How do you identify a myocardial infarction (MI) on an electrocardiogram (ECG) in a patient with chest pain and risk factors for cardiovascular disease, such as hypertension and diabetes?

How to Identify Myocardial Infarction on ECG

Measure ST-segment elevation at the J-point in two or more contiguous leads: ≥2.5 mm in men <40 years, ≥2 mm in men ≥40 years, or ≥1.5 mm in women in leads V2-V3, and ≥1 mm in all other leads—this is the hallmark finding that mandates immediate reperfusion therapy. 1

Primary STEMI Criteria

The diagnosis of ST-elevation myocardial infarction requires specific voltage thresholds that vary by lead location and patient demographics:

• Standard leads (I, II, III, aVL, aVF, V4-V6): ST elevation ≥1 mm (0.1 mV) at the J-point in at least two contiguous leads 1
• Leads V2-V3 in men <40 years: ST elevation ≥2.5 mm 1
• Leads V2-V3 in men ≥40 years: ST elevation ≥2 mm 1
• Leads V2-V3 in women: ST elevation ≥1.5 mm 1

Contiguity is defined anatomically: in the frontal plane by the sequence aVL, I, inverted aVR, II, aVF, III; in the precordial plane by consecutive V leads 1

NSTEMI and High-Risk Patterns

When ST elevation is absent, look for these diagnostic patterns:

• Horizontal or downsloping ST depression ≥0.5 mm (0.05 mV) in two or more contiguous leads 1
• Symmetric T-wave inversion ≥1 mm (0.1 mV) in at least two contiguous leads with prominent R waves or R/S ratio >1 1
• ST depression ≥1 mm in eight or more surface leads with ST elevation in aVR and/or V1 indicates left main or multivessel disease requiring urgent intervention 2

The magnitude of ST deviation directly correlates with prognosis: patients with ≥0.5 mm ST depression have a 16.3% rate of death or MI at one year compared to 8.2% in those without ECG changes 3

Critical Early Findings Often Missed

Hyperacute T waves are frequently the earliest ECG manifestation of acute coronary occlusion, appearing within minutes and preceding ST elevation 1, 2. These tall, peaked T waves are often overlooked but represent a window for intervention before irreversible myocardial damage occurs 2.

Territory-Specific Lead Placement

Standard 12-lead ECG misses up to 4% of acute MIs, particularly those involving the posterior wall or right ventricle 1. To avoid this pitfall:

Posterior MI Detection

• Record posterior leads V7-V9 at the fifth intercostal space when you see ST depression in V1-V3 with positive terminal T waves 1, 2
• Diagnostic threshold: ST elevation ≥0.5 mm in V7-V9 (≥1 mm in men <40 years) 1, 2
• Isolated posterior MI from circumflex occlusion often presents with only ST depression in anterior leads—this is an ST-elevation equivalent requiring immediate reperfusion 1

Right Ventricular MI Detection

• Record right precordial leads V3R and V4R in all patients with inferior MI (ST elevation in II, III, aVF) 1, 2
• Diagnostic threshold: ST elevation ≥0.5 mm in V3R-V4R (≥1 mm in men <30 years) 1, 2
• RV involvement predicts high rates of in-hospital complications and requires specific management 1

Reciprocal Changes Increase Diagnostic Certainty

ST depression ≥1 mm in leads electrically opposite to ST elevation confirms acute coronary occlusion and indicates larger myocardium at risk 2. Patients with reciprocal changes demonstrate 61% myocardial salvage with prompt revascularization versus only 17% without reciprocal changes (p<0.001) 2.

