In message <[log in to unmask]>, Darren Walter
<[log in to unmask]> writes
>Question to all.
>
>There was some discussion a few months ago about "false" ST elevation on a
>three lead monitor that does not show on a twelve lead and why this
>occurred. I'm afraid I read it and deleted and now need the info. Typical!
>
>Did anybody archive it? Grateful for any assistance.

I put up the original question a few months back and got some very interesting
replies, not least from Physio Control who were very helpful. I'm sure Sharon
O'Grady (from Physio) won't mind me publishing her explanation here, as this is
clearly a potential area of confusion. With the push towards more speedy onset-
to-needle targets, pre-hospital diagnosis will become more important, and so
will pre-hospital diagnostic equipment.

If anyone wants to view the other contribuations, they should be available on
the mailing list web site archive via:

http://www.mailbase.ac.uk/

Here's Physio Control's explanation (see their web site for more info: address
included below). Apologies to anyone who's read it before.
___________________________________________________

Dear Dr. G. Ray:

Your query Subject: Spurious ST elevation on 12-leads was picked up on a
bulletin board you posted to and I would be happy to help you with your
question.  As you identified in your posting, Frequency Response is a very
important element in the detection of ST elevation.  ECG Rhythm strips are
generally printed in a 1-30 Hz or .5-40 Hz frequency response since the
purpose of rhythm strips is, in general, to determine rate and rhythm.  When
you are looking for more diagnostic information that requires evaluation of
the waveform shape, it is important to use a diagnostic frequency response
which is .05 to 150Hz (note the decimal point difference in the low end .5
(monitor) vs. .05 (diagnostic)).  The electrical signal we know as "ST
segment" is generated by the heart's conduction system in the frequency
range between .05 and .5 Hz so the ECG you see on your printed rhythm strip
is not a true representation of PQRST morphology unless your device is set
to print in the diagnostic bandwidth.  ECG monitor screens are never
diagnostic quality, regardless of the setting on the printer.

The reason diagnostic frequency response (DIAG) is not widely used for all
ECGs is that the relative lack of filtering in DIAG creates a trace that is
often full of artifact, particularly in an EMS environment where there is a
lot of patient movement.  So the clinically useful information as I see it
is
*       use the routine monitoring frequency response when rate and rhythm
are the primary interest
*       use DIAG when, in addition to rate and rhythm, you wish to evaluate
PQRST morphology and, in particular, ST segment elevation

The American Heart Association ACLS guidelines for patients suspected of
acute myocardial infarction is to perform a 12-lead ECG.  Physio devices
that have the 12-lead feature always print the 12-lead with the .05
frequency.  In devices that do not have the 12-lead feature, the printer may
be able to switch from Monitor frequency to DIAG, depending on which Physio
device you are using.  Consult your operating manual or your local
representative for further information on this.

Of course, even when you use DIAG for all your ST segment evaluations, you
may see elevation at one time then a resolution of ST segment a few moments
later.  This goes hand in hand with the stuttering nature of coronary artery
occlusion/spasm.  It is a interesting (and frightening) to watch this
physiological phenomenon--reassuring to watch the ST segments come back to
normal during reperfusion therapy.

Some additional resources you may find helpful are:

The Physio booklet "Prehospital 12-lead ECG--What you should know" which is
published on the Physio website (link attached).  Follow the path through
[tab at top] Products&Services/Clinical Information/Clinical Booklets.

The Physio INSYNC article on Frequency response published in the Spring 1996
edition (only the current edition is available online but I can fax to you)

An article published in JEMS, Sept 1997 "When the Monitor and Tracing
Disagree".

Please spread the word on the importance of this issue.  I've personally
witnessed 12-lead ECGs performed in hospitals using the monitor bandwidth
rather than 
DIAG.  Clinical providers are not engineers (by and large) so this issue too
often gets overlooked.  

_____________________________________________________________

Physio's Web site:
http://www.physio-control.com/home.html

I've witheld Sharon O'Grady's Email address, but can tray and contact her if
there are further questions.


Dr G Ray
Staff Grade
A&E
Sussex
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