Blood pressure is measured

How Is Blood Pressure Measured and What Do the Numbers Mean?

When you visit your health care provider, a blood pressure measurement is one of the most important routine tests you’ll have.

Blood pressure is the force exerted by your blood against your arteries. As your heart pumps, it forces blood out through arteries that carry the blood throughout your body. The arteries keep tapering off in size until they become tiny vessels, called capillaries. At the capillary level, oxygen and nutrients are released from your blood and delivered to the organs.

Types of Blood Pressure

There are two types of blood pressure: Systolic blood pressure refers to the pressure inside your arteries when your heart is pumping; diastolic pressure is the pressure inside your arteries when your heart is resting between beats.

When your arteries are healthy and dilated, blood flows easily and your heart doesn’t have to work too hard. But when your arteries are too narrow or stiff, blood pressure rises, the heart gets overworked, and arteries can become damaged.

Measuring Blood Pressure

Blood pressure is measured with an instrument called a sphygmomanometer. First, a cuff is placed around your arm and inflated with a pump until the circulation is cut off. A small valve slowly deflates the cuff, and the doctor measuring blood pressure uses a stethoscope, placed over your arm, to listen for the sound of blood pulsing through the arteries. That first sound of rushing blood refers to the systolic blood pressure; once the sound fades, the second number indicates the diastolic pressure, the blood pressure of your heart at rest.

Blood pressure is measured in millimeters of mercury (mm Hg) and recorded with the systolic number first, followed by the diastolic number. For example, a normal blood pressure would be recorded as something under 120/80 mm Hg.

Blood pressure readings can be affected by factors like:

  • Smoking
  • Coffee or other caffeinated drinks
  • A full bladder
  • Recent physical activity

Blood pressure is also affected by your emotional state and the time of day. Since so many factors can affect blood pressure readings, you should have your blood pressure taken several times to get an accurate measurement.

What Is Normal Blood Pressure?

Experts consider normal blood pressure to be less than 120/80 mm Hg. Based on population data, about 42 percent of American adults have normal blood pressure. At one point, blood pressure at or above 120/80 and less than 140/90 was considered normal to high; these numbers are now considered pre-hypertensive. Blood pressure consistently at or above 140/90 is considered high blood pressure or hypertension.

Blood pressure normally rises as you age and grow. Normal blood pressure readings for children are lower than for adults, while blood pressure measurements for adults and older teenagers are similar.

Blood pressure can also be too low, a condition called hypotension. Hypotension refers to blood pressure lower than 90/60. Symptoms of hypotension include dizziness, fainting, and sometimes shock.

Checking Blood Pressure at Home

Many people can learn to check their blood pressure at home. You can buy blood pressure kits that use the cuffs or electronic digital technology at your pharmacy, a medical supply store, or an online retailer.

Since high blood pressure can exist without any symptoms, it is important to know your numbers. High blood pressure can cause stroke, heart attack, heart failure, and kidney failure.

Getting your blood pressure checked is quick, painless, and one of the most important things you can do to better your health.

Blood pressure test

How blood pressure is tested

A device called a sphygmomanometer will be used to measure your blood pressure.

This usually consists of a stethoscope, arm cuff, pump and dial, although automatic devices that use sensors and have a digital display are also commonly used nowadays.

It’s best to sit down with your back supported and legs uncrossed for the test. You’ll usually need to roll up your sleeves or remove any long-sleeved clothing, so the cuff can be placed around your upper arm. Try to relax and avoid talking while the test is carried out.

During the test:

  • you hold out one of your arms so it’s at the same level as your heart, and the cuff is placed around it – your arm should be supported in this position, such as with a cushion or arm of a chair
  • the cuff is pumped up to restrict the blood flow in your arm – this squeezing may feel a bit uncomfortable, but only lasts a few seconds
  • the pressure in the cuff is slowly released while a stethoscope is used to listen to your pulse (digital devices use sensors to detect vibrations in your arteries)
  • the pressure in the cuff is recorded at 2 points as the blood flow starts to return to your arm – these measurements are used to give your blood pressure reading (see Understanding your blood pressure reading)

You can usually find out your result straight away, either from the healthcare professional carrying out the test or on the digital display.

Blood pressure is one of the vital signs that doctors measure to assess general health. Having a high blood pressure, also called hypertension, that is not under control can result in heart problems, stroke, and other medical conditions.

Almost half of U.S. adults — 46 percent — have high blood pressure, according to the American Heart Association. High blood pressure is sometimes referred to as the “silent killer” because it often has no symptoms.

