Accurately
Measuring Blood Pressure
 Measuring blood
pressure is one of the most common measurements performed in
medical practice, but seldom is it measured accurately. While
blood pressure could apply to the pressure in the
arteries, veins, or within the heart, this article refers to the
pressure of the blood in the arteries, which is what is commonly
referred to as blood pressure or BP.
There are
normally two numbers provided for a blood pressure measurement.
A common measurement might be 120/80. The higher number is
called systole, and is the pressure produced when the heart
"beats" or contracts, producing the maximum pressure
in the artery. The lower number, diastole, is a measurement of
the minimum pressure that is present between beats of the heart.
The number is normally given in units of millimeters of mercury
(mmHg).
While we talk of
blood pressure as if it were a more or less fixed number similar
to body weight, it is not. It is much more similar (and related)
to heart rate. Blood pressure must, and does, change in order to meet the body's needs. The body needs
oxygen to function. The amount of oxygen needed depends on what
is happening. The higher number, systole, is particularly
variable with activity. Normal, healthy people can have systoles
well above 200 when they are exerting themselves physically or
even mentally. Diastole (the lower number) changes less, and has
been shown in studies to be less indicative of cardiovascular
risk (heart attack or stroke) than systole.
Does
Accuracy Matter?
It depends on
why BP is being measured. If it is being measured in an
emergency or hospital environment such as the operating room,
recovery, or critical care, then an error of 10 mmHg is not very
important. The reason for measuring blood pressure is to
determine if the pressure is within a wide range of acceptable
pressures. What is important is obtaining a reading quickly and determining if a sudden change is occurring. On the other hand
if it is being measured in a clinical environment, such as your
doctor's office, accuracy may be of high importance. Even a few
mmHg can make the difference between being prescribed medication or just having your blood pressure monitored. Once
being placed on medication, it is usually for a lifetime.
Frequently the medication is quite expensive, and has side
effects. Also, being diagnosed as hypertensive (a fancy term for
high blood pressure) will affect insurance rates, and may, for
instance in the case of an airplane pilot, mean the end of a
career. Unfortunately some of the least accurate measurements
take place in the clinical environment where accuracy is most
important.
Measuring
Blood Pressure
There are three
different ways that are commonly used to measure blood
pressure: invasive, ausculatory, and oscillometric. We will discuss each
and review some of the difficulties with each method.
Invasive
Measurement
 This is the only
direct measurement of blood pressure. In theory this measurement
could be highly accurate. However, in practice, it often is not
accurate, principally because of the measurement site, and not
paying attention to all of the various details concerning the
measurement.
In order to
perform this measurement, a tube (catheter) is inserted into an
artery of interest. A pressure transducer is connected to the
tube (or better, mounted on the tip of the catheter), and the pressure is measured. Because there is risk
associated with puncturing an artery, this invasive means of
measurement is not commonly used except in cases when it is
vitally important to quickly determine changes in blood
pressure. Most commonly, the catheter is inserted into the
radial artery (located at the wrist). This site is lower risk
than tapping into a larger artery. Also, unlike most of the
potential sites in the body, there is a second artery supplying blood
to the hand (the ulnar artery).
Compared to
brachial artery pressure, the blood pressure at the wrist is
higher for systole, lower for diastole, and about the same for
"mean" or average pressure. The amount of difference
depends on the stiffness of the arteries. Although the radial
artery is a
convenient site, it does not result in the same blood pressure
measurement as a site closer to the heart such as the brachial artery. An
additional accuracy issue is produced if the wrist is not at the
same elevation as the heart. For every inch, the pressure either
increases or decreases by 2 mmHg, depending on the elevation
being lower or higher. In any event, invasive measurements
cannot be routinely obtained, and, because of the many factors
that cause the measurement to differ from a brachial artery
measurement, may not be a suitable means to
determine if a subject has "high" blood pressure even
if available.
