Design of a
Blood Pressure Monitor
There are many
factors that go into the design of a blood pressure monitor depending on market needs. There are usually tradeoffs involving cost, measurement accuracy, measurement speed, ease of use, subject age range, device size, and artifact rejection. While all devices require consideration of most if not all of these factors, different markets require more attention to some of these variables.
Home blood pressure monitors require accuracy, ease of use, and low cost. Ambulatory monitors require accuracy, comfort, small size, and resistance to artifacts. Clinical monitors require accuracy and low cost. Critical care monitors require reasonably accurate measurements quickly obtained even if the subject is shivering. They also frequently require that they be suitable for use on children as well as adults. An ISO safety standard requires any automatic cycling device to limit maximum
pressure and maximum inflation time to certain limits depending on the age of the patient. In addition these limits must be met even in the presence of a single fault.
is relatively easy to design a monitor that will work well on a
subject that is sitting still, has a regular heartbeat, and is
motion free, as is usually the case in clinical or home
measurements. It is more difficult to design a
monitor that will quickly capture a subject's blood pressure
when the subject is moving or is being transported. Many of the
steps that can be taken to improve accuracy also extend the
Low cost digital circuitry has made the hardware design relatively simple. Even most adjustments (pots) can be eliminated, greatly reducing
the cost of calibration. The challenge comes in selecting the measurement method, and designing the firmware to provide the performance required.
Artifact and Arrhythmia
in Automatic Measurement
a. Muscle activity
b. Tendon movement
d. Vibration such as that caused by transportation
f. Elevation changes
a. Missed beats (Algorithm must accommodate)
b. PVC (Algorithm must accommodate)
d. Atrial Fibrillation
e. Ventricular fibrillation (ignore. BP not useful)
f. Erratic heart beats (Non-sinus rhythm)
strategy is to attack artifact and arrhythmia at five levels.
1. Pre-processing to eliminate artifact before collection
2. Pause at a pressure level until enough information is
collected if artifact is present
3. Pulse qualification scheme
4. Methods used to evaluate blood pressure "table"
5. Tests of sensibility after the best attempt to obtain BP
measurement scheme to a large extent determines how well the
device will function. There are three different schemes. Each
has its own advantages and disadvantages.
2. Measure during step-wise deflation
3. Measure during linear deflation
The design is
part art and part science. It usually takes some experimentation
before acceptable performance is achieved, especially if the
equipment is to be artifact tolerant.
In general, the
faster the measurement requirement, the more difficult it is to
obtain an accurate reading. Fortunately applications that
require a fast measurement usually do not have to be as precise
as applications that do not require a fast measurement. Perhaps
the most demanding for accuracy while being subjected to
considerable artifact is an ambulatory blood pressure device.
Such a device is normally worn by a subject for 24 hours, and
measures blood pressure several times an hour while the subject
engages in their normal activities. The author has extensive
experience in the design of the world's most popular ambulatory
blood pressure monitors (90202, 90207, 90217).
A blood pressure
monitor normally obtains the blood pressure by means of an
occluding cuff. International regulations require that an
automatic cycling device
be single-fault-tolerant to prevent too long of inflation, or
inflation to a pressure that is too high. These time and pressure limits
differ between adults and children. While the author has
designed systems that use a single processor and meet these
requirements, the easiest means is to use two processors in the
control of the device.
One of the
surprises we encountered early in our blood pressure career was
the need to teach health care professionals how to correctly
measure blood pressure. The company I was working for was
designing products in Oregon and testing them at a well-known
Eastern university. Our initial results were poor, and changes
in the algorithm were not successful at resolving differences.
It was only when we started using two "experts"
measuring blood pressure at the same time (using a teaching
stethoscope) that we located a major source of our problem. The
experts could not agree on the subject's blood pressure. So we
had to get really good at teaching the experts how to measure
pressure. The point of this discussion is that the design of the
clinical testing is every bit as important as the design of the
device. Also, the monitoring of the clinical validation by
an expert will usually catch lapses in following the protocol.
cuffs play a very important role in the indirect determination
of blood pressure, yet they are usually not well understood or
properly used. The size of the cuff (at least the inflatable
portion, or bladder) and the application of the cuff are of great
importance in obtaining an accurate measurement, yet this size
is frequently ignored. I have visited the clinics of luminaries
in the blood pressure field, and have asked them what size
bladder they use. Frequently they do not know. It is commonplace
to find that the size depends on the examination room, as
different brands of cuffs are employed, each being differently
sized. Yet the size of the cuff will greatly influence the
measurement results. Also, the way the cuff is applied to the
limb greatly influences the results. It is estimated that at
least 30% of blood pressure measurements are incorrect because
of cuff selection or application (Review: A century of
confusion; which bladder for accurate blood pressure
measurement? O'Brien J. Hum Hypertens 1996; 10:565-572). The most
frequent error is in using a cuff that is too small, which results in
blood pressure measurement that is too high, resulting in
millions of people in the US alone being placed on medication
that they do not need.
