Oscillatory Blood Pressure Monitoring Devices
BMJ 2001; 323 doi: https://doi.org/10.1136/bmj.323.7318.919 (Published 20 October 2001) Cite this as: BMJ 2001;323:919
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I thank Professor Grim for his comments, but I disagree with his
assertion that I was "incorrect" in my description of oscillometric
devices. My remit for this particular series is to use just 350 words to
describe things in an easy and understandable language. The series is
designed to appeal to readers who on the whole are "jobbing" doctors who
use such devices or refer patients for the tests in question, but who are
not experts in the intricacies of how they work. In each case I consult
with experts in the field.
For Professor Grim, the style and description used was clearly not
sufficiently detailed. Indeed what he says in his letter is entirely
accurate, but too detailed for the purposes of this series. There are of
course many problems with the auscultatory method, most of which relate to
individual user use. There are also potential problems with oscillatory
methods. The British Hypertension Society recommends that the deflation
rate for auscultatory devices should be 2-3 mm/second.
Competing interests: No competing interests
Mr Berger is incorrect in the assessement of how
oscillometric BP devices work. What he describes is close
to the vibrations that are set up and cause Korotkoff sounds
that are used in the ausculatory method not the
oscillometric method.
In virtually all oscillometric devices the sensor detects
the pulse wave form as blood moves under the cuff as the
cuff is deflated. The peak of maximal oscillation is
determined and this is set as the mean arterial pressure.
From this MAP reading the devices calculate both systloic
and diastolic pressures by propriatry methods that are not
published. What we do know is that none of these devices are
as accurate as trained humans using a stethoscope and a
mercury manometer.
The author states that the oscillometric devices deflate at
a slow rate of 4 mm per second which makes them seem slower
that the aneroid method. What he does not appear to know is
that the recommended rate of deflation is 2 mm/sec for an
accruate blood pressure. His observation that the 4 mm/sec
seems slower than what is usually used just reflets the
nearly univeral excess deflation rate of up to 10 mm/sec
wrongly done by most poorly trained human observers. This
is one of the reasons that many BP readings that expected
end in zero-the observer deflates so fast they cannot read
to the nearest 2 mm Hg.
Currently devloped oscillometric devices were designed to be
used in surgery or in the intensive care unit not in clinic
where even a 2 mm Hg error can have tremendous effects on
the number of person diagnosed with hypertnesion-or who have
their hypertension missed. In the US thers are about 6
million persons with a BP of from 90-92. And about 8
million with a BP of 88-90. If the office BP is off
systematicaly by only 2 mm Hg too low then the health care
system will miss 6 million people. If it overreads BP by 2
mm Hg then it will add another 8 million ot the ranks of
those who need to be treated--for a disease they do not
have.
For example Park et al (Arch Pediatr Adolesc Med 2001
Jan;155(1):50-3, Comparison of auscultatory and
oscillometric blood pressures) Found that the Dinamap
systolic pressure readings were 10 mm Hg higher (95%
confidence interval, -4 to 24 mm Hg) than the auscultatory
systolic pressure readings. Dinamap diastolic pressure
readings were 5 mm Hg higher (95% confidence interval, -14
to 23 mm). The results were also not linear. At higher
pressures the device underestimated BP. This widely used
device will over diagnose HTN in children by about 40%.
These same devices had a bizarre tendency to skip certain
values . (see Hypertension 2000 May;35(5):1032-6 Skip
patterns in DINAMAP-measured blood pressure in 3
epidemiological studies.). For example the following
systolic blood pressure values were consistently skipped by
the device: 89, 119, 120, 124, 125, 130, 140, 141, 150, 160,
170, 180, 190, and 200 mm Hg. These skip patterns are likley
related to the algorithm that the manufactures will not
reveal.
Why these unvalidated devices are so widely used in the US
and Britian is not clear but has the potential of doing
great harm to the public. Withoug accurate and reliable
blood pressure measurement instruments and observers the
proven benefits of the diagnosis and treatment of HTN will
not be transferred to the popuolation.
I would hope that in future sections of how does it work
that deals with blood pressure measurements be reviewed by
experts in blood pressure measurement befo
Competing interests: No competing interests
So how does it work, then?
BMJ readers need a little help from the Editor. On 20th October you
published an article "How does it work?" with an account of oscillometric
blood pressure measurement, with diagrams. One reader, Professor Clarence
Grim of Wisconsin, sent in a rapid response to say that was not how it
works at all - he said, reasonably politely, that the author of the
article was expounding on the subject without knowing what she was talking
about. He sounds as if he knows what he is talking about. I do not want to
see anyone pilloried, and right know I do not know how oscillometric blood
pressure measurement works, but I am curious and the BMJ is an
authoritative journal. I think a statement should be published settling
the matter.
Ed Cooper
Competing interests: No competing interests