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ECG Changes in Major Cardiac Emergencies

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Pathophysiology of ST Changes (The Foundation)

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1. STEMI — ST-Elevation Myocardial Infarction

Diagnostic Criteria (per Tintinalli's)

• ≥1 mm in all leads except V₂–V₃
• ≥2 mm in V₂–V₃ (men ≥40 y), ≥2.5 mm (men <40 y), ≥1.5 mm (women)

Localisation by Territory

Evolutionary ECG Sequence

Key Points

• Reciprocal ST depression in leads opposite the infarct zone indicates larger injury, more severe CAD, worse prognosis
• New LBBB with chest pain was historically a STEMI equivalent, but <10% actually have AMI — use Sgarbossa criteria
• RV infarction: always obtain right-sided leads (V₄R) in inferior STEMI; ST elevation ≥1 mm in V₄R is highly suggestive

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2. NSTEMI / Unstable Angina

ECG Findings

• ST depression ≥0.5 mm in ≥2 contiguous leads (subendocardial ischemia pattern)
• T-wave inversions — symmetric, deep
• ST elevation in aVR with diffuse ST depression elsewhere → suggests left main or proximal LAD occlusion (or severe 3-vessel disease)
• Normal ECG in 1–6% of confirmed NSTEMI — does not exclude the diagnosis

Wellens' Syndrome (Critical LAD Stenosis)

• Type A (25%): Biphasic T waves in V₂–V₃
• Type B (75%): Deeply symmetric T-wave inversions in V₂–V₃ (may extend to V₄–V₆)
• T-wave changes normalise when pain returns, reappear when pain subsides — dynamic pattern

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3. Acute Pericarditis

Stages of ECG Change

Distinguishing from STEMI

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4. Cardiac Tamponade

ECG Findings

• Sinus tachycardia — almost universal
• Low voltage (QRS amplitude <5 mm in limb leads, <10 mm precordial) — from pericardial fluid insulating electrical activity
• Electrical alternans — beat-to-beat alternation in QRS amplitude/morphology as the heart swings within fluid; pathognomonic when present

"The cardiac position will alternate, with the heart returning to its original position with every other beat, thus electrical alternans may be seen." — Rosen's Emergency Medicine

• Electrical alternans is highly specific but seen in only ~20% of tamponade; its absence does not exclude tamponade

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5. Pulmonary Embolism (PE)

Classic Pattern — S₁Q₃T₃

• S wave in lead I (right axis deviation)
• Q wave in lead III
• T-wave inversion in lead III

Other RV Strain Patterns

• Sinus tachycardia — most common finding
• New RBBB (complete or incomplete) — indicates severe RV strain/hypertension
• T-wave inversions in V₁–V₄ — RV strain
• Right axis deviation
• P pulmonale (peaked P in II >2.5 mm) — right atrial enlargement
• Atrial fibrillation/flutter
• ST elevation in V₁ ± aVR

"A new right bundle branch block suggests right ventricular strain and severe pulmonary hypertension." — Goldman-Cecil Medicine

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6. Hyperkalemia

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7. Wolff-Parkinson-White (WPW)

ECG Triad During Sinus Rhythm

1. Short PR interval (<120 ms) — rapid conduction via accessory pathway bypasses AV node delay
2. Delta wave — slurred upstroke of initial QRS (pre-excitation of ventricle)
3. Widened QRS complex — fusion of normal and accessory pathway depolarisation
4. Secondary ST-T changes — discordant to QRS direction

Tachyarrhythmias in WPW (Emergency)

Critical Warning: In WPW + AF, avoid AV-nodal blocking agents (adenosine, verapamil, digoxin, beta-blockers) — they block the normal pathway, forcing all conduction down the accessory pathway, potentially triggering VF.

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8. Ventricular Tachycardia (VT) vs. SVT with Aberrancy

VT Features on ECG (Favour VT)

• AV dissociation (P waves independent of QRS) — most specific sign
• QRS width >160 ms
• Concordance in precordial leads (all positive or all negative)
• Fusion beats and capture beats
• Extreme axis (Northwest axis, −90° to ±180°)
• Brugada criteria / Josephson's sign (notching near S-wave nadir)

SVT with Aberrancy

• QRS usually <160 ms
• Typical RBBB or LBBB morphology
• Responds to vagal manoeuvres or adenosine

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9. Aortic Dissection

• Most common: normal ECG or non-specific ST-T changes
• ST elevation in inferior leads — if dissection involves the RCA ostium (Type A with coronary involvement)
• LVH pattern — from underlying hypertension (common predisposing factor)
• Pericardial effusion changes (low voltage, electrical alternans) — from haemopericardium in Type A

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10. Brugada Syndrome

• Type 1 (diagnostic): Coved ST elevation ≥2 mm in V₁–V₂ with T-wave inversion — "coved pattern"
• Type 2: Saddle-back ST elevation ≥2 mm in V₁–V₂ (high takeoff) — not diagnostic alone
• Pattern can be concealed and unmasked by fever, sodium channel blockers, class IC antiarrhythmics

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11. Long QT Syndrome / Torsades de Pointes

• QTc >440 ms (men), >460 ms (women) — prolonged
• QTc >500 ms — high risk for Torsades de Pointes (TdP)
• Torsades: Polymorphic VT with rotating axis ("twisting of the points") around baseline — characteristically self-terminates but can degenerate to VF
• Triggers: hypomagnesaemia, hypokalaemia, QT-prolonging drugs (class IA/III antiarrhythmics, antipsychotics, antiemetics)
• Treatment: IV magnesium 2g, correct electrolytes, consider overdrive pacing

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Quick Reference Summary Table

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