What are the Why can't my blood pressure be read??

To make the best use of blood pressure monitoring equipment, it is helpful to have an insight into how the equipment works and the likely sources of error that can affect readings. Download a guide to these tips to keep with you for quick reference.

The most common blood pressure reading mistakes are:

Here’s what many of us do wrong, and how to take a blood pressure reading:

The most common error when using indirect blood pressure measuring equipment is using an incorrectly sized cuff. A BP cuff that is too large will give falsely low readings, while an overly small cuff will provide readings that are falsely high.

The American Heart Association publishes guidelines for blood pressure measurement . recommending that the bladder length and width (the inflatable portion of the cuff) should be 80 percent and 40 percent respectively, of arm circumference. Most practitioners find measuring bladder and arm circumference to be overly time consuming, so they don’t do it.

The most practical way to quickly and properly size a BP cuff is to pick a cuff that covers two-thirds of the distance between your patient’s elbow and shoulder. Carrying at least three cuff sizes (large adult, regular adult, and pediatric) will fit the majority of the adult population.  Multiple smaller sizes are needed if you frequently treat pediatric patients.

Korotkoff sounds are the noises heard through a stethoscope during cuff deflation.  They occur in 5 phases:

In their 1967 guidelines, the AHA recommended that clinicians record the systolic BP at the start of phase I and the diastolic BP at start of phase IV Korotkoff sounds. In their 1981 guidelines, the diastolic BP recommendation changed to the start of phase V .

The second most common error in BP measurement is incorrect limb position. To accurately assess blood flow in an extremity, influences of gravity must be eliminated.

The standard reference level for measurement of blood pressure by any technique — direct or indirect — is at the level of the heart. When using a cuff, the arm (or leg) where the cuff is applied must be at mid-heart level. Measuring BP in an extremity positioned above heart level will provide a falsely low BP whereas falsely high readings will be obtained whenever a limb is positioned below heart level.  Errors can be significant — typically 2 mmHg for each inch the extremity is above or below heart level.

A seated upright position provides the most accurate blood pressure, as long as the arm in which the pressure is taken remains at the patient’s side. Patients lying on their side, or in other positions, can pose problems for accurate pressure measurement. To correctly assess BP in a side lying patient, hold the BP cuff extremity at mid heart level while taking the pressure. In seated patients, be certain to leave the arm at the patient’s side.

Arterial pressure transducers are subject to similar inaccuracies when the transducer is not positioned at mid-heart level. This location, referred to as the phlebostatic axis, is located at the intersection of the fourth intercostal space and mid-chest level (halfway between the anterior and posterior chest surfaces.

Note that the mid-axillary line is often not at mid-chest level in patients with kyphosis or COPD, and therefore should not be used as a landmark. Incorrect leveling is the primary source of error in direct pressure measurement with each inch the transducer is misleveled causing a 1.86 mmHg measurement error. When above the phlebostatic axis, reported values will be lower than actual; when below the phlebostatic axis, reported values will be higher than actual.

The standard for blood pressure cuff placement is the upper arm using a cuff on bare skin with a stethoscope placed at the elbow fold over the brachial artery.

The patient should be sitting, with the arm supported at mid heart level, legs uncrossed, and not talking. Measurements can be made at other locations such as the wrist, fingers, feet, and calves but will produce varied readings depending on distance from the heart.

The mean pressure, interestingly, varies little between the aorta and peripheral arteries, while the systolic pressure increases and the diastolic decreases in the more distal vessels.

Crossing the legs increases systolic blood pressure by 2 to 8 mm Hg. About 20 percent of the population has differences of more than 10 mmHg pressure between the right and left arms. In cases where significant differences are observed, treatment decisions should be based on the higher of the two pressures.

Prejudice for normal readings significantly contributes to inaccuracies in blood pressure measurement. No doubt, you’d be suspicious if a fellow EMT reported blood pressures of 120/80 on three patients in a row. As creatures of habit, human beings expect to hear sounds at certain times and when extraneous interference makes a blood pressure difficult to obtain, there is considerable tendency to “hear” a normal blood pressure.

Orthostatic hypotension is defined as a decrease in systolic blood pressure of 20 mm Hg or more, or diastolic blood pressure decrease of 10 mm Hg or more measured after three minutes of standing quietly.

