ECG Changes in Myocardial Infarction
The ECG in acute MI evolves through a predictable sequence: hyperacute T waves appear first (within minutes), followed by ST-segment elevation (within hours), then pathologic Q waves, and finally T-wave inversion—with the specific pattern and timing determining whether immediate reperfusion therapy is indicated. 1, 2
Immediate Recognition: The STEMI Pattern
Activate emergency reperfusion immediately when you see ST-segment elevation ≥0.1 mV (1 mm) in two or more contiguous leads, except in leads V2-V3 where thresholds are higher: ≥0.25 mV in men <40 years, ≥0.2 mV in men ≥40 years, and ≥0.15 mV in women. 1, 2
Critical Early Signs Before Classic ST Elevation
- Hyperacute T waves are the earliest ECG sign, appearing within minutes of coronary occlusion—look for markedly increased amplitude with symmetrical, peaked morphology in ≥2 contiguous leads. 2
- Increased R-wave amplitude and width ("giant R waves") frequently accompany hyperacute T waves in leads that will later develop ST elevation, reflecting conduction delay in ischemic myocardium. 2
- Prolonged ST elevation >20 minutes, especially with reciprocal ST depression in opposite leads, signals complete coronary occlusion requiring immediate intervention. 1, 2
Reciprocal Changes: A High-Risk Marker
Reciprocal ST depression ≥1 mm in leads opposite to ST elevation indicates a larger area of myocardium at risk (salvage index 61% vs 17%, p<0.001) and mandates immediate reperfusion. 2 Do not misinterpret this as a second territory of ischemia—it reflects the electrical vector of the primary infarct zone. 2
Non-ST Elevation MI (NSTEMI) Patterns
NSTEMI encompasses all acute MI presentations lacking diagnostic ST elevation and carries significant mortality risk despite the absence of classic STEMI criteria. 1
Diagnostic ECG Criteria for NSTEMI
- Horizontal or downsloping ST depression ≥0.05 mV in two contiguous leads is the hallmark finding. 2
- T-wave inversion ≥0.1 mV in two contiguous leads with prominent R wave or R/S ratio >1. 2
- Transient ST changes ≥0.5 mm that appear during symptoms and resolve when asymptomatic strongly suggest severe coronary disease requiring urgent evaluation. 2, 3
- The ECG may be completely normal in up to 55% of NSTEMI cases initially, making serial recordings essential. 2
High-Risk NSTEMI Patterns Requiring Urgent Angiography
- ST depression ≥0.1 mV in eight or more surface leads indicates left main or multivessel disease. 2
- ST elevation in aVR and/or V1 also signals left main or multivessel disease. 2
- Widespread ST depression with ST elevation in aVR during chest pain represents diffuse subendocardial ischemia from severe coronary disease. 4
Territory-Specific ECG Patterns
Posterior MI: The Hidden Infarction
Record posterior leads V7-V9 at the fifth intercostal space whenever you see ST depression in V1-V3 with upright terminal T waves (ST elevation equivalent). 1, 2 ST elevation ≥0.05 mV in V7-V9 confirms posterior MI (use ≥0.1 mV threshold in men <40 years). 1, 2
Right Ventricular MI: Check Right-Sided Leads
In all inferior MIs, obtain right precordial leads V3R and V4R to assess right ventricular involvement. 1, 2 ST elevation ≥0.05 mV in these leads supports RV infarction (≥0.1 mV in men <30 years). 1, 2
Left Circumflex Territory: Frequently Missed
Circumflex occlusions are often overlooked because standard 12-lead ECG may be non-diagnostic—maintain high suspicion and record posterior leads when clinical presentation suggests ACS but initial ECG is unrevealing. 1
Serial ECG Protocol: Essential for Diagnosis
Obtain serial ECGs at 15-30 minute intervals in symptomatic patients with initially non-diagnostic ECGs, as dynamic changes are common during acute ischemic episodes. 1, 2
Monitoring Duration and Frequency
- First ECG within 10 minutes of emergency department arrival for all patients with chest discomfort. 2
- Continuous monitoring for 48-72 hours after confirmed MI to detect arrhythmias and ongoing ischemia. 2