Certain lifestyle factors, such as diet and smoking habits, can greatly impact a person’s risk of developing high blood pressure.

“Having a healthy lifestyle really makes a difference in your life because you can avoid high blood pressure,” said Dr. Mary Ann Bauman, an internist at Integris Baptist Medical Center in Oklahoma City. “If you do have high blood pressure, make sure take your medication. You may not necessarily have symptoms until your blood pressure gets really high.”

What is normal blood pressure?

Blood pressure reflects the force of blood as it hits the walls of the arteries. When the heart squeezes and pushes the blood out, the blood pushes against the walls of the blood vessels. People are born with very elastic vessels that can expand easily, and bounce back when the pressure on them is low.

As people age, they get plaque buildup inside the blood vessels, and the flexible walls of the arteries become stiff. Now, when the heart squeezes and pushes the blood out, the blood vessels can’t expand like they used to do and sustain higher pressure. Over time, the heart has to push so hard against the pressure that it starts to fail, Bauman said.

Blood pressure is recorded as two numbers and written as a ratio: the top number, called the systolic pressure, is the pressure as the heart beats. The bottom number, called the diastolic pressure, is the measurement as the heart relaxes between beats. According to guidelines announced in November 2017 by the American Heart Association (AHA), people’s blood pressure measurements fall into the following categories:

  • Normal: Less than 120 millimeters of mercury (mm Hg) for systolic and 80 mm Hg for diastolic.
  • Elevated: Between 120-129 for systolic, and less than 80 for diastolic.
  • Stage 1 hypertension: Between 130-139 for systolic or between 80-89 for diastolic.
  • Stage 2 hypertension: At least 140 for systolic or at least 90 mm Hg for diastolic.

A number of factors can raise blood pressure, including stress, smoking, caffeine, binge drinking, certain over-the-counter and prescribed medications and even cold temperatures.

Those who are fit — including those who regularly exercise and professional athletes — tend to have lower blood pressures and heart rates, as do those who do not smoke and are a healthy weight.

Checking your blood pressure

Starting at age 20, the AHA recommends a blood pressure screening at your regular healthcare visit or once every 2 years, if your blood pressure is less than 120/80 mm Hg.

People who have high blood pressure are encouraged to check their blood pressure at least three times a week, Bauman said.

People can check their blood pressure themselves. In fact, monitoring blood pressure at home may be better than doing so at the doctor’s office, partly because people are especially susceptible to a spike in their blood pressure when they visit a doctor, a situation known as white coat hypertension.

“We have many studies that indicate people taking their blood pressure at home is much more accurate than at the doctor’s office,” Bauman said.

A manual or digital blood pressure monitor (sphygmomanometer) typically comes with instructions that should be followed carefully to get the most accurate results.

The first step is to find your pulse by pressing your index finger on the brachial artery, which is at the bend of your elbow, slightly to the inside center. On a manual monitor, place the head of the stethoscope in the general area, or for a digital monitor, place the cuff in this area.

For a manual monitor, you have to hold the pressure gauge in one hand (your weaker hand) and the bulb in the other hand. Inflate the cuff until it reads about 30 points above your normal systolic pressure. At this point, you should not hear your pulse in the stethoscope. When you hear the first heart beat, this is the systolic pressure. As you deflate the cuff, keep listening for a heart beat. When you can no longer hear it, that is your diastolic pressure.

A digital monitor does the inflation and deflation and recording of the systolic and diastolic heart rates.

Risk factors

Risk factors for high blood pressure include too much salt in the diet, excess weight, inactivity, and smoking.

The dangers of high blood pressure include hardening of the arteries, or atherosclerosis, kidney disease and heart disease. High blood pressure can also result in stroke, either because of a blocked arteries or a burst in a blood vessel.

Low blood pressure, known as hypotension, can also cause health problems such as fainting and dizziness. Quick, dramatic drops in blood pressure can reduce the adequate blood supply to the brain. Most of the time, hypotension goes undetected and is typically not dangerous unless it produces symptoms.

Treating high blood pressure

Treating high blood pressure includes lifestyle changes and prescription medication for those with readings of 140/90 or higher, according to the AHA.

“The first thing we tell people to do if their blood pressure is in prehypertension range, is to lose weight, exercise more, and reduce salt in diet,” Bauman said. “If they reach higher levels, we then treat them with medications.”

Additional reporting by Kim Ann Zimmermann, Live Science Contributor.

Additional resources

  • American Heart Association: Understanding Blood Pressure Readings
  • CDC: High Blood Pressure

Sphygmomanometer: An instrument for measuring blood pressure, particularly in arteries. The two types of sphygmomanometers are a mercury column and a gauge with a dial face. The sphygmomanometer in most frequent use today consists of a gauge attached to a rubber cuff which is wrapped around the upper arm and is inflated to constrict the arteries.