There are circumstances when BP measurement is simply not possible. For many years, trauma resuscitation guidelines taught that rough estimates of systolic BP (SBP) could be made by assessing pulses. Presence of a radial pulse was thought to correlate with an SBP of at least 80 mm Hg, a femoral pulse with an SBP of at least 70, and a palpable carotid pulse with an SBP over 60. In recent years, vascular surgery and trauma studies have shown this method to be poorly predictive of actual blood pressure .

Noise is a factor that can also interfere with BP measurement.  Many ALS units carry doppler units that measure blood flow with ultrasound waves. Doppler units amplify sound and are useful in high noise environments.

BP by palpation or obtaining the systolic value by palpating a distal pulse while deflating the blood pressure cuff  generally comes within 10 – 20 mmHg of an auscultated reading. A pulse oximeter waveform can also be used to measure return of blood flow while deflating a BP cuff, and is as accurate as pressures obtained by palpation.

In patients with circulatory assist devices that produce non-pulsatile flow such as left ventricular assist devices (LVADs), the only indirect means of measuring flow requires use of a doppler.

The return of flow signals over the brachial artery during deflation of a blood pressure cuff in an LVAD patient signifies the mean arterial pressure (MAP). While a normal MAP in adults ranges from 70 to 105 mmHg, LVADs do not function optimally against higher afterload, so mean pressures of less than 90 are often desirable.

Clothing, patient access, and cuff size are obstacles that frequently interfere with conventional BP measurement. Consider using alternate sites such as placing the BP cuff on your patient’s lower arm above the wrist while auscultating or palpating their radial artery.  This is particularly useful in bariatric patients when an appropriately sized cuff is not available for the upper arm. The thigh or lower leg can be used in a similar fashion (in conjunction with a pulse point distal to the cuff).

All of these locations are routinely used to monitor BP in hospital settings and generally provide results only slightly different from traditional measurements in the upper arm.

Electronic blood pressure units also called Non Invasive Blood Pressure (NIBP) machines, sense air pressure changes in the cuff caused by blood flowing through the BP cuff extremity. Sensors estimate the Mean Arterial Pressure (MAP) and the patient’s pulse rate. Software in the machine uses these two values to calculate the systolic and diastolic BP.

To assure accuracy from electronic units, it is important to verify the displayed pulse with an actual patient pulse. Differences of more than 10 percent will seriously alter the unit’s calculations and produce incorrect systolic and diastolic values on the display screen.

Given that MAP is the only pressure actually measured by an NIBP, and since MAP varies little throughout the body, it makes sense to use this number for treatment decisions.

A normal adult MAP ranges from 70 to 105 mmHg. As the organ most sensitive to pressure, the kidneys typically require an MAP above 60 to stay alive, and sustain irreversible damage beyond 20 minutes below that in most adults. Because individual requirements vary, most clinicians consider a MAP of 70 as a reasonable lower limit for their adult patients.

Increased use of NIBP devices, coupled with recognition that their displayed systolic and diastolic values are calculated while only the mean is actually measured, have led clinicians to pay much more attention to MAPs than in the past. Many progressive hospitals order sets and prehospital BLS and ALS protocols have begun to treat MAPs rather than systolic blood pressures.

Finally, and especially in the critical care transport environment, providers will encounter patients with significant variations between NIBP (indirect) and arterial line (direct) measured blood pressure values.

In the past, depending on patient condition, providers have elected to use one measuring device over another, often without clear rationale besides a belief that the selected device was providing more accurate blood pressure information.

In 2013, a group of ICU researchers published an analysis of 27,022 simultaneous art line and NIBP measurements obtained in 852 patients . When comparing the a-line and NIBP readings, the researchers were able to determine that, in hypotensive states, the NIBP significant overestimated the systolic blood pressure when compared to the arterial line, and this difference increased as patients became more hypotensive.

At the same time, the mean arterial pressures (MAPs) consistently correlated between the a-line and NIBP devices, regardless of pressure. The authors suggested that MAP is the most accurate value to trend and treat, regardless of whether BP is being measured with an arterial line or an NIBP. Additionally, supporting previously believed parameters for acute kidney injury (AKI) and mortality, the authors noted that a MAP below 60 mmHg was consistently associated with both AKI and increased mortality.

Since 1930, blood pressure measurement has been a widely accepted tool for cardiovascular assessment.  Even under the often adverse conditions encountered in the prehospital or transport environment, providers can accurately measure blood pressure if they understand the principles of blood flow and common sources that introduce error into the measurement process.

Keep learning about blood pressure assessment by reading how to mitigate NIBP and auscultating innacuracies by watching the plethysmography waveform on your pulse oximeter and noting the mean arterial pressure.