Even when the
"correct" cuff is selected there is still a
significant difference in blood pressure measurement if the limb
size is in the range of two cuffs. Using the larger size cuff
will produce a reading significantly lower than using the smaller
There are two
schools of thought regarding the cuff. One believes the larger
the cuff (inflatable portion) the better, while the other school
believes too large of a cuff produces errors similar to a cuff
that is too small.
Heart Association (AHA www.americanheart.org) makes
recommendations about the cuff size, although it is not clear.
It appears that the battle between the two schools continues to
into their recommendations.
Cuff size has
been the topic of discussion for about 100 years. The first cuff
was a bicycle innertube width affair suitable for use only on
the smallest of arms. The next commonly-used size was about 12
cm in width, similar to the most commonly-used cuff size today. Length has varied from 50% to
100% of arm circumference. If the function of the cuff
is to apply the same pressure to the artery as is in the cuff,
then as postulated by Mike Cohen of CAS Medical (now deceased),
the bigger the better. The limb presents a back resistance to
the cuff trying to compress it. The maximum pressure point is in
the center of the cuff, and decreases towards the edges. The
wider the cuff, the closer the pressure exerted on the artery
approaches the pressure in the cuff.
Studies show that varying
the width from the nominal changes the blood
pressure reading. Too wide of a cuff results in too low of a
reading, and too narrow results in an artificially high reading.
The reason a wide cuff produces too low of a reading is not
obvious. Various researchers first did not believe it, because
it does not seem to make sense, and then begrudgingly accepted
it as true. The author believes an occluding cuff applied to the
limb prevents the reflection that adds to the incident wave.
This is referred to as "systolic heightening." This would result in too low
of a reading, unless compensation was provided. The narrower
cuff provides a first-order compensation.
For diastole, a similar compensation is occurring, but this time to compensate for using Korotkoff phase V instead of phase IV. This explanation is unknown in the medical field. You will only learn of it here.
Types of Cuffs
There are two
different types of cuffs in common use. One type has a two-piece
construction consisting of a cloth wrap that contains a rubber
bladder. The bladder will blow up much the same as a balloon if
not contained by the cloth wrap. This will cause inaccuracies if
the pocket holding the bladder is not sized to closely limit any
bladder expansion. Similar
to a balloon, the bladder will retain its shape and will be
resistant to expanding its volume until the first time it has
been over- inflated. This can occur if the cuff is attached to
an automatic device and the cuff is inflated without it being
wrapped around an arm. Once this has occurred, the bladder may
not be useable, unless the cuff pocket prevents any further
expansion when the cuff is used. This source of inaccuracy is
exacerbated if the cuff is loosely applied to the arm.
The other type
of cuff is a one-piece design where the bladder is formed as
part of the wrap. This bladder is non-distensible, and the
author has found that it will result in a lower reading even if
it is the same size as a cuff with a rubber bladder. Because the
bladder is non-distensible, the application technique is less
important for the one-piece style of cuff.
It has long been
known that it is important to apply the cuff snuggly. This is
because the space between the limb and the cuff must be filled
by the bladder before pressure is applied to the limb. If the
space is large, then the bladder inflation causes the bladder to
narrow where it contacts the arm, acting as a narrower bladder.
What is less appreciated is that the length of the bladder
influences the degree of narrowing. If the bladder almost fully
encircles the arm, then it does not narrow nearly as much as a
bladder does when it is only partially encircling the arm. Thus
the bladder length is not really important if the cuff is
applied correctly, and becomes more important as the cuff is
applied loosely. Most cuffs are designed such that the bladder
fully encircles the smallest arms and encircles less than 70% of
the largest arms within the specified arm range. In general, the
longer the bladder the better.
Cuffs are marked
such that the center of the bladder is positioned over the
artery if the cuff is correctly applied. A little analysis will
show the pressure applied by the cuff to the limb is the same
all of the way around, except where the transition occurs at the
ends of the bladder. At this point, there is an area where the
inflated bladder prevents the adjacent cuff material from
contacting the arm. If this area is over the artery,
considerably less pressure is applied resulting in a falsely
The Dilemma of
the Tapered Arm
exhibit a taper. That is, the arm near the shoulder is larger
than the arm near the elbow. Most practitioners apply the cuff
in a cylindrical manner such that the cuff is snug at the upper
part of the arm, and loose at the bottom edge. This will result
in a narrowing of the bladder as the bladder is inflated. The
amount of narrowing depends on the degree of taper, and the length of the
bladder. Usually it is better to wrap the cuff such that it is
of equal tightness at both the top and the bottom. This results
in less inflation needed. However, it also effectively results in
a narrower bladder, because the cuff is no longer symmetrical
around the arm. This narrowing is not as severe as is produced
in the former case of cylindrically wrapping the cuff.
The author has
invented a "one size fits all" cuff. This cuff was
used on the Pharma-Smart blood pressure device, and the clinical
results are detailed in Blood Pressure Monitoring 2004, 9:19-23.
The cuff has the added advantage of being quite difficult to misapply, and should eliminate most of the errors produced by cuff selection and application
at a low cost.
A patent is pending. More information will be provided to
qualified interested parties. After the patent is issued, we will
disclose all of the advantages of this new approach