In a previous post we discussed the problem of ST segment elevation.
Because acute myocardial infarction (STEMI) is not the most common cause of ST-segment elevation amongst chest pain patients, we need to consider other factors like reciprocal changes to shore up the diagnosis.
Itâ€™s also a good idea to be well versed in the typical appearance of the STEMI mimics (e.g., left ventricular hypertrophy, early repolarization, left bundle branch block, paced rhythm, pericarditis, ventricular aneurysm, 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 â€œtake-offâ€.Â When an ST segment loses its upward concavity and becomes straight or upwardly convex, it’s suggestive of acute myocardial infarction.
Consider this image from Brady W, Syverud S, Beagle C et al. Electrocardiographic ST-segment Elevation The Diagnosis of Acute Myocardial Infarction by Morphologic Analysis of the ST Segment. Acad Emergency 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 itâ€™s upwardly concave. If itâ€™s even with or above that line, then itâ€™s â€œnon-concaveâ€ (straight or upwardly convex) which is suspicious for acute myocardial infarction.
This phenomenon was described earlier by Pardee in An Electrocardiographic Sign of Coronary Artery Obstruction. Arch Intern Med. 1920;26:244-257 where upward convexity was referred to as â€œcovingâ€ of the ST-segment, and you still hear the ST-segment elevation of Brugada syndrome referred to in this way.
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. For example, the ST-segments retain their upward concavity 40% of the time with LAD occlusion. It is, however, fairly specific.
The STEMI mimics are more likely to presentÂ with upwardly concave ST-segments and an absence of reciprocal changes (although left ventricular hypertrophy, left bundle branch block, and paced rhythm can show pseudo-reciprocal changes due toÂ a pattern of T-wave discordance).
StephenÂ Smith, M.D. from Dr. Smithâ€™s ECG BlogÂ has shown me a couple of cases of left ventricular hypertrophy with upwardly convex ST-segments in the right precordial leads that were not experiencing STEMI so with a strain pattern it’s more important to consider the depth of the S-wave.
You might have noticed that I used the phrase â€œupwardly concaveâ€ as opposed to simply â€œconcaveâ€.Â Thatâ€™s because â€œconcaveâ€ is â€œconvexâ€ depending on your perspective. Thatâ€™s why I always mention the direction of the concavity or convexity.
This can get confusing! Consider this image from the AHAâ€™s STEMI Provider Manual.
The caption says â€œconcave downâ€ even though itâ€™s referring to the first window showingÂ upwardly concave ST-segments. ItÂ also indicatesÂ that the ST-segment elevation is “possibly due to early repolarization” but that is unlikely because the R-waves are not well developed and there is a tiny Q-wave in lead V3.
The second window shows the same patient 10 minutes later with non-concave (straight) ST-segments andÂ hyperacute T waves consistent with LAD occlusion.
The third window shows the same patient 1 hour later following PCI. Note the Q-waves and terminal T-wave inversion. If you didn’t know the history this would be a difficult ECG. It can be hard to distinguish between late presents and the so-called left ventricular aneurysm pattern (persistent ST-segment elevation from previous myocardial infarction).
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