Read next: Learn how to read a MAP.

References:

1. James PA, Oparil S, Carter BL, et al. 2014 Evidence-Based Guideline for the Management of High Blood Pressure in Adults: Report From the Panel Members Appointed to the Eighth Joint National Committee (JNC 8). JAMA. 2014;311(5):507-520.  (Available at: http://jama.jamanetwork.com/article.aspx?articleid=1791497)

2. Pickering TG, Hall JE, Appel LJ, et al. AHA Scientific Statement: Recommendations for blood pressure measurement in humans and experimental animals, part 1: blood pressure measurement in humans.  Hypertension.  2005; 45: 142-161. (Available at: https://hyper.ahajournals.org/content/45/1/142.full)

3. Deakin CD, Low JL.  Accuracy of the advanced trauma life support guidelines for predicting systolic blood pressure using carotid, femoral, and radial pulses: observational study.  BMJ. 2000; 321(7262): 673–674.  (Available at: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC27481/)

4. Lehman LH, Saeed M, Talmor D, Mark R, Malhotra A. Methods of blood pressure measurement in the ICU. Crit Care Med. 2013;41:34-40.

This article, originally posted Apr. 9, 2014, has been updated.

To determine whether you have hypertension, a medical professional will take a blood pressure reading. How you prepare for the test, the position of your arm, and other factors can change a blood pressure reading by 10% or more. That could be enough to hide high blood pressure, start you on a drug you don't really need, or lead your doctor to incorrectly adjust your medications.

National and international guidelines offer specific instructions for measuring blood pressure. If a doctor, nurse, or medical assistant isn't doing it right, don't hesitate to ask him or her to get with the guidelines.

Here's what you can do to ensure a correct reading:

Don't drink a caffeinated beverage or smoke during the 30 minutes before the test.

Sit quietly for five minutes before the test begins.

During the measurement, sit in a chair with your feet on the floor and your arm supported so your elbow is at about heart level.

The inflatable part of the cuff should completely cover at least 80% of your upper arm, and the cuff should be placed on bare skin, not over a shirt.

Don't talk during the measurement.

Have your blood pressure measured twice, with a brief break in between. If the readings are different by 5 points or more, have it done a third time.

There are times to break these rules. If you sometimes feel lightheaded when getting out of bed in the morning or when you stand after sitting, you should have your blood pressure checked while seated and then while standing to see if it falls from one position to the next.

Because blood pressure varies throughout the day, your doctor will rarely diagnose hypertension on the basis of a single reading. Instead, he or she will want to confirm the measurements on at least two occasions, usually within a few weeks of one another. The exception to this rule is if you have a blood pressure reading of 180/110 mm Hg or higher. A result this high usually calls for prompt treatment.

It's also a good idea to have your blood pressure measured in both arms at least once, since the reading in one arm (usually the right) may be higher than that in the left. A 2014 study in The American Journal of Medicine of nearly 3,400 people found average arm- to-arm differences in systolic blood pressure of about 5 points. The higher number should be used to make treatment decisions.

In general, blood pressures between 160/100 mm Hg and 179/109 mm Hg should be rechecked within two weeks, while measurements between 140/90 and 159/99 should be repeated within four weeks. People in the prehypertension category (between 120/80 and 139/89 mm Hg) should be rechecked within four to six months, and those with a normal reading (less than 120/80 mm Hg) should be rechecked annually. However, your doctor may schedule a follow-up visit sooner if your previous blood pressure measurements were considerably lower; if signs of damage to the heart, brain, kidneys, and eyes are present; or if you have other cardiovascular risk factors. Also, most doctors routinely check your blood pressure whenever you go in for an office visit.

Over time, uncontrolled hypertension damages the arteries, contributing to their stiffening. As the arteries become narrower and less flexible, the heart has to work harder to move blood through the body. Uncontrolled high blood pressure can lead to stroke, heart attack, heart failure and other heart conditions; cause damage to your kidneys, memory and vision; and contribute to erectile dysfunction.

In about 25 percent (1 out of 4) of people with resistant hypertension, there’s an identifiable, or secondary, cause. People whose blood pressure is raised by a medical condition are said to have secondary hypertension. Secondary hypertension will be very hard to control until those conditions are addressed. The more resistant the hypertension, the more likely there is to be a secondary cause.

Some common secondary causes of hypertension include:

In the other 75 percent of people with resistant hypertension, there is no known medical cause. These people are said to have primary or essential hypertension, and their treatment will focus on medication and lifestyle management.

