How do I differentiate ST elevation due to ischemia?

Differentiating ST Elevation Due to Ischemia from Non-Ischemic Causes

The most reliable way to identify ischemic ST elevation is to look for reciprocal ST depression in anatomically opposite leads—this finding strongly suggests acute coronary occlusion rather than benign mimics like early repolarization, pericarditis, or left ventricular aneurysm. 1

Key Discriminating Features for Ischemic ST Elevation

Reciprocal ST Depression (Most Specific Finding)

• In anterior ST elevation: Look for ST depression ≥0.025 mV in lead II—this occurs in 40% of STEMI cases but is absent in non-ischemic causes 1
• In inferior ST elevation: Look for ST depression ≥0.025 mV in lead I—this occurs in 83% of STEMI cases but is absent in non-ischemic causes 1
• Reciprocal changes reflect the spatial vector of injury current and are highly specific for acute coronary occlusion 2

Terminal QRS Distortion

• Terminal QRS distortion (loss of J-point and S-wave in leads with ST elevation) occurs in 40% of STEMI but only 7% of non-ischemic ST elevation 1
• This finding reflects severe transmural ischemia disrupting normal ventricular activation 3

ST Segment Morphology

• Convex (dome-shaped) ST elevation is more common in STEMI (22%) than non-ischemic causes (9%) 1
• In contrast, concave ST elevation with rapidly upsloping segments suggests benign early repolarization 2
• Ischemic ST elevation tends to be more horizontal or convex, while non-ischemic causes show steep upsloping 2

Features Suggesting NON-Ischemic ST Elevation

PR Segment Depression in Chest Leads

• PR depression in chest leads occurs in 38% of non-ischemic cases but only 12% of STEMI 1
• This finding is characteristic of pericarditis and argues against acute coronary occlusion 1

ST Depression in Lead aVR

• ST depression in aVR is associated with non-ischemic diagnosis in multivariable analysis 1
• In contrast, ST elevation in aVR with anterior changes suggests proximal LAD occlusion 2

T-wave to QRS Amplitude Ratios (for LVA vs. AMI)

• If the sum of T-wave amplitudes divided by sum of QRS amplitudes in V1-V4 is >0.22, predict AMI (sensitivity 91.5%, specificity 68.8%) 4
• If any single lead V1-V4 has T/QRS ratio ≥0.36, predict AMI (sensitivity 91.5%, specificity 81.3%) 4
• Left ventricular aneurysm typically shows persistent ST elevation with smaller T waves relative to QRS 4

Age and Gender-Specific Thresholds for Abnormal ST Elevation

Apply these thresholds to determine if ST elevation exceeds normal variants 2:

• Men ≥40 years: ≥2 mm in V2-V3, ≥1 mm in other leads
• Men <40 years: ≥2.5 mm in V2-V3, ≥1 mm in other leads
• Women (all ages): ≥1.5 mm in V2-V3, ≥1 mm in other leads
• V4-V5 (both sexes): ≥0.5 mm, except men <30 years where ≥1 mm is threshold
• Posterior leads V7-V9: ≥0.5 mm for both sexes

STEMI Equivalents to Recognize

Posterior MI (Often Missed)

• Horizontal ST depression maximal in V1-V3 with upright T waves and dominant R waves (R/S >1) in V2 suggests posterior MI 2
• Obtain posterior leads V7-V9 where ST elevation ≥0.5 mm confirms posterior STEMI 2, 5
• This represents a STEMI equivalent requiring immediate reperfusion 2

De Winter Sign

• Tall, prominent, symmetrical T waves arising from upsloping ST depression >1 mm at J-point in precordial leads 2
• May have 0.5-1 mm ST elevation in aVR 2
• Represents acute proximal LAD occlusion requiring immediate catheterization 2

Hyperacute T Waves

• Broad, asymmetric, peaked T waves may precede ST elevation in early STEMI 2
• Serial ECGs over short intervals (minutes) help detect progression to frank ST elevation 2

Localization of Culprit Vessel by ST Changes

Proximal LAD Occlusion

• ST elevation in V1-V6, I, aVL, and often aVR 2
• Reciprocal ST depression in II, III, aVF 2
• More ST elevation in aVL than aVR 2

Mid/Distal LAD Occlusion

• ST elevation in V3-V6 without elevation in V1, aVR, or aVL 2
• No reciprocal inferior ST depression (may even have inferior ST elevation) 2

RCA vs. LCx in Inferior MI

• RCA occlusion: Greater ST elevation in III than II, with ST depression in I and aVL 2
• Proximal RCA: Add right-sided leads V3R-V4R showing ST elevation ≥0.5 mm (indicates RV infarction) 2
• Record right-sided leads rapidly as RV ST elevation resolves quickly 2

Common Pitfalls to Avoid

• Do not dismiss ST depression as "non-specific" when present in multiple contiguous leads—this may represent posterior MI or severe ischemia requiring urgent angiography 5
• Never give fibrinolytics for isolated ST depression—this increases mortality 5
• Obtain posterior leads V7-V9 when ST depression is maximal in V1-V3 to avoid missing posterior STEMI 2, 5
• Consider dynamic/intermittent ST changes—34-40% of STEMIs show cycles of ST elevation and resolution in early hours, so a single normal ECG doesn't exclude MI 2
• Experienced cardiologists vary widely in STEMI vs. non-ischemic differentiation (sensitivity 50-100%, specificity 73-97%), so use systematic criteria rather than gestalt 6

Immediate Actions When Ischemic ST Elevation Identified

• Aspirin 162-325 mg immediately 5
• Dual antiplatelet therapy with clopidogrel 5
• Anticoagulation with LMWH or UFH 5
• Beta-blockers if no contraindications 5
• Activate catheterization lab for primary PCI 2, 5
• Continuous ST-segment monitoring for at least 24 hours 2, 5