Replacing the mercury sphygmomanometer
BMJ 2000; 320 doi: https://doi.org/10.1136/bmj.320.7238.815 (Published 25 March 2000) Cite this as: BMJ 2000;320:815
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Editor-It was with great interest that I read your editorial on
replacing the mercury sphygmomanometer with other non-invasive blood
pressure measuring devices 1. Far from agreeing with your editor on this
matter I would prefer
to have all electronic devices banned from the general surgical wards of
our hospitals.
Firstly non-invasive pressure devices, described in your article, are
designed for the management of hypertension. The emphasis is therefore on
the "mean" and diastolic blood pressures at the upper end of the spectrum
of pressure. These devices are therefore of no use in the management of
patients in whom an indication of their cardiac output or low blood
pressure is important. The relationship between blood pressure and oxygen
delivery or cardiac output is of course more or less non-existent
(r=0.2)2. At least with
a mercury sphygmomanometer the true systolic and diastolic can be
determined. The advantage of this is that the pulse pressure may then be
calculated. A wide pulse pressure indicates a good cardiac output and a
narrow pulse
pressure a poor cardiac output. (Wide pulse pressure equates to low
peripheral vascular resistance; low peripheral vascular resistance equates
to large stroke volume and cardiac output = stroke volume x heart rate.)
This is
also an argument for not recording blood pressures as a "mean". Two
patients with vastly different cardiac outputs may have the same "mean"
blood pressure.
One might argue that there are other signs of the cardiac output that one
could use to identify the clinical status of the patient other than the
pulse pressure. This is true and brings me to my next point.
The close
intimate contact required in order to successfully auscultate the
Korotkoff sounds associated with a hypovolaemic patient ensure that the
clamminess of the skin, damp sheets, tachypnoea, thready pulse, sunken
eyes, body odour and frown of the patient experiencing their impending
doom are not missed.
Secondly the very difficulty of measuring the blood pressure experienced
by the nurse or doctor is itself a clinical sign. Automated devices simply
keep mindlessly repeating the investigation until a pressure falls within
the
manufacturers algorithm.
Furthermore, ridding the wards of non-invasive
blood pressure devices ensures that the task of measuring the blood
pressure could no longer be assigned to the work experience youth or the
untrained auxiliary. With an automated device it is too often the case
that the cuff is applied, the button pressed and the staff goes off to
perform another task.
The numbers are charted and the whole thing is over with another
opportunity to do some real care of patients lost. I would also draw your
readers attention to the fact that once the non-invasive blood pressure
has fallen to a pressure of <100mm Hg the difference between the
pressure measured by
invasive devices, zeroed against the medulla and the non-invasive devices
may consistently be 20-25 mm Hg., the non-invasive device over-reading.
(This is independent of the manufactures.) If the use of such devices is
unavoidable
on the post-op wards then any systolic pressure below 90 mm Hg should read
"pressure unobtainable seek trained staff".
When the "no crisis" NHS is unable to provide any trained staff for post
operative surgical wards these issues are of paramount importance. When
one discovers your patient from the previous days operating list with a
stroke the next morning because the untrained staff did not realise that a
pressure of 70 mmHg for 8 hours is not normal for 80 year olds it is
doubly important not to put machines between staff and patient.
1. O'Brien E. Replacing the mercury sphygmomanometer. BMJ 2000;
329:815-6
2. Reinhart K. Principles and practice of SvO2 monitoring. Intensive
care world 1988; 5: 121-124.
E.G.Lawes.
Consultant Anaesthetist
Critical Care
Competing interests: No competing interests
SIR - I couldn't agree more with Robert Salaman's note on non
conventional uses of the sphygmomanometer.
They can also be invaluable for difficult AV fistula needling and venflon
insertion, when the compression provided by a simple tournique is too
crude.
No doubt such functions could be incorporated into automated devices, but
where is the will of the industry?