Treatment options for resistant hypertension or pseudo-resistant hypertension (described below) depend on your underlying conditions and how well you tolerate various medications. Treatments include:

Pseudo-resistant (seemingly resistant) hypertension is high blood pressure that seems to be resistant to treatment, but other factors are actually interfering with proper treatment or measurement. Specifically:

Pseudo-resistant hypertension is also very important to diagnose as true resistant hypertension, because both conditions raise your risk of heart attack and stroke.

For each person there’s an ideal combination of medications and dosages that would best control their hypertension. Some people haven’t yet received that ideal combination. It’s important to work with a doctor who is familiar with the range of medications and who knows what works best for each individual.

Many medications and supplements raise blood pressure. Examples include various pain medications, antidepressants, decongestants, aspirin at high doses and birth control pills. Stimulants — from caffeine and ADHD medications to cigarettes — as well as recreational drugs and excessive alcohol can also raise blood pressure. So can many “natural” or “herbal” supplements, as well as licorice-containing candies or drinks.

It’s important to give your doctor the full picture of the medicines and supplements you take, whether you take them every day or just once in a while.

Your lifestyle can make high blood pressure hard to control or may have caused it in the first place. Taking blood pressure medications without changing your habits means your medicine is lowering your blood pressure while your actions raise it back up. Such actions may include:

Many people have higher blood pressure in the doctor’s office than they have during their regular day. If your doctor suspects white-coat effect, you may need to wear a small, portable, 24-hour blood pressure monitor to see what your pressure looks like over time during your daily activities, or measure or BP with an automated device at home.

Some physicians consider the most authentic form of pseudo-resistant hypertension to be caused by stiffened brachial (arm) arteries that prevent the blood pressure cuff from obtaining a true reading. If your doctor suspects this form of pseudo-resistant hypertension, they might consider other ways to measure your blood pressure.

Measure your blood pressure regularly to help your health care team diagnose any health problems early. You and your health care team can take steps to control your blood pressure if it is too high.

Measuring your blood pressure is the only way to know whether you have high blood pressure. High blood pressure usually has no warning signs or symptoms, and many people do not know they have it.

You can get your blood pressure measured

Take this form  with you on your first blood pressure visit to record important blood pressure-related information.

Many things can affect a blood pressure reading, including:

It’s important to get an accurate blood pressure reading so that you have a clearer picture of your risk for heart disease and stroke.

A reading that says your blood pressure is lower than it actually is may give you a false sense of security about your health. A reading that says your blood pressure is higher than it actually is may lead to treatment you don’t need.

Learn the correct way to have your blood pressure taken, whether you’re getting it checked at the doctor’s office or checking it yourself at home. Use this checklist:

If you are keeping track of your blood pressure at home, use these additional tips.

First, a health care professional wraps an inflatable cuff around your arm. The health care professional then inflates the cuff, which gently tightens on your arm. The cuff has a gauge on it that will measure your blood pressure.

The health care professional will slowly let air out of the cuff while listening to your pulse with a stethoscope and watching the gauge. This process is quick and painless. If using a digital or automatic blood pressure cuff, the health care professional will not need to use a stethoscope.

The gauge uses a unit of measurement called millimeters of mercury (mmHg) to measure the pressure in your blood vessels.

If you have high blood pressure, talk to your health care team about steps to take to control your blood pressure to lower your risk for heart disease and stroke.

Use this list of questions to ask your health care team  to help you manage your blood pressure.

Talk with your health care team about regularly measuring your blood pressure at home, also called self-measured blood pressure (SMBP) monitoring.

SMBP means you regularly use a personal blood pressure measurement device away from a doctor’s office or hospital—usually at home. These blood pressure monitors are easy and safe to use. A health care team member can show you how to use one if you need help.

Evidence shows that people with high blood pressure are more likely to lower their blood pressure if they use SMBP combined with support from their health care team than if they don’t use SMBP.3

Use these additional tips for SMBP:4

Talk with your health care team about how often you should have your blood pressure measured or when to measure it yourself. People who have high blood pressure may need to measure their blood pressure more often than people who do not have high blood pressure.

Acute meal ingestion, caffeine or nicotine use can all affect BP readings, leading to errors in measurement accuracy. If the patient has a full bladder, that can lead to an error in systolic BP of up to 33 mm Hg, and the white-coat effect can have an error of up to 26 mm Hg.