The other day I was driving for the shift. My partner and I responded to a high school
for the report of syncope on the football field. Practice was going on and one of the coaches
had fainted. In checking him out, my
partner found out that coach was 55 years old, had felt weak and lightheaded,
and thinks he just got overheated out in the sun. His vital signs were normal and the coach was
only complaining of residual weakness.
We put him on the monitor.
My partner hit print (he apparently hadn’t read the Quick Six rant from
a few weeks ago):
Dude was very worried about the inferior ST
depression. I finished putting the precordial leads onto the patient. The 12-lead that resulted, cut down for
privacy of course:
Take a closer look at the two ECGs. First, the third beat of Lead II in the print
view:
Now, look at the second beat in Lead II from the 12-lead:
Do you see the difference?
The printed view has one and a half or two millimeters of ST
depression. In the 12-lead, though, the
depression is gone. Maybe there is half
a millimeter of depression in the 12-lead, but there isn’t much. What caused the difference? (Hint: Look at the whole strips above, rather
than the individual pictures. The
difference is spelled out.)
The signals collected by an ECG monitor are very small. The heart’s electrical signal is around 1
millivolt, and many waveforms are much less than that. So the ECG signal can be easily drowned out
by other “noise.” The extraneous signal
can be from other muscular movements, nearby electrical equipment, nearby power lines, poor
electrode-skin interface, and from equipment in the monitor itself. The collected ECG signal must have the extra
noise filtered out so that a clean picture of the heart’s electrical activity
can be shown more clearly.
I need to take a minute here and explain that I am a little
embarrassed. You see, my plan for this
week’s post was to explain how ECG filtering works. So I spent all week researching the
topic. And failed. Miserably.
I don’t understand the technical aspects of how filtering works on an
ECG. For those of you who are smarter
than I am, or have a better technical background, there are several
explanations on the internet. Please
take a few minutes to go check them out.
This one, for
example. Or this
one. Here is a powerpoint
presentation in pdf form that tries to explain the topic. All they did is confuse me and make me feel dumb. So now I am sad.
Good friggin’ luck to you if you want to get into the specifics of signal filters.
If you can understand the topic (understand it, not just regurgitate
it), then you are a better person than I.
If it is something that interests you, to read about log scales, low
pass filters, Fourier Analysis, superposition, and phase distortion, well then
knock your bad self out with the links above.
Here is what the average medic needs to know about ECG
filtering: There is a difference between
the print view and the 12-lead. By the
way, the screen on the monitor shows the print view. The 12-lead is diagnostic. You can make decisions based on a
12-lead. It is the same view as on a
12-lead in the hospital. The same thing
cannot be said for the print view.
Look at the bottom left of the printed ECG and the 12-lead
above. The print view is filtering at
the 0.5-40 hertz level. That is, there
is more aggressive filtering going on to smooth the waveforms. That filtering, though, can do weird things
to the ST segments like, say, depress them by 2 millimeters.
The bottom left of the 12-lead shows the filtering is being
performed at the 0.05-150 hertz level.
This is a diagnostic view. It is
going to result in more artifact, baseline wander, and general noise. But it is also going to give you the accurate
view of the patient’s heart’s electrical activity. ST segments are accurate at this level of
filtering. In order to cut down on the
noise in the ECG, we have to be very precise with electrode application,
electrode location choice, vehicle movement, patient movement, and those kinds
of things. We have to make physical
changes to improve the quality of the ECG, rather than relying on the monitor
to filter out the noise for us.
One way or the other, make sure that you are making
decisions based on a diagnostic-level view of the patient’s ECG. It would be pretty embarrassing to STEMI
alert a patient based on falsely elevated print view ST segments.
The print view can give you false positive and false
negative findings.
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