You've seen it countless times: a patient walks into the ED with chest pain, and the EKG isn't screaming STEMI. But in follow-up, they end up having a massive myocardial infarction. What is the hidden culprit lurking behind those non-alarming EKGs? Welcome to the Occlusive Myocardial Infarction (OMI) world, where traditional STEMI criteria fall short, and a new paradigm shifts our approach to acute cardiac ischemia.
All STEMIs are OMIs
Acute cardiac ischemia has long been divided into ST Elevation MI (STEMI) and Non-ST Elevation MI (NSTEMI), with many quality control metrics developed around this concept. A better way to conceptualize cardiac ischemia may be to use an Occlusive Myocardial Infarction (OMI) vs Non-Occlusive Coronary Occlusion (NOMI) paradigm.
- STEMI: Characterized by ST-segment elevation (meeting specific ECG criteria), indicating a complete coronary artery blockage.
- OMI: Focuses on the underlying pathology of coronary occlusion rather than ECG changes. An OMI involves significant occlusion of a coronary artery that might not always present with ST-segment elevation.
All STEMIs fall under the category of OMIs. However, not all OMIs present as STEMIs on an ECG. Some OMIs may present with non-ST-segment elevation myocardial infarction (NSTEMI) or with more subtle ECG changes that do not meet the strict criteria for STEMI yet still involve significant coronary artery occlusion.
Why does finding an OMI matter?
Patients with OMI have similar infarct size and mortality to patients with STEMI. When OMIs go unrecognized due to not meeting STEMI criteria, there is a risk of significant delays to cardiac catheterization.
The size of the QRS helps find an OMI due to LAD lesion, the dreaded widowmaker.
Traditional STEMI criteria focus on how many millimeters of ST elevation are present on the EKG. However, how the ST elevation or T wave size compares to the preceding QRS complex size is more important than strict millimeter criteria.
If the QRS complex is small, more subtle changes like submillimeter ST elevation or T wave inversions may be more significant than when the QRS complex is large. As the myocardium dies, the QRS complex becomes smaller, the R waves shrink, and the T wave gets bigger in comparison. The ST segment may begin to elevate, giving us another clue.
Who doesn’t love a calculation tool?
A four-variable calculation tool can determine which subtle changes represent OMI, differentiating the findings from benign early repolarization (sensitivity 88%, specificity 94%.) The calculator will tell you either “likely anterior STEMI” or “likely benign early repolarization.” The exact number calculated isn’t important.
1) Measure the amplitude of the QRS in V2 (smaller QRS → OMI)
2) Measure the ST segment elevation in lead V3 (higher ST elevation → OMI)
3) Measure the R wave amplitude in V4 (smaller amplitude → OMI)
4) Measure the QTc (longer QTc → OMI)
Additionally, get a repeat ECG. Dynamic changes are concerning for OMI, not benign early repolarization. Point of care ultrasound (POCUS) showing anterior wall motion abnormalities indicates an OMI.
Who does not qualify for the use of this tool?
This calculator cannot be used when there is:
- Anything that distorts the QRS complex (e.g., LVH, left bundle branch blocks)
- T wave inversions in any of V2-V6
- Convex or coved morphology in any of V2-V6
- Pathologic Q waves in V2-V4
- Anterior or inferior ST segment depression
- Terminal QRS distortion - loss of S wave and J wave (see example below)
How do you take this with you to your next shift?
- Size matters: The relative size of the QRS, T wave, and ST elevation is important. Absolute measurements are less meaningful, especially when teasing out subtle OMI.
- Context matters: ST elevation in the 20-year-old patient is more likely to be early repolarization, and the calculator can help you feel more comfortable with that. ST elevation in the 50-year-old smoker is more likely to represent ischemia, and this calculator can help confirm your suspicions.
- But remember: No calculator should drastically alter your workup or override your clinical gestalt. If the history, physical exam, and ECG are concerning for ischemia, but the calculator doesn’t agree, proceed with troponins and the usual chest pain workup!
Learn more by listening to the "The Hidden Widowmaker" segment on ERcast.
