Overview
This lecture explains how myocardial infarctions (MIs) are classified based on EKG findings and affected heart wall layers, and reviews key EKG segment changes to recognize MI severity.
MI Classification by Heart Wall Involvement
- Subendocardial infarction affects the inner layer of the heart wall and is classified as a non-STEMI.
- Intramural infarction occurs in the middle of the heart wall and is also a non-STEMI.
- Transmural infarction spans the full thickness of the heart wall and is called a STEMI, with the highest risk of complications.
- Subepicardial infarction involves the outer layer and is another type of non-STEMI.
EKG Patterns in MI
- Non-STEMI (NSTEMI) shows ST segment depression or no ST elevation on EKG.
- STEMI (ST segment elevation MI) presents with ST segment elevation, indicating more severe heart damage.
- ST segment depression can also result from reversible myocardial ischemia, not just MI.
Recognizing EKG Changes
- The ST segment on an EKG should be flat and at baseline in a healthy heart.
- Minimal ST segment depression indicates ischemia, while more pronounced depression suggests non-STEMI.
- During an acute MI, hyperacute (tall) T waves occur first, followed by ST segment elevation.
- T wave inversion and Q waves may remain on EKG for years, signaling a past MI.
Key Terms & Definitions
- MI (Myocardial Infarction) — Heart muscle injury due to blocked blood supply.
- Non-STEMI (NSTEMI) — MI without ST segment elevation, typically with ST depression.
- STEMI — MI showing ST segment elevation, indicating severe full-thickness heart injury.
- ST segment — EKG portion between QRS complex and T wave, normally flat.
- Ischemia — Reduced blood flow causing tissue damage, often seen as ST depression.
- Hyperacute T wave — Early EKG sign of MI with unusually tall T wave.
- T wave inversion — EKG change after MI, may indicate old heart damage.
Action Items / Next Steps
- Review and memorize the location and EKG features of each MI type.
- Practice identifying normal and abnormal ST segments on sample EKGs.
- Revisit the heart wall diagram and EKG tracings for understanding.