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10 Diseases with ECG Changes

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1. ST-Elevation Myocardial Infarction (STEMI)

• ST elevation ≥ 1mm in 2+ contiguous leads in the territory of the occluded artery
• Hyperacute (tall, broad) T waves in early phase
• Reciprocal ST depression in mirror leads
• Pathological Q waves develop within hours (>40ms wide, >25% of R wave height)
• T wave inversion follows ST elevation

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2. Acute Pericarditis

• Diffuse, saddle-shaped (concave up) ST elevation in most leads (except aVR and V1)
• PR segment depression in the same leads (very specific finding)
• PR elevation in aVR (reciprocal)
• No reciprocal ST depression (unlike STEMI)
• Evolves through 4 stages: ST elevation → ST normalization → T wave inversion → T wave normalization

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3. Brugada Syndrome

• Coved-type ST elevation ≥ 2mm in V1-V3
• Followed by negative T wave (no isoelectric separation)
• Spontaneous or drug-induced (sodium channel blockers, fever)
• Right bundle branch block pattern may be present
• Predisposes to ventricular fibrillation and sudden cardiac death

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4. Hyperkalemia

• K+ 5.5-6.5 mmol/L: Tall, peaked (tented) T waves, narrow-based
• K+ 6.5-7.5 mmol/L: Prolonged PR interval, widening QRS
• K+ 7.5-9.0 mmol/L: P wave flattening/disappearance, wide QRS, sine wave pattern
• K+ >9 mmol/L: Sine wave, ventricular fibrillation, asystole

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5. Hypokalemia

• Prominent U waves (most characteristic; best seen in V2-V3)
• Flattened or inverted T waves
• ST segment depression
• Apparent QT prolongation (actually QU prolongation)
• Prolonged PR interval
• Predisposes to torsades de pointes and atrial/ventricular ectopy

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6. Pulmonary Embolism (PE)

• S1Q3T3 pattern - deep S in lead I, Q wave in III, inverted T in III (classic but only ~20% sensitive)
• T wave inversions in right precordial leads V1-V4 (most specific finding)
• New incomplete or complete RBBB
• Sinus tachycardia (most common finding)
• Right axis deviation
• P pulmonale (peaked P waves in II)
• Negative T waves in BOTH lead III and V1 have 88% sensitivity and 99% specificity for PE vs ACS

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7. Wolff-Parkinson-White (WPW) Syndrome

• Short PR interval (< 120ms) - pre-excitation via accessory pathway
• Delta wave - slurred upstroke of the QRS
• Widened QRS (> 120ms)
• Pseudo-infarction Q waves (negative delta waves can mimic Q waves)
• Secondary ST-T changes (discordant from QRS)
• Susceptible to SVT (AVRT) and, dangerously, AF with rapid ventricular conduction

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8. Hypertrophic Cardiomyopathy (HCM)

• Left ventricular hypertrophy (increased QRS voltage: S in V1 + R in V5/V6 > 35mm)
• Deep, narrow ("dagger") Q waves in lateral leads (I, aVL, V5-V6) due to septal hypertrophy
• Left axis deviation
• T wave inversions (especially in apical HCM - giant T wave inversions in V3-V5)
• Left atrial enlargement (broad, notched P waves in II)
• Apical HCM specifically: giant T wave inversions (>10mm) in precordial leads

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9. Complete Heart Block (Third-Degree AV Block)

• P waves and QRS complexes are completely dissociated (no relationship between P and R)
• Regular P-P intervals and regular R-R intervals, but at different rates
• P rate > ventricular rate
• Wide, bizarre QRS if ventricular escape rhythm (junctional escape gives narrow QRS ~40-60 bpm, ventricular escape gives wide QRS ~20-40 bpm)
• Cannon A waves on JVP clinically

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10. Long QT Syndrome (Congenital or Acquired)

• Prolonged QTc: ≥ 450ms in men, ≥ 460ms in women (QTc = QT / √RR interval)
• T wave morphology abnormalities: notched T waves, bifid T waves, biphasic T waves
• Predisposes to torsades de pointes (polymorphic VT with twisting around the baseline)
• T wave alternans may be seen
• Causes: congenital (LQTS1-3), drugs (antiarrhythmics, antipsychotics, antibiotics), electrolyte abnormalities (hypoK, hypoMg, hypoCa), hypothyroidism, intracranial hemorrhage

