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    Subtle Anterior STEMI Calculator

    Differentiates early repolarization from anterior STEMI.
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    INSTRUCTIONS

    NOTE: Cannot be used with LVH or bundle branch block. Not to be used for obvious anterior STEMI.
    Pearls/Pitfalls

    This cannot be used in patients with LVH or bundle-branch block and should not be used in obvious anterior STEMI. It also may not be applicable in patients with other signs of anterior STEMI, like inferior/anterior ST depression, convexity, terminal QRS distortion, Q waves.

    mm
    msec
    mm
    About the Creator
    Dr. Stephen Smith
    Content Contributors
    • Stephen Smith, MD

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    Critical Actions

    subtle anterior stemi
    • ST elevation at 60 ms after the J-point in lead V3, relative to the PR segment
      • Find the J-point (lower blue arrow)
      • Locate the time point of that J-point (left-hand green arrow)
      • Go 60 ms to the right (right hand green arrow)
      • Go up to the tracing (upper blue arrow)
      • Draw a horizontal line at this point (upper green line)
      • Draw a horizontal line at the level of the PR interval (lower green line)
      • Measure the voltage between these two lines (in this case, 3.5 mm or 0.35 mV)
    • R-wave amplitude in lead V4
      • Mark the level of the PR interval (lower green line)
      • Mark the top of the R-wave (upper green line)
      • Measure the distance between the two: (17 mm of grid lines plus + 7.5 mm off the grid, Total = 24.5 mm

    Formula

    Subtle Anterior STEMI Calculation = (1.196 × [ST-segment elevation 60 ms after the J point in lead V3, in mm]) + (0.059 × [QTc in ms]) – (0.326 × [R-wave amplitude in lead V4 in mm])

    Facts & Figures

    • This study was undertaken to help differentiate two very different entities that are commonly encountered in the emergency department: STEMI vs "benign" early repolarization, particularly in the anterior/precordial leads.
    • This study excluded anyone with bundle branch block or obvious anterior STEMI, and cannot be used in patients with LVH. It also may not be applicable in patients with other signs of anterior STEMI, like inferior/anterior ST depression, convexity, terminal QRS distortion, Q waves.
    • Note: The QTc in this equation is the one calculated by the EKG machine, not manually calculated by the Bazzett QTc correction formula.
    • The sensitivity and specificity of this equation are ~90%, so always consider false negatives and false positives.
    Dr. Stephen Smith

    From the Creator

    Why did you develop the Subtle Anterior STEMI Calculator? Was there a clinical experience that inspired you to create this tool for clinicians?
    I found that I could always diagnose subtle anterior STEMI with better sensitivity and specificity than anyone else. To me, it was like looking at a face. So I had to figure out what it was I was seeing that others were not, and I easily identified that feature as the ratio of the T-wave amplitude to the R-wave amplitude. It so happens that anterior MI diminishes R-wave amplitude and early depolarization has very well-formed R-waves. I also knew that early depolarization depolarizes EARLY, and so would have a shorter QT. And I figured that ST elevation at 60 ms after the J-point would be higher because the ascending part of the T-wave would rise more rapidly in MI.
    There were several other features that I thought would differentiate them, such as degree of upward concavity, upright T-wave in lead V1, and T-wave amplitude in V1 > V6.
    So, we just needed a study group of subtle anterior MI (so exclude those which are obvious, such as those with massive ST elevation, T-wave inversion, inferior “reciprocal” ST depression, precordial ST depression, or something called terminal QRS distortion, which is the obliteration of the S-wave in V2 or V3, without the presence of a J-wave), and we needed a group of ED patients with early repolarization.
    Then we tested the hypotheses and found they were right, and the best differentiator was R-wave amplitude, but that each had some value on its own. Then we sought to create the best rule using logistic regression and found that the only variables that were significantly and independently predictive were R-wave, QTc, and STE at 60 ms after the J-point in lead V3. We did find that T- to R-wave ratio was a good predictor, but it was only due to R-wave amplitude; the T-wave amplitude in the two groups was NOT significantly different.
    Then we had to validate the rule, and did so.
    What pearls, pitfalls and/or tips do you have for users of the Subtle Anterior STEMI Calculator? Are there cases in which it has been applied, interpreted, or used inappropriately?
    It is used inappropriately frequently!
    1. Mostly on those who already met exclusions because they had obvious MI anyway, and especially in patients who already have inferior or anterior ST depression, which cannot be present in normal variant precordial ST elevation (early repolarization). One must pay attention to the exclusions.
    2. Also on patients who have ST elevation due to LVH or LV aneurysm. These ECGs do not look like subtle STEMI or like early repolarization, so they should not be included. ST elevation due to LVH especially may result in false positives.
    3. On patients with no ST elevation in any lead.
    Finally, the rule is not meant to be a firm diagnosis. It is meant to initiate intensive evaluation: frequent serial ECGs, emergent formal echo to look for wall motion abnormality.
    A patient with possible ischemic symptoms and a positive formula should not be just admitted for serial troponin to rule out MI. They should be thought of as “MI needing immediate treatment unless proven otherwise.”
    What recommendations do you have for health care providers once they have applied the Subtle Anterior STEMI Calculator? Are there any adjustments or updates you would make to the score given recent changes in medicine?
    There are some false negatives - most have a value between 22.0 and 23.4. At the 23.4 cutoff, sensitivity and specificity are about 90%. It is important to remember that this is sensitivity for SUBTLE MI. In the study, the obvious ones were already excluded. So the true sensitivity is about 96%. By changing the cutoff to 22.0, the sensitivity is improved to 96% for subtle MI, but 98.5% for all LAD occlusion.
    Any other comments? Any new research or papers on this topic in the pipeline?
    None. I would love to see an external validation. Or a study in which students, medics, residents, faculty, cardiology fellows, cardiologists, and interventionalists are given 100 ECGs, some subtle LAD occlusion, some early repol. They give their diagnosis. Then they are given another chance with the rule in hand. See how much they improve (if at all), stratified by experience. We know that interventionists are the best, but only have about 70% accuracy!

    About the Creator

    Stephen W. Smith, MD, is a physician in the Emergency Medicine Residency at Hennepin County Medical Center (HCMC). He is also a professor of emergency medicine at the University of Minnesota School of Medicine. Dr. Smith research focuses include the ECG and troponin in acute myocardial infarction and health issues related to gamma hydroxybutyrate (GHB), and he runs the Dr Smith's ECG Blog.

    To view Dr. Stephen Smith's publications, visit PubMed

    Content Contributors
    • Stephen Smith, MD