The signs of myocardial infarction (MI), or heart attack, on an electrocardiogram (ECG) can vary depending on the timing and severity of the event. Here's a breakdown of the typical ECG changes seen in MI:
Early Signs
- Hyperacute T Waves: One of the earliest signs, often seen within minutes to hours of the onset of symptoms. These are characterized by tall, broad, and symmetrical T waves, indicating early ischemia.
Evolving MI (Over Hours to Days)
- ST-Segment Elevation: This is a hallmark sign of acute ST-segment elevation myocardial infarction (STEMI). The ST segment, which represents the period between ventricular depolarization and repolarization, becomes elevated above the baseline. The elevation needs to meet specific criteria (e.g., ≥1 mm in limb leads or ≥2 mm in precordial leads).
- T Wave Inversion: As the MI progresses, T waves may become inverted in the leads overlying the affected area. This usually happens after ST-segment elevation.
- Q Waves: Development of significant Q waves (wider than 0.04 seconds and/or a depth greater than one-third the height of the R wave) indicates irreversible myocardial damage and necrosis. Q waves may persist indefinitely after an MI.
Later Stages (Days to Weeks)
- ST-Segment Resolution: The ST segment may return to baseline, but T wave inversions and Q waves often remain.
Reciprocal Changes
- ST-Segment Depression: ST-segment depression may be seen in leads opposite the area of infarction. This is referred to as reciprocal changes and can help confirm the diagnosis of MI.
Summary Table
| ECG Change | Timing | Significance |
|---|---|---|
| Hyperacute T Waves | Earliest (Minutes to Hours) | Earliest sign of ischemia. |
| ST-Segment Elevation | Early (Hours) | Significant sign of acute STEMI, indicating complete or near-complete occlusion of a coronary artery. |
| T Wave Inversion | Later (Hours to Days) | Indicates ischemia or injury; often follows ST-segment elevation. |
| Q Waves | Days to Weeks | Indicates irreversible myocardial damage and necrosis; may persist indefinitely. |
| ST-Segment Depression | Can occur anytime during the episode | Usually occurs as a reciprocal change. |
It is important to remember that ECG findings should always be interpreted in the context of the patient's clinical presentation, risk factors, and other diagnostic tests. Furthermore, not all MIs present with classic ST-segment elevation (NSTEMI).
