From: yogi@cs.ubc.ca (Yossi Gil)
Subject: Q&A on blood pressure monitoring (long).
Summary: Compilation of replies to my query
Keywords: Gauge blood pressure monitor medical consumer report summary
Organization: Computer Science, University of B.C., Vancouver, B.C., Canada
Date: Sat, 5 Sep 92 17:21:07 GMT

Few days ago I posted to misc.consumers a query on Blood Pressure
Monitors.  Soon after that informed replies started flowing in.
I believe many others will be interested in this topic.  Gladly I
fulfill my duty of compiling a summary for the net. Perhaps someone
would care enough to put this on a FAQ or on an archival site.

My gratitude to the kind users who replied;

1.  <rgw@potato.hq.ileaf.com> 
     leafusa!potato.HQ.Ileaf.COM!rgw@uunet.UU.NET
2. Zheng Chen <zheng@matt.ksu.ksu.edu>
3. Jon <jon@linkers.med.utah.edu>
4. Denis McKeon <galway@chtm.eece.unm.edu>
5. Eric C. Jung <jung@base.bellcore.com>
6. Andrew Klossner <andrew@frip.wv.tek.com>
                   <uunet!tektronix!frip.WV.TEK!andrew>

MY QUERY (slightly edited):
===========================
My father suffers from a high blood pressure, and he needs to monitor
his blood pressure constantly.  He currently uses to different gauges,
and he is very displeased with the results.  The devices give different
readings even if he doesn't pause at all as he switches from one to
the other.


QUESTIONS AND ANSWERS (heavily edited):
=======================================

1. Is it a good idea to take two consecutive measures?
------------------------------------------------------
The instructions that come with certain blood pressure monitors
specifically advise against taking consecutive measurements.  They
recommend waiting at least 15 minutes between measurements in order to
allow the blood buildup in the lower arm from the first measurement to
dissipate.  They say that failure to do this can result in inaccurate
readings.

2. Is it normal to see varying results in consecutive measurements?  
-------------------------------------------------------------------
A variability of < 6 mm Hg may not be worth worrying about.  Just
because one gauge says 176.4/89.6 and another says 173.7/92.3 doesn't
mean they are both wrong -- it might mean that both should really
say 175/90 and skip the extra digits of false "accuracy".  You could
record a number of readings over a week or two and discuss the topic
of gauge accuracy with your doctor.

Also you should ask yourself the following questions: Varying by how
much?  In which reading (or both)?  Is there a trend?  Does using the
same device several times in a row give the same reading, or widely
varying readings?  Do you read from the same arm each time?  In the
same body position?  Do you read upon waking, or during an animated
discussion of how inaccurate and unsophisticated the gauges are?
(gentle :-))


3. How accurate could we expect the measurements to be?
------------------------------------------------------
An important distinction must be made between "accuracy" and
"precision."  Accuracy reflects how close the mean of the measurements
is to the true value of the variable being measured, without regard
for the variability between measurements.  Precision reflects the
variability associated with repeated measurement, without regard for
the mean.

The body's regulation of blood pressure is very complex.  Slight
changes in posture can significantly affect blood pressure.  Blood
pressure also varies with the breathing cycle.  Even touching your
throat can alter your blood pressure, if the baroreceptors in your
carotid sinuses are stimulated.  It may even make a difference which
arm the cuff is on.


4. Which is the most reliable, accurate and precise device?
-----------------------------------------------------------
Probably the most accurate, precise sphygmomanometers are the
wall-mounted ones that actually contain mercury.  These models are
usually quite expensive, though, and cannot be moved easily.  The
aneroid type is cheaper, and more practical.


5. How sophisticated can blood pressure measurement be?
------------------------------------------------------
How much external pressure does it take to block blood flow?  How
little external pressure does it take to have no effect on blood flow?
That's it.

The devices themselves can be very, *very* sophisticated.  Blood
pressure is often one of the most valuable variables that a physician
can monitor, and a great deal of research has gone into the accurate,
precise determination of blood pressure.

Even the very best methods of blood pressure measurement require
both a calibrated instrument, and a competent technician to use the
instrument.


6. What is the most recommended device for personal use?
--------------------------------------------------------
For most personal purposes, though, it seems that a conventional
cuff-type aneroid sphygmomanometer should suffice.  There was a general
consensus that "sophisticated" is exactly what you don't want.  If
you want to the most accurate and reliable readings, you should use a
mercury-column manometer and stethoscope, since it *directly* measures
what you are trying to determine, namely millimeters of mercury.
A second choice would be a mechanical gauge and stethoscope.  Avoid
the automatic electronic models since (it seems that) the electronic
pressure transducers that they use are not all that stable, and
that the pulse-monitoring algorithms they use for determining when
to capture the systolic and diastolic readings are not completely
foolproof.

