ECG Evolution During Myocardial Infarction
The ECG changes during myocardial infarction follow a predictable temporal sequence: hyperacute T waves appear first (within minutes), followed by ST-segment elevation (within 20-30 seconds to hours), then T-wave inversion develops (subacute phase), and finally pathologic Q waves may persist indefinitely. 1, 2, 3, 4
The Four Phases of ECG Evolution
Hyperacute Phase (Minutes to Early Hours)
- Hyperacute T waves are the earliest ECG sign, appearing as tall, peaked, symmetrical T waves in at least two contiguous leads before ST elevation develops 5, 1, 2
- Increased R-wave amplitude and width (giant R-waves with S-wave diminution) often accompany these changes, reflecting conduction delay in ischemic myocardium 5, 1
- This phase occurs within minutes of coronary occlusion and may be missed if the ECG is not obtained immediately 1, 3
Acute Phase (Hours)
- ST-segment elevation becomes prominent, measured at the J-point in two or more contiguous leads 5, 1, 2
- ST elevation develops within 20-30 seconds after coronary occlusion and persists when occlusion is complete 3, 4
- Reciprocal ST depression appears in opposite leads, confirming acute coronary occlusion 1, 2
- The magnitude of ST elevation correlates with the extent of myocardial ischemia and worse prognosis 1
Subacute Phase (Days to Weeks)
- ST segments begin to normalize as the infarction evolves 2
- T-wave inversion develops as a characteristic finding, appearing as the ST elevation resolves 1, 2, 3
- T-wave inversion may persist for weeks to months after the acute event 1, 2
Chronic Phase (Weeks to Permanent)
- Pathologic Q waves develop in many (but not all) patients, defined as Q waves ≥0.03 seconds duration and ≥0.1 mV deep in at least two contiguous leads 5, 1
- Q waves may appear early in the acute phase or develop later, and typically persist indefinitely as markers of prior infarction 5, 1, 3
- T-wave inversion may persist chronically 2
Important Clinical Caveats
The Sequence Is Not Always Linear
- Serial ECGs at 15-30 minute intervals are essential because changes evolve rapidly and the initial ECG may be non-diagnostic 5, 1
- The ECG can be completely normal in NSTEMI, where ST depression or T-wave inversion (rather than ST elevation) predominates 1, 2, 6
- Transient ST elevation that resolves may indicate aborted MI with successful spontaneous or therapeutic reperfusion 6
Non-STEMI Patterns Differ
- In NSTEMI, the progression involves horizontal or down-sloping ST depression ≥0.05 mV and/or T-wave inversion ≥0.1 mV in two contiguous leads, without the classic ST elevation sequence 1, 6
- These patients may never develop ST elevation or Q waves 1, 6
Timing Matters for Intervention
- Continuous ECG monitoring for 48-72 hours is mandatory to capture evolving changes and detect life-threatening arrhythmias 1
- Dynamic ST-segment shifts in the first 4 hours correlate with worse outcomes and guide reperfusion decisions 1
Common Pitfalls to Avoid
- Do not wait for Q waves to develop before initiating reperfusion therapy—hyperacute T waves and ST elevation are sufficient 1, 2
- The ECG alone is insufficient for diagnosis, as ST deviation occurs in other conditions (pericarditis, LV hypertrophy, LBBB, Brugada syndrome, early repolarization) 5, 1
- Always compare with previous ECGs when available to identify new changes 1, 7
- Consider posterior leads (V7-V9) and right precordial leads (V3R-V4R) when standard leads are non-diagnostic but clinical suspicion remains high 1, 2