As to kilo Pascals for blood pressure measurement, I welcome them.
They are long overdue. we should have relegated them to the status of
firkins and furlongs long ago.
Competing interests: No competing interests
In the context of a discussion of the mercury sphygmomanometer, the
claim that "we measure what we see" is false (1). What is being measured
is the difference in height between the top of the mercury column and the
level of mercury in the reservoir, which is usually not visible. The "mm"
scale alongside the column is not in true millimetres since it allows for
the fall in reservoir level as mercury is pumped up the column. Two
hundred "mm" on this scale is roughly 180 millimetres; I say ‘’roughly’
since the exact figure depends on the diameters of the column and the
reservoir.
This subtlety is of course well known to the manufacturers!
1. O’Brien E. Replacing the mercury sphygmomanometer. BMJ
2000;320:815-816.
Competing interests: No competing interests
It's amazing the thought given to such a simple procedure as getting
blood pressure non invasively. It saddens me to think that the logistics
are too complicated for the people involved in this article. I suggest
you all stop thinking about it and get some engineers to think about it.
After all, what qualifications do most doctors have regarding the physics
and the engineering of this? Please be more professional by noting your
shortcomings and going to those who know better: bioengineers, etc. This
is baby stuff. This article is so out of touch. All you have to do is go
to a library ( a good medical library, if you can find it) and look this
up. It's been done more than you can imagine! Of course you know that what
you believe regarding the accuracy of blood pressure only has value when
there are clinical correlations between pressure readings and pathology.
^^O^^
Competing interests: No competing interests
EDITOR - Accurate automated blood pressure measuring devices may well
be the way forward for the recording of systemic blood pressure. This does
not mean however that the old "mercury sphyg" can be relegated to the
hospital museum just yet.
Measurement of ankle arterial Doppler pressures is a cheap, accurate and
non invasive method of assessing the severity of peripheral vascular
disease and had been in use for many years (1). As Eoin O'Brien says,
aneroid devices are unreliable (2) leaving no other alternative for the
vascular surgeon than a mercury column.
Our vascular clinics retain these archaic devices specifically for this
purpose. The wards however long ago replaced these old friends with
electronic automated devices. The most time consuming part of a ward
consultation is no longer spent with the patient but in the often
fruitless search for a manual sphygmomanometer.
By all means develop better and safer blood pressure measuring devices but
please do not chuck out your sphygs just yet. They have more uses than the
one for which they were designed.
Robert A. Salaman
SpR in Surgery
Dept of Vascular Surgery,
Selly Oak Hospital,
Birmingham
1. Yao ST, Hobbs J, Irvine NT. Pulse examination by an
ultrasonographic method. BR. Med J. 1968; 4: 555-557
2. O'Brien E. Replacing the mercury sphygmomanometer. Br.Med. J.2000;
320: 815-6
Competing interests: No competing interests
Eoin O'Brien points out some of the issues of accuaracy and
validation surrounding the tranisition to electronic sphygmomanometers.
There are two further issues which should concern physicians.
It is clear that strong marketing of these new devices is
accelerating the change and local guidelines groups are being bombarded
with requests for information.There is a huge issue of toxicity from
obsolete mercury machines, each one containing a substantial amount of
this very toxic substance. Urgent measures are required to ensure
collection and safe disposal of these machines in the community and no
such measures appear to be organised at present.
Secondly, the machines are sold with a standard cuff size which only
just fits a slim female arm. Surely the lessons of the previous generation
of machines should have taught us that these cuffs are too small for the
majority of arms. Perhaps the 'kite-marking'of British Hypertension
Society valdity could ensure either that that both cuffs are supplied or
that the larger cuff is supplied. The Medical Devices Agency is due to
bring out recommendations and perhaps both issues could be addressed.
Competing interests: No competing interests
I was interested to read Eoin O'Brien's editorial advocating the replacement of the mercury sphygmomanometer, and with it, the introduction of the kilopascal as the standard unit of pressure in medicine.
