Because acute myocardial infarction (STEMI) is not the most common cause of ST segment elevation in chest pain patients, we need to consider other factors like reciprocal changes to shore up the diagnosis.
It's also a good idea to be well versed in the typical appearance of the STE-mimics (paced rhythms, left ventricular hypertrophy, benign early depolarization, pericarditis, left ventricular hypertrophy, hyperkalemia, and so on).
Another factor that can assist you is an analysis of the morphology of the ST segment.
The normal ST segment should not be flat. It should have an upward concavity sometimes referred to as a "take-off".
When an ST segment loses its concavity and becomes straight or upwardly convex, it can indicate acute myocardial infarction.
Consider this image from WJ Brady, SA Syverud, C Beagle et al. Electrocardiographic ST-segment Elevation: The Diagnosis of Acute Myocardial Infarction by Morphologic Analysis of the ST Segment Acad Emerg Med 2001; 8(10):961-967
You can draw an imaginary line between the J point and the apex of the T wave. If the ST segment is below that line, then it's upwardly concave. If it's even with or above that line, then it's "non-concave" (straight or upwardly convex) which is suspicious for acute myocardial infarction.
If it helps you to remember, an upwardly concave ST segment makes a "smiley face" (good) and an upwardly convex ST segment makes a "frowny face" (bad).
Does that mean that acute myocardial infarction always presents with non-concave ST segments when ST segment elevation is present?
Not at all! This finding is not particularly sensitive. It is, however, fairly specific. When non-concave ST segments are present, it's another piece of the puzzle.
The STE-mimics almost always present with upwardly concave ST segments and an absence of reciprocal changes.
You might have noticed that I used the phrase "upwardly concave" as opposed to simply "concave".
That's because "concave" is "convex" depending on your perspective. That's why I always mention the direction of the concavity or convexity.
Sometimes this can get confusing! Consider this image from the AHA's new STEMI book.
The caption says "concave down" even though it's referring to an ST segment that is upwardly concave. This may have been a typo, but I think it's always helpful to use standardized definitions/language when it comes to medicine (or incident command)!
Regardless, if you look at this image from the STEMI book, the second window shows ST segments with a loss of upward concavity (ST segment straightening) and hyperacute T waves.
After PCI, you can see the development of Q waves and terminal T wave inversion (which usually indicates a STEMI that's been around for a while).
It's tough when a chest pain patient presents with an ECG with ST segments like we see in the third window. It's often difficult to determine the age of an ECG abnormality like that.
See also:
ECG mimics of acute STEMI
14 comments:
Thanks for presenting the concave vs. non-concave in a format easier to remember. I admit I always had difficulty in this when discussing pericarditis.
You're welcome, Shaggy!
One of the most important clues for pericarditis is ST elevation in leads I and II (two leads that are normally reciprocal).
Add in PR segment depression and an absence of reciprocal changes (with the exception of lead aVR) and the history, and the correct dx emerges!
At a recent AHA ML meeting, a paramedic said that his medical director didn't expect paramedics to be able to determine whether or not ST elevation was due to acute STEMI or pericaditis (for example).
I thought this was unfortunate! Paramedics can identify the mimics with a high degree of accuracy with the right education!
We should help spare our patients from unnecessary and potentially risky procedures.
Tom
"Paramedics can identify the mimics with a high degree of accuracy with the right education!"
The key is, do medics currently have the "right education". If not, that MD is not incorrect in his opinion. I am not sure if all medic programs are currently on the same page when it comes to 12 lead interpretation.
Besides, have you ever considered doing more advanced 12 lead programs for pre-conference workshops on the continent? Something to think about. If Mike Smith, and other big names can be in hot demand just to make people laugh or feel good about themselves, I am sure there would be a demand for you to actually teach people critical knowledge that may be lacking in some regions.
Tom,
I think it would be helpful to use a systematic approach to rule out mimics. Just as we look at the 12-lead to interpret the rhythm and axis every time, we should have a system to rule out early repol, hyperkalemia, pericarditis, hypertrophy, brugada etc...
How would you go about this? From most common to least common[mimics], or do you just know right away?
to clarify, I mean a step by step approach, using the rules we know/you've taught us.
That is a good idea, but I guess with time, you should be able to look at it like you look at a 3 lead. That is of course if you are very proficient like Tom. People like me could use any additional help we can.
Shaggy -
What I thought was disappointing was the fact that it was the paramedic's Medical Control Physician who didn't expect paramedics to be able to tell the mimics apart.
I could sympathize with an ED physician who was only recognizing that paramedics could not routinely differentiate pericarditis from STEMI.
If you're building a STEMI system, and you're a Medical Control Physician, then I think you have a responsibility to make sure that your paramedics can identify the most common STE-mimics.
You're certainly correct that paramedics are not being adequately taught in school. I'm not entirely satisfied with the aftermarket education, either.
Tom
Shaggy -
I agree with you that a pre-conference workshop would be a good place for some advanced 12 lead ECG interpretation.
Tom
Adam -
You're correct in that screening for STE-mimics should be a systematic part of every 12 lead ECG interpretation.
I can spot them fairly easily now, but in the beginning I would follow a 6-step method.
1.) Rate and rhythm
2.) Axis
3.) QRS duration (intervals)
4.) Morphology
5.) STE-mimics
6.) Ischemia, Injury, Infarct
I do look for the most common STE-mimics first, and I combine some of them (LBBB/paced, BER/pericarditis).
It's also a good idea to familiarize yourself with the so-called "strain patterns" of LVH.
Left ventricular aneurysm is a tough one!
I'll definitely be revisiting the topic of STE-mimics in the months ahead!
Tom
"I'll definitely be revisiting the topic of STE-mimics in the months ahead! "
Can't Wait! Can we hold you to that? :)
You sure can! That's the whole point of the PH12ECG blog.
Tom
"After PCI, you can see the development of Q waves and terminal T wave inversion (which usually indicates a STEMI that's been around for a while)".
It's worth pointing out that these changes only occur after PCI if the patient develops a large infarct (for example, if there is a delay from inital symptoms to the procedure, or if the procedure is not successful).
If the patient is rapidly revascularized, the resulting infarct is smaller and these changes are not typically seen.
MRI studies have shown that the presence of Q-waves is associated with larger infarcts, not with a full-thickness infarct (which is what's commonly taught).
Excellent point. Thanks for the comment!
Tom
Post a Comment