Non-Invasive Blood Pressure
Measurement Techniques
Issues and Challenges that affect us
Ausculatory and Oscillometric
Ausculatory (manual or automated) uses detection of
Korothoff sounds to determine blood pressure. Detection is
either by a clinician’s ear using a stethoscope or a
microphone inside of an automated unit. Pressure is
displayed by either a dial(Aneroid) or column of mercury.
Oscillometric measures the oscillations in the air contained
in the machine’s umbilical cable caused by the arterial
pressure pulse. The machine measures MAP then calculates
systolic and diastolic pressure. This reading is usually
displayed by LED's (Light Emitting Diodes) on a display.
Which one is better?
Oscillometric Pros and Cons
Movement,muscle tremors and abnormal heart rhythms,
a very weak pulse or pressure can cause some
automated units to fail to obtain a reading.
And most devices need a reasonably steady BP during
deflation to accurately detect the appearance, maximum
and disappearance of pressure oscillations. And the
initial price may restrict it’s wide spread use.
But they are easily transportable, very easy to use, can
give multi-parameter readings (temperature, SpO2), can
have a built-in paper recorder, have no observer bias and
are mercury-free. And when used properly give accurate
and repeatable readings.
The oscillometric method does not determine the blood pressure
instantaneously but it determines it from the curves of the changes in the
pressure and its oscillation.
This feature has anti-noise characteristics as it is not affected by external
noise or electric surgical units.
Even when the
Korotkoff sounds are
hardly detected due to
hypotension, the
oscillometric method
is capable of
determining BP
because the cuff
oscillates as long as
the artery pulsation
exists.
Which one is better?
Ausculatory Pros and Cons
Two types of manometers are used in this facility,
Aneroid and Mercury. Aneroids require annual checks to
insure that mechanical wear or shocks haven’t affected
readings.
Mercury is the “Gold Standard” of NIBP but needs
accuracy checks as well.
Most of the problems with ausculatory are operator
induced such as digit preference, observer bias and
differences with operators in auditory acuity.
But now the major problem with mercury manometers is
the mercury used inside the manometers.
What’s this I am hearing about
getting rid of mercury?
The VHA’s policy is to comply with the EPA’s Pollution Prevention Act,
Resource Conservation and Recovery Act and all state and local
environmental regulations in controlling and reducing elemental
mercury and mercury containing compound in the health care waste
stream with the goal of eliminating mercury from the health care waste
stream by 50% by 2005 and eventually achieving a mercury-free
environment.
Locally, we are conducting an inventory to document the number and
types of medical and non-medical devices containing mercury then
where practical utilize mercury-free products. Also promoting
awareness of the problems associated with mercury is a large part of
the solution.
Why do we need to do this?
Mercury is found in many medical devices and uses such as
thermometers, BP manometers, esophageal dilators as well as many
non-medical products. If these products are broken, spilled or
disposed of improperly, mercury can be released into the
environment. Incinerators release mercury into the environment
when waste containing mercury is burned. Hospitals contribute 4-5%
of the total wastewater mercury load. There is up to 50 times more
mercury in medical waste that in general municipal waste. Mercury
from thermometers contribute about 17 tons of mercury in landfills
annually.
Even if properly recycled its reuse in new products pose the same
eventual risk. Inside the hospital mercury spills are costly because of
the special training, equipment and disposal required.
Surely Mercury isn’t that bad!
Exposure to mercury can occur three ways: inhalation,
ingestion and absorption. The most likely route of inorganic
mercury is through inhalation after a spill.
Short term exposure to mercury vapor can cause nausea, shortness of
breath, pneumonitis and bronchitis. Long term exposure can lead to
personality changes, memory loss, stupor and coma.
The elemental mercury used in thermometers and manometers is
converted by microorganisms in the environment to methlymercury, the
most toxic form. Exposure to organic mercury is through the water we
drink by contamination of groundwater or by the food we eat because of
contamination of the environment.
Methlymercury is a neurotoxin and especially dangerous to fetuses and
young children. A 2001 CDC study found that 10% of women have
mercury levels high enough to cause neurological damage to their
children.
Wouldn’t keeping mercury out of the
environment make these questions easier to
answer?
Is this fish mercury-free?
Has mercury contaminated
this lake?
What do I do if I think my equipment is
not working properly?
The most common problems affect both ausculatory and
oscillometric types.
First make sure that you don’t have an external air leak
either in the cuff or tubing connecting the cuff to the
unit. Aging BP cuffs are the source of most air leaks
[replacements are available from SPD (both single and
dual tube types)].
If you suspect a mechanical or electronic problem then
call the work order desk at extension 5018. The more
information (type of unit , problems experienced) you
can provide will speed up any repairs.
What Affects A Reading?
No matter which method you use there are common
issues that affect both methods of measuring blood
pressure. First the equipment:
Is the cuff you are using sized for the patient? A cuff too
large can cause reading to be lower than actual and a cuff
too small can cause reading to be higher than actual.
Cuff sizes are:Small Adult 17-25cm/Standard Adult 2333cm/Large Adult 31-40cm/Thigh 38-50cm.
Is the equipment (either sphygmomanometer or vital
signs monitor) working properly? Air leaks in the cuff,
tubing or inside the unit can affect the deflation rate.
An mercury or aneroid manometer that is improperly
zeroed will affect the entire reading.
How does the patient’s state affect
the pressure reading?
Is your patient ready?
If your patient has finished a cigarette or an
alcoholic beverage within the last 15 minutes
the readings will be altered.
If they haven’t sat quietly for at least 5 minutes
or are talking during the procedure, the
readings will be altered.
And if you have placed the cuff over a shirt
sleeve the readings will not be reliable.
What can I do to insure an
accurate reading?
Follow the Hypertension guidelines
for BP measurement.
Daily before using any type of
manometer verify that it is in good
working order.
And if you feel that the device you are
using is not providing an accurate
reading, tag the device and notify the
work order desk at extension 5018.
My Critikon won’t take a BP
reading!!
Is it the cuff
OR
Is it the Critikon???
Connect each one of the cuffs to
the Critikon, push start and see
what happens!
What’s Your Rate?
Deflation that is…...
Inflate the cuff to 200mm/Hg
Tell the timer to start the clock, then deflate
the cuff at the recommended 2-3 mm/sec
rate.
After 20 seconds when the timer says stop,
close the valve.
We tell you what your rate is!