Ausculatory
Measurement
 This is the way
blood pressure is most commonly measured in a clinical
environment. A cuff is put on an arm and inflated to a pressure
above what is expected to be the systolic pressure. A
stethoscope is held against the arm down the arterial stream
from the cuff, hopefully directly above the artery.
The pressure in the cuff is then gradually reduced until the
person listening to the stethoscope hears an indication that
blood flow has started to occur at the peak of the blood
pressure cycle (systole), and continues to reduce the pressure
in the cuff until the blood is flowing throughout the cardiac
cycle (diastole). This method takes a minimum of equipment, and
is the method used in large population studies to determine what
normal blood pressure is for humans. Every medical professional
"knows" how to perform this measurement. Unfortunately
few know how to perform it correctly. Perhaps the
four most common errors are:
- Using the wrong size cuff.
- Applying the cuff incorrectly
- Not positioning the stethoscope directly above the artery. Incorrect placement will result in too low of a
systolic and too high of a diastolic pressure measurement.
- Incorrect interpretation of the sounds heard.
There are many more ways to obtain inaccurate results such as
trying to obtain a measurement in a noisy environment, not
supporting the arm at the correct level, wrong definition of
systole or diastole, not pumping high enough, releasing the
pressure too rapidly, too much (or even too little) finger
pressure on the stethoscope, manometer not at eye level, hand
not palm up, etc. The point is that it is easy to obtain an
inaccurate measurement without realizing it, and a correct
measurement is only obtained with a great deal of attention
being paid to every detail. This attention is seldom present in
a clinical environment.
The American
Heart Association publishes recommendations on how to perform
this measurement. The author has had his pressure measured many
times, and not once has it been in accordance with these
recommendations. Studies comparing invasive to ausculatory
measurements have been all over the map (see AAMI SP10 for
comparison). The only agreement that is normally reached is that
these measurements differ. The author believes much of this
"differing" is caused by poor measurement techniques,
and varying measurement locations on the body. Few people know
how to accurately measure blood pressure either invasively or
indirectly to the degree needed for a comparison study. Also
most of the invasive measurements have been performed using the
radial artery for reasons previously stated. An invasive radial
artery measurement cannot be expected to be the same as an
ausculatory brachial artery measurement except for the
"mean" blood pressure. More on this later.
Oscillometric
Measurement
This is the
method most frequently used by automatic devices to measure
blood pressure. A cuff is placed on the arm, and is inflated to
a pressure that is above the systolic pressure, blocking the
flow of blood. The pressure is then decreased in a controlled
fashion. During this deflation, the pressure in the cuff is
monitored for minor pressure fluctuations caused by the artery
encircled by the cuff opening and closing. This method of blood
pressure determination is older than the ausculatory method, but
is very difficult to perform accurately without involving a
microprocessor.
Some automatic
devices use the ausculatory method. In this case, a microphone
is used to detect the blood flow sounds (Korotkoff sounds). When
using the ausculatory method, placement of the microphone over
the brachial artery is quite important to get a correct
measurement. The popularly of the oscillometric method can be
mostly attributed to the lack of critical cuff placement, and
better performance when the subject is in shock. This method has
been modified and refined over the years in order to closely
match other blood pressure measurements, however the reference
standard has been a major subject of disagreement.
One school of
thought is that only an invasive measurement can be thought of
as being the "real" blood pressure, and therefore it
should be the standard of comparison. While this school has much
to recommend it, the author strongly believes that for the
diagnosis of hypertension the ausculatory method should be the
"gold" standard, because all the population studies
that give us the knowledge of what is normal and high blood
pressure have been based on the ausculatory method. This is not
likely to change, as it would be unethical to perform large
population studies using an invasive method. If future studies show that properly performed ausculatory measurements agree closely with properly performed brachial artery invasive measurements, then this becomes a moot point.