A blood pressure reading consists of two numbers: systolic and diastolic. Systolic refers to systole, the phase when the heart pumps blood out into the aorta. Diastolic refers to diastole, the resting period when the heart refills with blood. At each heartbeat, blood pressure is raised to the systolic level, and, between beats, it drops to the diastolic level.

With the cuff inflated with air, a stethoscope is placed over an artery (the brachial artery) in the crook of the arm. As the air in the cuff is released, the first sound heard through the stethoscope marks the systolic pressure. As the release of air from the cuff continues, a point is reached when the sound diminishes and then is no longer heard. The point where the sound disappears marks the diastolic pressure. The blood pressure reading might show the systolic and diastolic pressures to be, for example, 120 and 78mm of mercury (Hg) respectively — written 120/78 and said to be “120 over 78.”

A typical blood pressure reading for an adult might, in fact, be 120/78. Readings vary depending on age and many other factors. Children and adults with smaller or larger than average-sized arms may need special-sized pressure cuffs.

The sphygmomanometer was introduced in 1896 by the Italian physician Scipione Riva-Rocci (1863-1937). The American physiologist Joseph Erlanger (1874-1965) studied the principles of sphygmomanometry and devised a recording sphygmomanometer.

The word “sphygmomanometer” (pronounced sfig·mo·ma·nom·e·ter) was put together from the Greek sphygmos, the beating of the heart or the pulse + manometer, a device for measuring pressure or tension.

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Blood pressure monitors for home

Your health care provider may ask you to keep track of your blood pressure at home. To do this, you will need to get a home blood pressure monitor. The monitor you choose should be of good quality and fit well.

MANUAL BLOOD PRESSURE MONITORS

  • Manual devices include a cuff that wraps around your arm, a rubber squeeze bulb, and a gauge that measures the blood pressure. A stethoscope is needed to listen to the blood pulsing through the artery.
  • You can see your blood pressure on the circular dial of the gauge as the needle moves around and the pressure in the cuff rises or falls.
  • When used correctly, manual devices are very accurate. However, they are not the recommended type of blood pressure monitor for home use.

DIGITAL BLOOD PRESSURE MONITORS

  • A digital device will also have a cuff that wraps around your arm. To inflate the cuff, you may need to use a rubber squeeze ball. Other kinds will inflate automatically when you push a button.
  • After the cuff is inflated, the pressure will slowly drop on its own. The screen will show a digital readout of your systolic and diastolic blood pressure.
  • After showing your blood pressure, the cuff will deflate on its own. With most machines, you must wait for 2 to 3 minutes before using it again.
  • A digital blood pressure monitor will not be as accurate if your body is moving when you are using it. Also, an irregular heart rate will make the reading less accurate. However, digital monitors are the best choice for most people.

TIPS FOR MONITORING YOUR BLOOD PRESSURE

  • Practice using the monitor with your provider to make sure you are taking your blood pressure correctly.
  • Your arm should be supported, with your upper arm at heart level and feet on the floor (back supported, legs uncrossed).
  • It is best to measure your blood pressure after you rest for at least 5 minutes.
  • DO NOT take your blood pressure when you are under stress, have had caffeine, or used a tobacco product in the last 30 minutes, or have recently exercised.
  • Take at least 2 readings 1 minute apart in the morning before taking medicines and in evening before eating supper. Try to measure and record BP daily for 5 days and then report your results to your provider.

Bookshelf

Technique

The most accurate measurement of arterial blood pressure is obtained by direct methods that involve sophisticated and expensive equipment as well as cannulation of an artery. Although these methods are necessary in some settings, sphygmomanometric measurements are much easier and safer, and are accurate enough for most clinical situations.

The standard blood pressure cuff must be of the proper size to minimize errors in blood pressure determinations. The width of the bladder ideally should be 40% of the circumference of the limb tested. Most standard cuffs have a bladder length that is twice its width. This ensures that the length is the recommended 80% of the limb circumference. Cuffs that are too small give results erroneously high; cuffs that are too large give results erroneously low.

The patient should be comfortably seated and the deflated cuff applied with the bladder centered over the brachial artery. It should be high enough on the arm to allow the stethoscope to be placed in the antecubital fossa without touching the cuff. Pressure is then rapidly increased to at least 30 mm Hg higher than that which eliminates a palpable radial pulse.