I am sure that each generation of medical students, like Pavlov's ducklings, become fixated on 120/80mmHg as the first and only "normal" blood pressure. One day they will be taught that the normal adult blood pressure is approximately 16/11kPa.
The multitude of disparate units of pressure in medicine is a real impediment to the proper understanding of the relative pressures within the human body. In my own field of anaesthesia, it is traditional to measure gas cylinder pressures in pounds per square inch, gas pipeline pressures in bars, breathing circuit pressures in centimetres of water, blood pressures in mmHg, CSF pressures in millimetres of CSF, vacuum pipeline pressures in inches of mercury and decompression chamber pressures in atmospheres, or feet (or metres) of water! What is the normal blood pressure in pounds per square inch?
In the meantime, here is a poem which appeared in Verse and Worse, published by Faber many years ago and long since out of print:
Willie, from his bedroom mirror
Licked the mercury right off,
For he thought in childish error
It would cure the whooping cough.
At the funeral, his mother
Brightly said to Mrs Brown,
"It was a chilly day for Willie
When the mercury went down."
Competing interests: No competing interests
I applaud any attempt to reduce the use of chemicals in medicine
which are toxic to ourselves or our environment. However, I don't agree
with O'Brien's assertion that the use of the millimetre of mercury as the
unit of measurement for blood pressure is an irksome anomaly.
The kilopascal is O'Brien's preferred choice, so I considered the
benchmark value for blood pressure so favoured by O'Brien and colleagues
in the British Hypertension Society, namely 160/90 millimetres of mercury.
Its equivalent in kilopascals is 21.28/11.97. Now that is what I call
irksome.
Competing interests: No competing interests
We have been using a number of automated devices for a few years and
have experienced some difficuties with them, however they also eliminate
some problems associated with using Mercury sphygmomanometers. Manually
you need to get the cuff around the biceps to listen to the brachial
artery. Automated machines can have the cuff placed lower, directly over
the brachial artery and this seems to give consistent readings to the cuff
placed over the biceps. The Omron 705CP does give Error readings, and
these are more common in people with obese arms. If an Error reading
occurrs, try lowering the cuff and repeat.
If I am concerned about a reading, it is also possible to do a manual
reading using the displayed pressure reading from the Automated machine.
This can be compared to the Automated reading. There may be a benefit from
seeking the manufactures to produce a pressure gauge which is electronic,
to replace the anaeroid and mercury sypgmomanometers which are inaccurate
and a health risk, but still permit us to use the traditional method of
listening for the Korotkoff sounds.
Competing interests: No competing interests
Re: Automated Devices are Inaccurate in Hospitalised Elderly with Pre Existing Disability
A lively debate followed the publication of O'Brien's editorial last
year voicing the advantages of automated blood pressure
measurement devices. I would also be concerned about 'putting machines
between staff and patients' in the population that we serve which are the
acutely ill elderly with preexisting disability.
I looked at 40 consecutive inpatient referrals for consultation to
the Department of Geriatric Medicine. These patients had a mean age of 82
years and a median MMSE of 20. Referrals were from other medical
specialities and orthopaedics. Manual measurement of blood pressure
revealed half of this group to have significant postural hypotension as
defined by at least a greater than 20mmHg drop in systolic blood pressure
on standing. Automated measurement only recognised a third of those with
postural hypotension. In nearly two thirds of those with postural
hypotension on manual measurement the standing blood pressure was less
than 100mmHg.
Automated devices are not suitable for the hospitalised elderly with
acute illness with preexisting disability. These patients often have
multifactorial gait and balance disorders and find it difficult to stand
independantly with their arm straight and still. The movement artifact
renders the automated device unable to analyse accurately. These machines
underestimate the prevalence of this important
clinical sign in these patients. These patients should all have manual
measurement of their standing blood pressure throughout their hospital
admission.
Competing interests: No competing interests