- Repeat ECG at 60-180 minutes after fibrinolytic therapy to assess reperfusion success. 2
- Patients with diagnostic changes on serial ECG have 2.5× greater risk of ACS, 9.6× greater risk of life-threatening complications, and 12.3× greater risk of death. 2
Challenging ECG Scenarios
Left Bundle Branch Block (LBBB)
New or presumed new LBBB with ischemic symptoms warrants immediate reperfusion therapy. 2, 5 When LBBB is present, look for:
- Concordant ST elevation (ST elevation in leads with positive QRS deflection) strongly suggests acute MI. 1, 2
- Apply modified Sgarbossa criteria: ST elevation ≥5 mm in leads with negative QRS, ST elevation ≥1 mm in leads with positive QRS, or ST depression ≥1 mm in V1-V3. 5
Ventricular Pacing
Pacing fundamentally obscures ST-T changes essential for MI diagnosis due to altered ventricular depolarization. 5 Maintain a low threshold for urgent angiography when clinical suspicion is high despite non-diagnostic ECG, and consider reprogramming the pacemaker in non-dependent patients to evaluate intrinsic rhythm. 5
Right Bundle Branch Block (RBBB)
ST-T abnormalities in V1-V3 are common with RBBB, complicating ischemia assessment—however, new ST elevation or Q waves in the presence of RBBB should raise immediate suspicion for MI. 1, 2
Evolution of ECG Changes Over Time
Temporal Sequence
- Hyperacute T waves: Within minutes of occlusion 2
- ST-segment elevation: Within hours 2
- Pathologic Q waves: May develop early or late, persist indefinitely 2
- T-wave inversion: May persist for weeks to months 2
Pathologic Q Wave Criteria Indicating Prior MI
| Lead Territory | Q Wave Criteria | Interpretation |
|---|---|---|
| V2-V3 | Q ≥0.02 sec or QS complex | Prior anterior MI |
| I, II, aVL, aVF, V4-V6 (≥2 contiguous) | Q ≥0.03 sec and ≥0.1 mV deep or QS complex | Prior MI in corresponding territory |
| V1-V2 | R ≥0.04 sec with R/S ≥1 and concordant positive T | Prior posterior MI |
Critical Pitfalls to Avoid
Do Not Dismiss ST Depression as "Rate-Related" in Symptomatic Patients
Horizontal or downsloping ST depression ≥0.05 mV in ≥2 contiguous leads indicates ischemia until proven otherwise, regardless of heart rate. 3 Upsloping ST depression with steep upward slope from the J-point is the hallmark of benign, rate-related changes. 3
Do Not Rely on a Single Normal ECG
A normal resting ECG does not exclude acute coronary syndrome—1-6% of patients with normal ECG still experience MI. 3 Serial recordings are mandatory when clinical suspicion persists. 1, 2
Do Not Confuse Early Repolarization with STEMI
Early repolarization shows upward-concave, rapidly upsloping ST segments in 95% of cases, with terminal QRS notching or slurring at the J-point, most commonly in V3-V4. 2 STEMI shows convex or horizontal ST elevation without these features. 2
Do Not Overlook Pseudo-Normalization of T Waves
Previously inverted T waves that become upright during chest pain indicate acute ischemia (pseudo-normalization) and require immediate evaluation. 1
Risk Stratification Based on ECG Findings
The magnitude of ST-segment deviation provides independent prognostic information even after adjusting for clinical findings and biomarkers. 2 More profound ST-segment shift involving multiple leads/territories correlates with greater myocardial ischemia and worse prognosis. 1
- ST-segment depression on presenting ECG carries the highest risk of death at 6 months. 2
- Small variations of ST-segment shift in the first 4 hours correlate with worse outcomes. 2
- Extensive ST shifts involving multiple territories indicate larger ischemic burden and poorer prognosis. 2
Integration with Biomarkers
The ECG by itself is often insufficient to diagnose acute MI, as ST deviation may occur in other conditions including acute pericarditis, left ventricular hypertrophy, Brugada syndrome, stress cardiomyopathy, and early repolarization. 1 Cardiac troponin measurement is mandatory when ischemia is suspected, with repeat measurement at 6-12 hours if initial values are normal. 1