What are the signs of myocardial infarction (MI) on a 12-lead electrocardiogram (ECG)?

Signs of Myocardial Infarction on 12-Lead ECG

The hallmark ECG finding of acute myocardial infarction is ST-segment elevation ≥0.1 mV (1 mm) in at least two contiguous leads, with higher thresholds in leads V2-V3 (≥0.25 mV in men <40 years, ≥0.2 mV in men ≥40 years, ≥0.15 mV in women). 1

Primary ECG Findings in STEMI

ST-Segment Elevation Criteria:

• Measure ST elevation at the J-point in two or more contiguous leads 1
• Standard leads (other than V2-V3): ≥0.1 mV (1 mm) elevation 1
• Leads V2-V3 in men <40 years: ≥0.25 mV (2.5 mm) 1
• Leads V2-V3 in men ≥40 years: ≥0.2 mV (2 mm) 1
• Leads V2-V3 in women: ≥0.15 mV (1.5 mm) 1
• Persistent ST elevation lasting >20 minutes, particularly with reciprocal ST depression, indicates acute coronary occlusion 1

Hyperacute Changes (Earliest Signs):

• Hyperacute T waves appear within minutes of coronary occlusion, often preceding ST elevation 2
• Increased R-wave amplitude and width in leads with ST elevation 2

ECG Findings in NSTEMI

Non-ST elevation patterns include: 1

• Transient ST-segment elevation
• Persistent or transient ST-segment depression (≥0.05 mV in two contiguous leads) 2
• T-wave inversion ≥0.1 mV in two contiguous leads with prominent R wave or R/S ratio >1 2
• Flat T waves or pseudo-normalization of T waves 1
• The ECG may be completely normal in NSTEMI 1

Pathological Q Waves (Established Infarction)

Q waves develop in many patients with MI and indicate myocardial necrosis: 1, 2

• Duration ≥0.03 seconds (30 milliseconds)
• Depth ≥0.1 mV (1 mm) or ≥25% of R-wave amplitude
• Must appear in at least two contiguous leads
• Q waves may persist indefinitely after MI 2

Territory-Specific ECG Patterns

Inferior MI:

• ST elevation in leads II, III, and aVF 3
• Record right precordial leads V3R and V4R to detect right ventricular involvement 1
• ST elevation ≥0.05 mV in V3R/V4R indicates RV infarction 2

Anterior MI:

• ST elevation in leads V1-V4 3
• May show Q waves or QS complexes in V1-V3 for septal involvement 4

Posterior MI:

• ST depression in leads V1-V3 with positive terminal T waves (ST elevation equivalent) 1
• Confirm with posterior leads V7-V9 showing ST elevation ≥0.05 mV (≥0.1 mV in men <40 years) 1, 2

Lateral MI:

• ST elevation in leads I, aVL, V5-V6 2

Atypical ECG Presentations Requiring Urgent Management

Left Bundle Branch Block (LBBB):

• Concordant ST elevation (in leads with positive QRS deflections) strongly suggests acute MI 1
• New or presumed new LBBB with clinical suspicion warrants immediate reperfusion therapy 1
• Modified Sgarbossa criteria improve diagnostic accuracy 5

Left Main or Multivessel Disease:

• ST depression ≥0.1 mV in eight or more surface leads 1
• ST elevation in aVR and/or V1 1
• Often associated with hemodynamic compromise 1

Ventricular Pacing:

• Pacemaker rhythm prevents ST-segment interpretation 1
• Consider reprogramming to assess intrinsic rhythm if patient is not pacemaker-dependent 1
• Otherwise, proceed directly to emergency angiography 1

Right Bundle Branch Block (RBBB):

• RBBB typically does not hamper ST-segment interpretation 1
• New ST elevation or Q waves with RBBB indicate MI 2

Critical Diagnostic Pitfalls

Normal Variants That Mimic MI:

• QS complex in lead V1 is normal 2
• Q wave <0.03 sec and <25% of R-wave amplitude in lead III is normal if frontal QRS axis is between 0-30° 2
• Early repolarization can mimic anterior STEMI—use 3- or 4-variable formulas to differentiate 5

Patients Without Diagnostic ST Elevation:

• 10-30% of STEMI patients present with atypical symptoms and may lack classic ECG findings 1
• Repeat ECG every 15-30 minutes in symptomatic patients with initially non-diagnostic ECG 1, 2
• Look for hyperacute T waves that precede ST elevation 1, 2
• Ongoing chest pain despite medical therapy mandates emergency angiography even without ST elevation 1

Essential Diagnostic Actions

Immediate Steps:

• Obtain and interpret 12-lead ECG within 10 minutes of first medical contact 1
• Compare with previous ECGs when available 2
• Initiate continuous ECG monitoring to detect life-threatening arrhythmias 1

When Standard ECG is Non-Diagnostic:

• Record posterior leads V7-V9 for suspected circumflex occlusion 1, 2
• Record right precordial leads V3R-V4R for suspected RV involvement in inferior MI 1, 2
• Perform serial ECGs at 15-30 minute intervals if symptoms persist 1, 2
• Consider continuous 12-lead ST-segment monitoring, which detects MI in an additional 16.2% of patients 2

Critical Timing:

• Do not wait for cardiac biomarker results to initiate reperfusion therapy in STEMI 1
• Blood sampling for cardiac markers is routine but should not delay treatment 1
• The ECG alone is often insufficient—integrate with clinical presentation and biomarkers 2

Evolution of ECG Changes

Temporal Sequence: 2

• Hyperacute T waves: minutes after occlusion
• ST-segment elevation: within hours
• Q wave development: hours to days (may never develop in all patients)
• T-wave inversion: days to weeks
• ST segments normalize: days to weeks
• Q waves may persist indefinitely

The absence of ECG changes does not exclude MI, particularly for posterior or lateral wall infarctions. 3