ST-Segment Elevation Diagnostic Criteria for Myocardial Infarction
ST-segment elevation must be measured at the J-point (where the QRS complex meets the ST segment) and present in at least two contiguous leads, with specific voltage thresholds that vary by lead location, age, and sex. 1, 2
Standard Voltage Criteria by Lead Location
The diagnostic thresholds differ based on anatomical location and patient demographics:
Precordial Leads V2-V3
- Men under 40 years: ≥2.5 mm (0.25 mV) 1, 2
- Men 40 years and older: ≥2.0 mm (0.20 mV) 1, 2
- Women (all ages): ≥1.5 mm (0.15 mV) 1, 2
All Other Leads
- ≥1.0 mm (0.1 mV) in at least two contiguous limb leads (I, II, III, aVF, aVL) or precordial leads V1, V4-V6 1, 2
These criteria apply only in the absence of left ventricular hypertrophy or left bundle branch block, as these conditions alter normal repolarization patterns. 1
Essential Technical Requirements
The ST elevation must appear in at least two anatomically contiguous leads to qualify as diagnostic. 1 Contiguous leads are defined as:
- Inferior leads: II, III, aVF 1
- Lateral leads: I, aVL, V5-V6 1
- Anterior leads: V1-V4 1
- Septal leads: V1-V2 1
Special Anatomical Considerations
Right Ventricular Infarction
Record right precordial leads V3R and V4R in all patients with inferior STEMI to identify concomitant right ventricular involvement. 1, 3, 2 ST elevation ≥0.5 mm in V3R-V4R (≥1 mm in men under 30 years) confirms right ventricular infarction. 3
Posterior Myocardial Infarction
ST-segment depression in leads V1-V3 with positive terminal T-waves suggests posterior MI (an "ST-elevation equivalent"). 1, 2 Confirm by recording posterior leads V7-V9, where ST elevation ≥0.5 mm (some sources use ≥1.0 mm) is diagnostic. 1, 2 Posterior MI represents 15-21% of all MIs and is frequently missed on standard 12-lead ECG. 1
Critical Pitfalls and How to Avoid Them
Left Bundle Branch Block (LBBB)
In the presence of LBBB, look for concordant ST elevation ≥1 mm in leads with positive QRS deflections (sensitivity 73%, specificity 92%). 1 Additional findings that predict MI in LBBB include:
- ST-segment depression ≥1 mm in leads V1-V3 (sensitivity 25%, specificity 96%) 1
- ST-segment elevation ≥5 mm discordant with QRS complex (sensitivity 19%, specificity 82%) 1
However, most LBBB patients in the emergency department do not have acute coronary occlusion. 1 When clinical suspicion is high with new or presumed new LBBB, proceed directly to emergency angiography rather than relying solely on ECG criteria. 1
Ventricular Paced Rhythm
Apply the same concordance/discordance rules as LBBB: ST elevation ≥1 mm concordant with QRS (sensitivity 18%, specificity 94%) or ≥5 mm discordant with QRS (sensitivity 53%, specificity 88%) predicts MI. 1
Non-Diagnostic Initial ECG
Up to 30% of STEMI patients present with atypical symptoms, and some have non-diagnostic initial ECGs. 1 The ECG may be non-diagnostic because:
- Very early presentation (look for hyperacute T-waves preceding ST elevation) 1, 2
- Circumflex artery occlusion (46-61% sensitivity with standard leads) 4
- Posterior wall involvement 1
Perform serial ECGs every 10-15 minutes or use continuous ST-segment monitoring when clinical suspicion remains high despite non-diagnostic initial ECG. 1 In one study, 72.4% of patients with initially non-diagnostic ECGs showed diagnostic changes within 90 minutes. 2
Measurement Errors
Automated ECG interpretation has suboptimal performance (sensitivity only 35% in one prospective study), missing STEMIs due to incorrect measurement points (30%), borderline elevations (36%), and timing issues (25%). 5 Always manually verify ST-segment measurements at the J-point rather than relying solely on automated interpretation. 1, 2
Non-Ischemic Causes of ST Elevation
ST elevation can occur in multiple non-ischemic conditions, creating diagnostic challenges. 6 Even experienced cardiologists show wide variation in differentiating STEMI from non-ischemic ST elevation (sensitivity 50-100%, specificity 73-97%). 6 Common mimics include:
- Left ventricular aneurysm 7
- Pericarditis 1
- Early repolarization 1
- Takotsubo cardiomyopathy 1
- Wolff-Parkinson-White syndrome 1
When symptoms are consistent with acute coronary syndrome and ST elevation meets voltage criteria, assume STEMI and activate reperfusion protocols immediately without waiting for biomarkers. 2 The clinical context—particularly symptom timing and character—is essential for interpretation. 2
Prognostic Value of ST-Segment Magnitude
Greater magnitude of ST elevation correlates with larger infarct size and worse outcomes. 1, 4 Patients with ST deviation have higher mortality than those with isolated T-wave changes (who in turn have higher mortality than those with normal ECGs). 1 This gradient of risk persists even after adjusting for biomarkers and clinical findings. 1