With the stethoscope in place over the brachial artery in the antecubital fossa, the cuff is slowly deflated at a rate of 2 mm Hg per second. As the pressure declines, the Korotkoff sounds are noted. These sounds (Table 16.2) are low pitched and are sometimes more easily auscultated with the bell of the stethoscope. Palpating the loss of the radial pulse is essential because some patients, particularly those with hypertension, may have an auscultatory gap. This is a complete loss of Korotkoff sounds at pressures higher than the true diastolic. Careful auscultation in these patients will reveal return of sounds, allowing accurate pressure determinations (Figure 16.1).

Table 16.2

Sequence of Korotkoff Sounds Obtained during Sphygmomanometry.

Figure 16.1

Temporary loss of all Korotkoff sounds is called an auscultatory gap.

Systolic blood pressure is best estimated using phase I Korotkoff sounds (i.e., the first sounds auscultated). Phase V Korotkoff sounds (i.e., loss of all sounds) estimates diastolic pressures. Usually phase IV and phase V sounds occur very near each other. If, however, they are widely separated, the blood pressure may be written to signify both (e.g., 128/80/30). Under these circumstances, phase IV sounds more accurately predict diastolic pressures.

Considerable controversy exists concerning prediction of the diastolic blood pressure using Korotkoff sounds. Kirkendall et al. (1981) addresses these controversies and recommends phase V sounds in adults and phase IV in children.

Systolic pressure normally varies with respirations. During inspiration, the negative intrathoracic pressure causes pooling of blood in expanding pulmonary vessels and a delay of flow to the left ventricle. Thus, systolic pressure falls as cardiac output falls momentarily.

The following procedure checks for a paradoxical pulse. During normal respirations, the pressure is noted at which Korotkoff sounds are first heard. Commonly, these first sounds are audible only during expiration. Cuff pressure is slowly lowered until Korotkoff sounds are heard continually. If the difference between these two pressures exceeds 10 mm Hg, a pulsus paradoxus exists. A paradoxical pulse occurs most commonly with clinical situations involving large negative intrathoracic pressures like heavy breathing, asthma, or emphysema. Pulsus paradoxus also occurs in pericardial tamponade, but the mechanisms are more complex and not as well understood.

The blood pressure is usually taken with the patient seated. Additional information may be gained by checking the patient in the lying and standing positions. A supine blood pressure should be compared to that obtained after the patient has been standing for a sufficient time to allow the pulse to stabilize. Normally, systolic blood pressure should not drop more than 10 mm Hg, and diastolic pressure should remain unchanged or rise slightly. Significant orthostatic changes in blood pressure may indicate dehydration or an adverse drug reaction. When correlated with an inadequate rise in pulse, it may indicate autonomic nervous system dysfunction.

All patients should have the blood pressure checked in the left and right arm at least once to detect anatomical abnormalities. Pressure differences greater than 15 mm Hg may indicate obstruction of flow to one of the brachial arteries, such as occurs in coarctation of the aorta.

Vital Signs (Body Temperature, Pulse Rate, Respiration Rate, Blood Pressure)

How to check your pulse

As the heart forces blood through the arteries, you feel the beats by firmly pressing on the arteries, which are located close to the surface of the skin at certain points of the body. The pulse can be found on the side of the neck, on the inside of the elbow, or at the wrist. For most people, it is easiest to take the pulse at the wrist. If you use the lower neck, be sure not to press too hard, and never press on the pulses on both sides of the lower neck at the same time to prevent blocking blood flow to the brain. When taking your pulse:

  • Using the first and second fingertips, press firmly but gently on the arteries until you feel a pulse.

  • Begin counting the pulse when the clock’s second hand is on the 12.

  • Count your pulse for 60 seconds (or for 15 seconds and then multiply by four to calculate beats per minute).

  • When counting, do not watch the clock continuously, but concentrate on the beats of the pulse.

  • If unsure about your results, ask another person to count for you.

If your doctor has ordered you to check your own pulse and you are having difficulty finding it, consult your doctor or nurse for additional instruction.

What is the respiration rate?

The respiration rate is the number of breaths a person takes per minute. The rate is usually measured when a person is at rest and simply involves counting the number of breaths for one minute by counting how many times the chest rises. Respiration rates may increase with fever, illness, and other medical conditions. When checking respiration, it is important to also note whether a person has any difficulty breathing.

Normal respiration rates for an adult person at rest range from 12 to 16 breaths per minute.

What is blood pressure?

Blood pressure is the force of the blood pushing against the artery walls during contraction and relaxation of the heart. Each time the heart beats, it pumps blood into the arteries, resulting in the highest blood pressure as the heart contracts. When the heart relaxes, the blood pressure falls.

