Monday, March 4, 2019

How accurate are the Monitor leads for ST Elevation?

This was a healthy young man who did not have a cardiac issue, but was on the monitor:

Lead MCL1, which is mid-precordial, is the bottom tracing.
There is a huge amount of ST Elevation.
What does it mean?


It does not mean much.

Because of different electronic processing (which I do not well understand), the ST segments on the monitor leads are not reliable.

We recorded a simultaneous 12-lead.  Here it is:
As you can see, there is very little ST elevation in the mid-precordial leads.



Learning Point:

Do not be alarmed by ST elevation on the monitor leads.  It is probably an artifact of different electronic processing.

On the other hand, it is prudent to obtain a 12-lead to assess the validity of ST Elevation seen on the monitor.


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Comment by KEN GRAUER, MD (3/4/2019):
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The important point highlighted in this post is that ST-T wave changes seen on a monitor may have little to do with the actual appearance of ST-T wave changes on a standard 12-lead ECG.
  • As per Dr. Smith — When something of potential concern is seen on a monitor (ie, significant ST elevation or depression) — then immediately obtain a complete 12-lead ECG to determine if what you see is (or is not) a “real” effect.
  • The chance that an abnormal appearance of the ST-T wave on monitoring reflects acute ischemia or infarction is greater IF — you have been monitoring a patient over time, and without any change in lead placement, you all-of-a-sudden note a difference in ST-T wave appearance. But regardless of whether there has been ongoing monitoring, or you just this moment began telemetry monitoring — even marked ST segment depression or elevation may not be “real”.
  • BOTTOM LINE: The only way to determine if ST elevation or depression on a monitoring lead is real — is by immediately obtaining a 12-lead ECG.
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COMMENT: While far from expert in the area Filter Settings — the basic concepts that I keep in mind are the following:
  • Different settings are typically used for ECG recording depending on whether emphasis is placed on rhythm determination vs diagnostics (with focus for diagnostics being placed on interpreting 12-lead waveforms).
  • Greater filtering is generally used in monitor mode, with a common frequency setting being between 0.5 Hz and 40 Hz (Hz = Hertz). Doing so has the advantage of minimizing artifact and baseline wander that may affect rhythm interpretation.
  • In contrast — a broader passband (typically from 0.05 Hz to 150 Hz) is recommended for diagnostic mode, for which emphasis is on optimally accurate ST segment analysis.
For example — imagine you are looking at a 12-lead ECG that indicates, “40 Hz” as the frequency setting given at the bottom of the tracing. This 40 Hz value represents the high-pass filter setting — without mention of a low-pass filter limit. If the low-pass filter setting is typical for monitoring (ie, 0.5 instead of 0.05 Hz) — this 0.5-to-40 Hz filter setting might contribute to ST-T wave distortion and/or false appearance of ST segment elevation, when in fact there is none. Other alterations in the fine points of Q wave, QRS complex and ST-T waveform appearance may also be introduced by use of this overly narrow filter range.
  • The generally recommended frequency setting for 12-Lead ECG Diagnostic Mode ECG interpretation in adults is 0.05-to-150 Hz. An even higher upper frequency limit may be used in infants or young children. If muscle artifact is severe, one may need to compromise with a narrower range setting of 0.05-to-100 Hz (or 0.05-to-40 Hz) — but the recommended range 0.05-to-150 Hz should be tried first. KEY Point: If compromise in filter settings is needed, then at least be aware that there may be less than optimal fidelity for ST segment analysis. This explains why monitoring lead rhythm strips should not be used for ST segment analysis. One may be alerted to possible/probable ST segment elevation or depression based on changes seen on a rhythm strip — but when clinically important, this must always be verified on a 12-lead tracing.
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For comparison — I have put the 2-lead monitoring strip from this patient together with the simultaneously-obtained 12-lead ECG (Figure-1). Unfortunately, monitoring specifics (ie, filter frequency setting — paper speed — voltage calibration) were cut off from the 12-lead tracing.
  • For illustrative purposes — I have taken the liberty of adding in the optimal frequency filter range for a 12-lead ECG (ie, 0.05-to-150 Hz— standard paper speed in the U.S. (ie, 25 mm/sec— and standard voltage calibration (ie, 10 mm/mV).
Figure-1: What the monitor shows (TOP) — and what a simultaneously-obtained 12-lead ECG on this patient show (See text).
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SUGGESTION: If you previously have not been aware of filter settings — Began to look for them! Usually they will be seen under the ECG (similar to the way they appear in Figure-1). Confession: I fully acknowledge that I was many years into my career as an “ECG enthusiast” until I finally began to pay attention to these settings. This is important — because use of a different filter setting can be the reason why ST segment deviation (elevation or depression) may give false impression of a difference in ST elevation or depression changes between serial ECGs done on the same patient!
  • The beauty of the 2 tracings in this case (Figure-1) — is that the monitoring strip suggests marked J-point ST elevation, when in fact there is no more than minimal ST elevation in the simultaneously-obtained 12-lead ECG.
  • BOTTOM LINE: The only way to determine if ST elevation or depression on a monitoring lead is real — is by immediately obtaining a 12-lead ECG.

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David Richley, who is truly expert in this area (former Cardiac Physiologist at NHS — UK) — wrote the following in the EKG Club (3/5/2019). I have copied his comment here, and added Figure-2, which Dave provided.
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What causes artefactual ST elevation is the use of a digital high-pass filter in a real time ECG display, as when recording a manual rhythm strip or when monitoring. In the old analogue days, to preserve the lower frequency components of the ECG waveform (eg, ST segment) — the recommended lower frequency response of ECG machines was 0.05 Hz. Nowadays, in a digital machine, the recommended lower limit is 0.67 Hz — because this will help to suppress the low frequency artefact of baseline wander (such as that caused by perspiration or deep breathing) whilst not distorting the ST segment. This is because the machine applies a high-pass filter when it acquires the signal in order to reduce the low frequency artefact — but because this distorts the ST segments, it then re-applies the filter in the opposite direction to reverse the distortion. However, it can only do this when a fully automatic ECG is recorded in its memory. If the ECG is recorded in real-time — the filter can only be applied unidirectionally, and the ST distortion cannot be corrected. For manual rhythm strips or ECG monitoring, the lower frequency response should therefore be 0.05 Hz if artefactual ST elevation is to be avoided.


Figure-2: Illustrative figure provided by David Richley.

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4 comments:

  1. absolutely. gotten burned by this a lot.

    ReplyDelete
  2. All congratulations to professors on a wonderful explanation. I have a question for you, refer to at 24h holter monitoring. Often, during the reading of Holter Ecg, ST segment disorders are seen, is this explanation upside also applies to 24h holter monitoring and can be adjusted so that the frequency is 0.05-150Hz during the recording of Holter Ecg.
    Once again, thanks to the exhaustive explanation, it means a lot to us which want to understand ecg as much as possible.

    ReplyDelete
  3. jeez,
    that was super helpful... how often has the question come up (that's rhetorical) when the nurse comes over quite worried about the monitor strip.... this sheds light. lots of light. thanks guys
    tom

    ReplyDelete

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