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Quick Reference Summary

Sources: LITFL ECG Library - Life in the Fast Lane; Braunwald's Heart Disease, 11th Ed.; ROSEN's Emergency Medicine

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How to Read an ECG at OPD - A Systematic Approach

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Before You Start: Know Your ECG Paper

• Paper speed: 25 mm/sec (standard)
• Amplitude: 10 mm = 1 mV
• 1 small square = 1 mm = 40 ms
• 1 large square = 5 mm = 200 ms

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The ECG Waveform - Know What You're Looking At

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STEP 1 - RATE

Count large squares between two R waves → Rate = 300 ÷ number of large squares

• 1 large square = 300 bpm
• 2 large squares = 150 bpm
• 3 large squares = 100 bpm
• 4 large squares = 75 bpm
• 5 large squares = 60 bpm
• 6 large squares = 50 bpm

Count QRS complexes in a 10-second strip × 6 = bpm

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STEP 2 - RHYTHM

1. Regular P waves - one before every QRS
2. P waves upright in lead II, inverted in aVR
3. Constant PR interval (120-200ms)
4. Regular R-R intervals

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STEP 3 - AXIS (Cardiac Axis)

OPD tip: If Lead I is "thumbs up" and aVF is "thumbs up" = normal axis. Easy!

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STEP 4 - P WAVE (Atrial abnormalities)

• Normal P wave: Smooth, rounded, < 2.5mm tall, < 120ms wide in Lead II
• Right Atrial Enlargement (P pulmonale): Peaked tall P wave > 2.5mm in Lead II (and III, aVF)
• Left Atrial Enlargement (P mitrale): Broad, notched (bifid) P wave > 120ms in Lead II; deep negative component in V1
• No P waves: Consider AF or junctional rhythm

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STEP 5 - PR INTERVAL

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STEP 6 - QRS COMPLEX

1. Width - narrow vs wide
2. Voltage - tall vs low
3. Q waves - pathological Q waves?
4. R wave progression - V1 to V6 (R gets taller, S gets smaller)

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STEP 7 - ST SEGMENT

Key rule: ST elevation in a "territory" with reciprocal changes = STEMI until proven otherwise. Act immediately.

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STEP 8 - T WAVE

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STEP 9 - QT INTERVAL

• Prolonged QTc: Drugs (amiodarone, antipsychotics, macrolides), hypokalemia, hypomagnesemia, hypothyroidism, congenital LQTS → risk of torsades de pointes
• Short QTc < 340ms: Hypercalcemia, digoxin, congenital short QT syndrome

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The OPD Quick Reference Card

R - RATE       (>100 tachy, <60 brady)
R - RHYTHM     (regular? P before every QRS?)
R - (R wave progression V1→V6)
P - P WAVE     (tall, broad, absent?)
Q - PR interval (120-200ms normal)
R - QRS        (narrow <120ms, Q waves, voltage)
S - ST segment (elevation? depression?)
T - T waves    (peaked, inverted, flat?)
  + QT interval (< half RR = ok)

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What a Normal ECG Should Look Like

• Rate: 60-100 bpm
• Regular rhythm, upright P in II before every QRS
• PR 120-200ms, narrow QRS <120ms
• R wave grows from V1 to V5/V6
• ST segment isoelectric in all leads
• Upright T waves in I, II, V3-V6
• QTc within normal limits

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Practical OPD Tips

1. Always look at the patient first - ECG findings must match clinical context
2. Compare with old ECG when available - a "new" change matters much more than a "known" one
3. Rate and rhythm first - if the patient is unstable, this is what matters immediately
4. A normal ECG does not rule out ACS - if chest pain is present, serial ECGs and troponins are needed
5. aVR is the forgotten lead - ST elevation in aVR + diffuse ST depression = left main / proximal LAD occlusion (critical!)
6. Calibration check - always confirm 1mV = 10mm marker is present on the ECG before interpreting voltage

Sources: LITFL ECG Library - Life in the Fast Lane; Braunwald's Heart Disease, 11th Ed.