Two numbers are recorded when measuring blood pressure. The higher number, or systolic pressure, refers to the pressure inside the artery when the heart contracts and pumps blood through the body. The lower number, or diastolic pressure, refers to the pressure inside the artery when the heart is at rest and is filling with blood. Both the systolic and diastolic pressures are recorded as “mm Hg” (millimeters of mercury). This recording represents how high the mercury column in an old-fashioned manual blood pressure device (called a mercury manometer or sphygmomanometer) is raised by the pressure of the blood. Today, your doctor’s office is more likely to use a simple dial for this measurement.

High blood pressure, or hypertension, directly increases the risk of heart attack, heart failure, and stroke. With high blood pressure, the arteries may have an increased resistance against the flow of blood, causing the heart to pump harder to circulate the blood.

Blood pressure is categorized as normal, elevated, or stage 1 or stage 2 high blood pressure:

  • Normal blood pressure is systolic of less than 120 and diastolic of less than 80 (120/80)

  • Elevated blood pressure is systolic of 120 to 129 and diastolic less than 80

  • Stage 1 high blood pressure is systolic is 130 to 139 or diastolic between 80 to 89

  • Stage 2 high blood pressure is when systolic is 140 or higher or the diastolic is 90 or higher

These numbers should be used as a guide only. A single blood pressure measurement that is higher than normal is not necessarily an indication of a problem. Your doctor will want to see multiple blood pressure measurements over several days or weeks before making a diagnosis of high blood pressure and starting treatment. Ask your provider when to contact him or her if your blood pressure readings are not within the normal range.

Why should I monitor my blood pressure at home?

For people with hypertension, home monitoring allows your doctor to monitor how much your blood pressure changes during the day, and from day to day. This may also help your doctor determine how effectively your blood pressure medication is working.

What special equipment is needed to measure blood pressure?

Either an aneroid monitor, which has a dial gauge and is read by looking at a pointer, or a digital monitor, in which the blood pressure reading flashes on a small screen, can be used to measure blood pressure.

About the aneroid monitor

The aneroid monitor is less expensive than the digital monitor. The cuff is inflated by hand by squeezing a rubber bulb. Some units even have a special feature to make it easier to put the cuff on with one hand. However, the unit can be easily damaged and become less accurate. Because the person using it must listen for heartbeats with the stethoscope, it may not be appropriate for the hearing-impaired.

About the digital monitor

The digital monitor is automatic, with the measurements appearing on a small screen. Because the recordings are easy to read, this is the most popular blood pressure measuring device. It is also easier to use than the aneroid unit, and since there is no need to listen to heartbeats through the stethoscope, this is a good device for hearing-impaired patients. One disadvantage is that body movement or an irregular heart rate can change the accuracy. These units are also more expensive than the aneroid monitors.

About finger and wrist blood pressure monitors

Tests have shown that finger and/or wrist blood pressure devices are not as accurate in measuring blood pressure as other types of monitors. In addition, they are more expensive than other monitors.

Before you measure your blood pressure:

The American Heart Association recommends the following guidelines for home blood pressure monitoring:

  • Don’t smoke or drink coffee for 30 minutes before taking your blood pressure.

  • Go to the bathroom before the test.

  • Relax for 5 minutes before taking the measurement.

  • Sit with your back supported (don’t sit on a couch or soft chair). Keep your feet on the floor uncrossed. Place your arm on a solid flat surface (like a table) with the upper part of the arm at heart level. Place the middle of the cuff directly above the bend of the elbow. Check the monitor’s instruction manual for an illustration.

  • Take multiple readings. When you measure, take 2 to 3 readings one minute apart and record all the results.

  • Take your blood pressure at the same time every day, or as your healthcare provider recommends.

  • Record the date, time, and blood pressure reading.

  • Take the record with you to your next medical appointment. If your blood pressure monitor has a built-in memory, simply take the monitor with you to your next appointment.

  • Call your provider if you have several high readings. Don’t be frightened by a single high blood pressure reading, but if you get several high readings, check in with your healthcare provider.

  • When blood pressure reaches a systolic (top number) of 180 or higher OR diastolic (bottom number) of 110 or higher, seek emergency medical treatment.

Ask your doctor or another healthcare professional to teach you how to use your blood pressure monitor correctly. Have the monitor routinely checked for accuracy by taking it with you to your doctor’s office. It is also important to make sure the tubing is not twisted when you store it and keep it away from heat to prevent cracks and leaks.

Proper use of your blood pressure monitor will help you and your doctor in monitoring your blood pressure.

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