Common reciprocal patterns:

• Anterior MI (V1-V4 elevation) → inferior depression (II, III, aVF)
• Inferior MI (II, III, aVF elevation) → anterior depression (V1-V3) or lateral depression (I, aVL)

Serial ECG Protocol for Non-Diagnostic Initial Tracings

A completely normal ECG does not exclude MI—1% to 6% of patients with normal initial ECG are ultimately diagnosed with NSTEMI 1. When clinical suspicion remains high despite non-diagnostic initial ECG:

• Repeat ECG every 15-30 minutes during ongoing symptoms 1, 2
• Continue for at least 48-72 hours of continuous monitoring after diagnosis 2
• Serial 12-lead monitoring detects injury in an additional 16.2% of patients, representing a 34% relative increase in those eligible for emergency reperfusion 2

Patients with diagnostic changes on serial ECG have 2.5 times greater risk of acute coronary syndrome, 9.6 times greater risk of life-threatening complications, and 12.3 times greater risk of death 2

Confounding Patterns Requiring Special Interpretation

Left Bundle Branch Block (LBBB)

LBBB typically obscures Q waves and causes baseline ST-T abnormalities 1. To diagnose acute MI in LBBB:

• Look for concordant ST elevation (ST elevation in same direction as QRS complex) 1, 2
• Compare with prior ECG when available 1
• New or presumed new LBBB with ischemic symptoms warrants immediate reperfusion therapy 2

Right Bundle Branch Block (RBBB)

ST-T abnormalities in V1-V3 are common with RBBB, but new ST elevation or Q waves should raise suspicion for acute MI despite these baseline changes 1

Left Ventricular Hypertrophy

Patients with LVH and confounding baseline ECG patterns are at highest risk for death 1. An unchanged ECG compared to prior tracing significantly reduces the likelihood of acute MI even in the presence of LVH 1

Pathological Q Waves Indicating Prior or Evolving MI

Q waves develop in many (but not all) patients with MI and indicate transmural myocardial necrosis 1, 2:

Diagnostic criteria for pathological Q waves:

• Duration ≥0.03 seconds (30 ms) AND depth ≥0.1 mV (1 mm) in at least two contiguous leads 1, 4
• Q/R ratio ≥0.25 in two or more contiguous leads (except III and aVR) 4
• Any Q wave ≥0.02 seconds or QS complex in V2-V3 4

Normal Q waves that should NOT be considered pathological:

• QS complex in lead V1 1, 4
• Small septal Q waves <0.03 seconds and <25% of R-wave amplitude in I, aVL, aVF, V4-V6 1, 4
• Q wave in lead III when frontal QRS axis is between 30° and 0° 1, 4

Up to 25% of NSTEMI patients with elevated CK-MB develop Q waves during hospitalization, converting to Q-wave MI 1, 4

Common Pitfalls and How to Avoid Them

1. Dismissing subtle ST elevation in young patients or those with early repolarization: Use the 3- and 4-variable formulas to differentiate benign early repolarization from subtle LAD occlusion 5

2. Missing posterior MI: Always record V7-V9 when you see isolated ST depression in V1-V3, especially with positive terminal T waves 1

3. Overlooking RV involvement in inferior MI: Routinely record V3R-V4R in all inferior MIs—RV involvement dramatically changes management 1, 2

4. Relying on a single ECG: Serial tracings at 15-30 minute intervals detect evolving changes in 16% of patients initially missed 2

5. Misinterpreting reciprocal changes as a second territory of ischemia: Reciprocal ST depression reflects the electrical vector of the primary infarct zone, not a separate area requiring intervention 2

6. Waiting for biomarkers in STEMI: Do not delay reperfusion therapy for troponin results when ECG meets STEMI criteria 2, 6

7. Improper lead placement: Verify correct positioning—high lead placement commonly creates pseudo-septal infarct patterns with Q waves in V1-V2 4

Risk Stratification Based on ECG Findings

The ECG provides independent prognostic information even after adjusting for clinical findings and biomarkers 1:

Highest risk (22.9% death/MI at 1 year): LBBB with ischemic symptoms 3

High risk (16.3% death/MI at 1 year): ST deviation ≥0.5 mm 3

Intermediate risk (11% death/MI at 1 year): ST deviation ≥1 mm 3

Lower risk (6.8% death/MI at 1 year): Isolated T-wave inversion 3

Lowest risk (8.2% death/MI at 1 year): Normal or nonspecific changes 3

Transient ST-segment changes during symptomatic episodes that resolve when asymptomatic strongly suggest severe coronary disease requiring urgent evaluation 2