There is
widespread misconception on how oscillometry works. It does not
"determine the mean pressure and then calculate systole and
diastole" as many think. Rather it determines the relative
volume of the artery surrounded by the cuff, determines when the
artery is starting to allow blood to pass (systole) by
determining the pressure at which the artery volume is at 50% of
the maximum, and determines when blood is flowing throughout the
cardiac cycle (diastole). The better implementations are much
more accurate than those of almost any clinician, and may be even better
in some cases than those of the best clinician. This technique is especially good in the cases
where there is a trail-off of Korotkoff phase V sounds such that
it is difficult to determine diastole.
What is
"Normal" Adult Blood Pressure?
Cardiovascular
disease is the leading cause of deaths in the United States.
Little was known about the risk factors until a study was
instituted in Framingham MA in 1948 (see http://www.nhlbi.nih.gov/about/framingham).
Initially 5,209 residents were enrolled into the study, which
was about two-thirds of the population of this small town. These
subjects were interviewed and tested for various factors in
order to better understand the risk factors. Another 5,124
residents were enrolled in 1971. These subjects were the
offspring of the original subjects. Every two years the
surviving subjects returned for detailed medical history,
physical examination and laboratory testing. By testing and
keeping track of these subjects, it was possible to determine
which of the various risk factors contribute most to
cardiovascular events. They determined that blood pressure
higher than 120/80 increased the risk for a cardiovascular
event. The higher the pressure, the more the risk. The blood
pressure was determined using the ausculatory method.
In medical
practice, frequently 140/90 becomes the dividing line where
patients are put on medication. Why 140/90? Increased risk with
increased blood pressure is a gradual curve, and the prognosis
of 139/89 is not much different than 140/90, but the decision
point must be somewhere.
What is the
"Real" Blood Pressure?
 While this seems
like a simple question, it is quite complex. First the question
of what use is to be made of the pressure needs to be addressed.
If blood pressure is being measured in order to determine the
short-term prognosis of the subject, then it makes little
difference what pressure is used. In the operating room, the
anesthesiologist is not likely to treat a pressure of 140/90 much
differently than a pressure of 115/70. What is required is trend
information to assure that the patient is properly maintained at a
safe level of blood pressure. However in clinical practice,
consistent pressures above 140/90 are usually treated with
medication, while a pressure below this figure is not treated.
It is easy to
argue that a properly performed invasive measurement can
determine the pressure at the measurement site. There are many
pitfalls in obtaining accuracy, but in theory, if all are
addressed, one could determine the actual pressure at the site
of the catheter. The questions now becomes one of what is an
appropriate site? It has been shown by many investigators that
invasive pressure is greatly influenced by the measurement site.
Systole in particular changes as the site in moved from the
aorta away from the heart towards the extremity. Usually it is a
surprise to the newcomer to this field that systole increases as
one moves down the arterial stream. This increase (systolic
heightening) is caused by the reflection of the incidence
waveform adding to the pressure. The amount of this increase
depends on the transmission speed of the artery, which is
affected by the arterial compliance. Hard arteries produce more
heightening than compliant arteries.
The pressure in
the aorta is not much influenced by reflections, and may be the
best indication of "true" blood pressure.
Unfortunately this is a very difficult place to measure blood
pressure except when undergoing cardiac evaluation or surgery,
and there are no population studies to indicate the
cardiovascular risk factors.
Resources
http://www.nhlbi.nih.gov/about/framingham
- This site reviews the history of the Framingham study, and the
results. It also offers risk assessment worksheets and much
more.
http://www.dableducational.com/index.html
- A must visit site if you are interested in blood pressure
measurements.
http://marketplace.aami.org
- Location to order
AAMI SP10 which specifies the requirements for blood pressure
measuring equipment.
http://ash-us.org/
- Home site of The American Society of Hypertension
http://smipdx.com Website of a development
partner who has designed blood pressure devices and many other types of equipment.
Sources for some of the illustrations on this
website include:
http://health.allrefer.com/pictures-images/blood-pressure.html
http://www.merck.com/mmhe/sec03/ch022/ch022a.html
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