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Optimizing Simulation Results With Your SimCube Oscillometric
NIBP Simulator
by Karl Ruiter, Pronk Technologies Inc.
Introduction
The SimCube NIBP simulator provides
accurate NIBP simulation in an amazingly
convenient, rugged, and affordable package.
As you use your simulator on various patient
monitors it is important to understand what sort
of results to expect, what they mean, and how
to optimize them.
NIBP Device Accuracy
The accuracy of NIBP devices is governed by
an AAMI standard (SP-10). This standard
specifies two different types of accuracy: static
and dynamic.
The static accuracy represents the accuracy
with which the internal pressure gauge in the
device can measure cuff pressure and it is
required to be within +/-3mmHg or +/-2%,
whichever is greater. The static accuracy of
the device can be easily checked by placing
the monitor in cal or check mode and verifying
the accuracy against the SimCube's
manometer mode.
The dynamic accuracy represents the
accuracy of the device in taking actual readings
on patients, and it is specified in a relatively
vague way. The AAMI specification calls for
automated (device) readings to be taken on a
large group of patients meeting certain criteria
and the results to be compared with manual
readings taken by nurses on the same patients.
The manual and automatic readings are then
compared statistically.
To be acceptable under the standard the
mean error on systolic and diastolic readings
must be no more than 5mmHg and the
standard deviation of the error must be no
more than 8mmHg.
What this means is that, under the specifica-
tion, roughly 68% of all patients must be within
13mmHg of the manual readings, but 5% might
be only within 21mmHg and 1% might be as far
out as 29mmHg. While these specifications
may seem very loose they can be quite difficult
to meet.
The errors in the readings come from several
basic sources:
1.Patient to Patient Physiological Variations:
Different patients have different arterial pulse
shapes, arterial compliance, flesh rigidity and
other factors which simply make the BP cuff
respond differently. The oscillometric signal is
complex and changes not only in size but in
shape with cuff pressure and it is simply slightly
different from patient to patient. Additionally, a
patient's actual BP values are often not
perfectly stable, but change over time, often
during a reading or a series of readings.
2. Extrinsic Noise: During the testing process
patients may be moving or talking. The signal
to noise ratio on the oscillometric signal is
never very good and it does not take very much
additional noise to affect readings.
3. Intrinsic Noise: Even on a perfectly still
patient with perfectly static blood pressure
there would still be significant reading to
reading variations. The biggest factor on this is
sampling error introduced by the cuff pressure
bleed rate or step size. It is no accident that
step-down NIBP monitors step in 8mmHg
steps and the standard deviation of the error is
specified at 8mmHg. In addition to this both
bleeding and stepping introduce pneumatic
noise which can affect the oscillometric signal.
NIBP Device Differences
In manual NIBP readings there is a published
AHA standard for how the readings should be
performed. In practice the process is somewhat
subjective, but at least there is the intent of
uniformity.
With automated NIBP readings the situation is
quite different. The first companies in the
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