpful heart!

How do we measure blood pressure?

Your blood pressure can vary quite a lot during the day and night as your level of activity changes, so a single, instantaneous measurement of blood pressure isn't that useful. To get a sense of what your blood pressure is really doing, you need to measure it repeatedly over days, weeks, months, or even years.

When physicians measure blood pressure, they quote two numbers, known as systolic and diastolic pressure. Systolic blood pressure is effectively your maximum blood pressure (while your heart is pumping blood); diastolic blood pressure is your minimum blood pressure (when your heart is resting). Measuring both these things indicates the range of your blood pressure (a bit like a maximum and minimum thermometer that gives a better idea of the weather by showing a day's highest and lowest temperature readings).

Blood pressure is measured in the same units as air pressure, water pressure, or any other kind of pressure. Typically, blood pressures are quoted in relatively old-fashioned units called millimeters of mercury (written "mmHg"; see our article on barometers, used for measuring air pressure, if you're not sure what this means), with the systolic measurement followed by the diastolic measurement. So if your blood pressure is "120 over 80" (often written 120/80) it means your systolic blood pressure is 120mmHg and your diastolic pressure is 80mmHg.

The air pressure gauge on the top of a typical foot pump

If you find these numbers quite meaningless, it might help if you bear in mind that standard atmospheric pressure (the pressure of the air pushing in on your body at sea level) is about 760mmHg. So a blood pressure of 80mmHg is about one tenth that much, while a pressure of 160mmHg is about one fifth normal atmospheric pressure, and a dangerously high pressure of 200mmHg would be about a quarter normal air pressure.

Photo: Feel the pressure. Here's an easy way to appreciate what blood pressure measurements really mean. When you inflate tires on a car or a bicycle, you probably use a foot pump, which has a gauge like this on top showing the pressure. This gauge is marked out in units called bars (black scale) and pounds per square inch (psi, red scale). One bar (about 14.5 psi) is very roughly equal to one atmosphere (normal atmospheric pressure) or about 760mmHg. If you're inflating tires and you see pressures of about 1–3 bars (14.5–43.5 psi), the kind of pressure your blood is exerting inside your body is about one tenth as much.

How to measure blood pressure

Photo of a thumbtack (drawing pin) being pushed into a wall.

In science, pressure is defined as the force something exerts on a certain area of a surface—and it's something we use in everyday life, all the time. When you press on a thumbtack, for example, the large metal head concentrates your pushing force onto the tiny pin behind it, increasing the pressure so the tack enters the wall more easily. Cars and bicycles use pressure too: they have tires filled with pressurized air to make them ride more smoothly over bumps in the road. When you measure the air pressure in a bicycle or car tire, you put a little meter on the air valve and let a tiny amount of air escape. As the air rushes out, it exerts more or less force on the meter and that tells you what pressure the air inside the tire is under.

In theory, you could use the same trick to measure blood pressure: you could poke a hole in a major blood vessel and see how much pressure the blood exerted as it gushed out all over the floor. In practice, that would be highly dangerous and very messy—so we have to measure blood pressure a different way, without actually letting any blood escape from the body. People generally prefer it that way!

Photo: Right: Pressure is the force acting on a certain area. When I push this thumbtack (drawing pin), my thumb applies a certain force to the large flat area of the head. The same force acts at the pointed end but, because that has a much smaller area, it's under much greater pressure—and that's what makes the tack enter the wall so easily.

An arm cuff pulled tight during blood pressure measurement.

Aneroid measurements

Photo: Blood-pressure measurement can be a little uncomfortable—the cuff has to be tight enough to restrict the flow of blood in your arm—but it's not what you'd call painful. Photo by Jason R. Zalasky courtesy of US Navy.

Until recently, blood pressure was almost always measured with what's called an aneroid sphygomomanometer. A sphygmomanometer is the name given to any blood measuring instrument, while aneroid, in this context, simply means using a dial (strictly speaking, it means "using no liquid"). It has an inflatable cuff that fits around your arm, a rubber bulb for pumping in air, a little dial on top for measuring pressure, and (quite separate from the other items) a stethoscope. How does it work? Once the cuff is fastened securely in place, the physician pumps the rubber bulb to inflate the cuff rapidly and cut off the blood flow in your arm. Then an air valve on the cuff is released so the blood rushes back along your arm, while the physician uses the stethoscope to listen to your pulse. When blood is first heard pumping, the physician notes the pressure reading on the dial: that's your systolic pressure. As the blood pumps back and the cuff deflates, there comes a point where the blood pumping can no longer be heard through the stethoscope: reading the dial again now gives the diastolic pressure.

Photo: Left: It's a bit more tricky to have your blood pressure measured the old-fashioned way. Photo by Michael R. McCormick courtesy of US Navy. Right: Looking more closely at the apparatus. A traditional aneroid sphygmomanometer has (from left to right) a rubber air pump, inflatable arm cuff, pressure-measuring dial, and stethoscope. Picture courtesy of Wikimedia Commons.

A patient having his blood tested using a cuff, stethoscope, and traditional aneroid sphygmomanometer Photo of traditional aneroid sphygmomanometer

Digital measurements

Aneroid instruments are generally very accurate but quite difficult to use by yourself. That's why many people now monitor their pressure using automatic, digital sphygmomanometers. They're amazingly simple to use and, at first sight, seem to consist of little more than a fabric cuff with an electronic, liquid-crystal display (LCD) panel on top. You can see a typical example in the top photo of this article.

To measure your blood pressure with one of these devices, you sit quietly for a minute or two to calm yourself down. Then you simply fasten the cuff around your arm and press a button on the instrument panel. You hear a kind of whirring noise (the sound of an electric motor) and a tiny air pump inside the gadget makes the cuff inflate. Once the cuff is blown up, the machine automatically releases air through an exhaust valve, measures your systolic and diastolic pressure, and displays the two values on the LCD screen. Some monitors have electronic memories and keep a record of your blood pressure measurements, so you can see how your pressure is changing over a day, week, or longer period of time. If you are monitoring your blood pressure, it's a good idea to draw your own chart of how your readings change from day to day. That gives you a much better idea of your overall blood pressure than occasional isolated readings, which may be abnormally high or low for all kinds of reasons. You can also take your chart along to show your physician next time you visit.

How electronic sphygmomanometers work

Electronic meters measure your blood pressure by listening to your blood flow. When the fabric cuff inflates around your wrist, it cuts off your blood flow altogether. That's because the pressure exerted by the cuff is higher than the (systolic) blood pressure in the brachial artery in your upper arm. As the cuff slowly deflates, the pressure it exerts reduces until it's equal to your systolic blood pressure. At that point, your blood starts to flow, making "thumping" noises known as Korotkoff sounds, which is when the meter measures your systolic blood pressure. As the cuff continues to deflate, the Korotkoff sounds stop altogether, and this corresponds to the point when the meter measures your diastolic blood pressure. Electronic sphygmomanometers typically work by measuring the cuff pressure at the points when the Korotkoff sounds start and stop. So one of their key components is a sound sensor and some electronic circuitry that can analyze noises and accurately detect the beginning and end of the Korotkoff sounds. Exactly the same thing happens with an aneroid sphygmomanometer, except that the physician or nurse is listening out for the sounds with a stethoscope and making manual measurements instead.

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