None of the automatic gauges are worth squat if you don't calibrate
them often, and home units are not easily calibrated.  To get reliable
readings, use a "low tech" sphygmomanometer and a stethescope.

You can buy a stethoscope and sphygmomanometer ($10 word for blood
pressure cuff) for about $25.  Then you'll be using the same equipment
a doctor or nurse would be using.  It doesn't take too much practice
to use.  Get these at a medical supply place, not at a drug store.

You would very soon learn (the hard way) that getting a blood pressure
reading is a pretty inexact art (and pretty awkward to do one-handed).
Try getting him focused on the trends by keeping good (and regular)
records over the weeks.

According to Consumer Report, May 1992, all finicky finger units
tested were not acceptable!

7. How much should I worry about the accuracy the measurement?
-------------------------------------------------------------
At the risk of jumping in and making some reckless guesses, you might
explore the possibility that you:

    are worried and unhappy about your high blood pressure and the
    possible consequences of it,

    had gotten very focused on the "numbers" part of a high blood
    pressure, rather than on the "change of habits & lifestyle" part,

    are viewing the blood pressure readings and whatever goals or
    critical levels your doctor had given you as something like an
    inverse high-jump (with all the intensity and hoopla of the Dan
    & Dave Olympic track-shoe "competition") to the point that his
    days are now "good" or "bad" depending on his being over or under
    a particular critical level.

Apologies in advance the above guesses are wide of the mark.

It is possible that your absolute blood pressure reading (if there
is such a thing) is less important than any long-term trend in your
blood pressure.  Think how someone on a diet might use a single scale
to watch their weight over weeks and months rather than using 3 or 4
different scales to monitor their apparent weight changes over hours
and days.

Or recall the saying: "A man with two watches can never be sure which
one is right."  Or the related one - "An old sailor will never put
to sea with two compasses - one, or three - but never two."


8. Any more good advises for a personal blood pressure monitoring?
------------------------------------------------------------------
* Have your physician (or a nurse) instruct your on the proper
measurement technique.  It is not enough for them to just tell him
how to do it; have them observe your father's technique, and correct
any errors.  Many factors can decrease the accuracy and precision
of measurement, including releasing the pressure in the cuff too
quickly, improper placement of the cuff, and improper placement of
the stethoscope.

* During measurement, try to hold constant as many physical variables
as possible.  Make your measurements under the most rigidly regulated
conditions possible, including such subtle things as his posture and
his anxiety level.  If you start to get mad about the performance of
the blood pressure instruments, you can forget about getting a good
number.  ;->

* Have your instrument calibrated by a reliable technician.  If the
instrument won't calibrate satisfactorily, replace it.  You probably
don't need to spend a fortune on an instrument with lots of bells
and whistles, but you will need to treat any instrument delicately,
and calibrate it regularly.

9. Where can one get more information about blood pressure gauges?
------------------------------------------------------------------
Consumer Report magazine in its May 1992 issue did an evaluation of
blood pressure monitors that rated them on the basis of consistency
of readings (among other things).  You can probably find a copy
of Consumer Report in your neighborhood library.  [Please consider
subscribing to this highly recommended, non profit, magazine.  You
will enjoy valuable and interesting information on many consuming
products presented in a very readable way.  Subscription is quite
cheap and will give you your money's worth and much much more.]


10. What models of which brands are the best?
---------------------------------------------
From Consumer Report, May 1992 (page 298):

A. Mechanical models:

	Marshall 104, Omron HEM-18, Walgreens 2001 and Sunmark 100
	were very consistent and relatively not too difficult to use.

	Lumiscope 100-021 was equally consistent but difficult to use.

	Sunbeam 7627-10 was not so consistent. In addition it was
	difficult to use.

B. Electronic models.

	Sunbeam 7621 (now called Sunbeam 7621-10) was the best in
	terms of consistency. Second best were Sunbeam 7650 (now called
	Sunbeam 76510-10) and AND UA-701. Note that the AND brand
	gives only a 90 days warranty.

C. Electronic finger models.

	All of the following were tested and found unacceptable:
	Lumiscope 1083, Marshall F-89, Omron HEM-815F and Sunbeam
	7655-10.
--
Joseph (Yossi) Gil                 {alberta,uw-beaver,uunet}!ubc-cs!yogi
Dept. of Computer Science          yogi@cs.ubc.ca (cs.ubc.ca=137.82.8.5)
6356 Agricultural Road., Univ. of B.C.,             Tel: +1-604-822-8175
Vancouver, British Columbia, V6T 1Z2, Canada.       Fax: +1-604-822-5485
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