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Professional ECG Interpretation - Complete Systematic Guide

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Part 1 - The Foundation: What the ECG Measures

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Part 2 - The ECG Paper (Grid Fundamentals)

• Count the number of big boxes between two R waves
• Divide 300 by that number (the "300 method")
• e.g., 4 big boxes between R-R = 300/4 = 75 bpm
• For irregular rhythms: count QRS complexes in a 10-second strip × 6

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Part 3 - The Waves, Intervals & Segments

Normal Values Table (Goldman-Cecil Medicine)

Wave-by-Wave Breakdown

• Should be upright in I, II, aVF
• Always inverted in aVR (normal)
• Duration < 120 ms, amplitude < 2.5 mm
• If absent: AF, junctional rhythm, SA block
• If wide/notched ("P mitrale"): left atrial enlargement
• If tall/peaked ("P pulmonale", >2.5 mm in II): right atrial enlargement

• 120-200 ms (3-5 small boxes)
• Short PR (<120 ms): Wolff-Parkinson-White (WPW), junctional rhythm
• Long PR (>200 ms): 1st degree AV block (delayed AV conduction)
• Variable PR: 2nd or 3rd degree heart block

• Duration 75-110 ms (< 3 small boxes)
• Capital letters (Q, R, S) = amplitude ≥ 5 mm
• Lowercase (q, r, s) = amplitude < 5 mm
• QRS > 120 ms = bundle branch block or ventricular rhythm
• Normal: small r in V1, growing to tall R in V5, then back down in V6 ("R-wave progression")

• Should be isoelectric (flat, at baseline)
• Elevation > 1 mm in limb leads or >2 mm in precordial: STEMI pattern
• Depression > 0.5 mm: ischemia, NSTEMI, digoxin effect

• Normally upright in I, II, V3-V6; inverted in aVR
• Inverted T waves: ischemia, LVH strain, RBBB, LBBB
• Peaked/hyperacute T waves: early STEMI, hyperkalemia

• Must be corrected for heart rate (QTc) using Bazett's formula: QTc = QT ÷ √RR
• Prolonged QTc: risk for Torsades de Pointes (drugs, electrolyte abnormalities, congenital long QT)

• Prominent U wave: hypokalemia, bradycardia
• Do NOT include in QT measurement

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Part 4 - The 12-Lead System

Limb Leads (Frontal Plane)

• Lead I: Left arm (+) vs Right arm (-) - horizontal axis
• Lead II: Left foot (+) vs Right arm (-) - inferior view
• Lead III: Left foot (+) vs Left arm (-) - inferior view
• aVR: Right arm - looks at base of heart (normally negative)
• aVL: Left arm - high lateral view
• aVF: Left foot - inferior view

Precordial (Chest) Leads - Horizontal Plane

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Part 5 - Axis Determination

• LAD: LBBB, left anterior fascicular block, inferior MI, LVH
• RAD: RBBB, RVH, left posterior fascicular block, lateral MI, WPW (right-sided)

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Part 6 - The 8-Step Systematic Interpretation

Step 1 - Rate

• Use 300/number of big boxes method (regular rhythms)
• Or: QRS complexes in 10 sec × 6 (irregular rhythms)
• Bradycardia < 60 bpm | Normal 60-100 | Tachycardia > 100

Step 2 - Rhythm

Step 3 - P Wave

• Present? Upright in II? Inverted in aVR?
• Morphology: peaked (RAE), bifid/notched (LAE), absent (AF), retrograde (junctional)

Step 4 - PR Interval

• Normal = 3-5 small boxes (120-200 ms)
• Long: 1st degree block (constant prolongation), Wenckebach (progressively lengthening then dropped QRS), Mobitz II (sudden dropped QRS, constant PR), Complete heart block (complete AV dissociation)
• Short + delta wave: WPW

Step 5 - QRS Complex

• Duration: narrow (<120 ms) or wide (>120 ms)?
• Wide QRS: RBBB (rsR' in V1, wide S in I/V6), LBBB (broad notched R in I/V6, no septal q), ventricular ectopic, pacemaker
• Q waves: pathological if > 1 small box wide (40 ms) OR > 1/4 height of R wave in same lead
• Voltage: LVH (S in V1 + R in V5 ≥ 35 mm, or R in aVL ≥ 11 mm), RVH (R>S in V1, RAD)

Step 6 - ST Segment

• Elevation: STEMI (convex/tombstone), pericarditis (saddle-shaped, all leads except aVR/V1), early repolarization (concave, young patients, benign)
• Depression: ischemia/NSTEMI, reciprocal changes, digoxin ("reverse tick" or scoop), LBBB

Step 7 - T Wave

• Inversion: ischemia (symmetric, deep), LBBB/RBBB (expected in certain leads), LVH strain (I, aVL, V5-V6), PE (V1-V4 + SIQIIITIII)
• Peaked/tall: hyperkalemia (first sign), hyperacute STEMI

Step 8 - QTc + Context

• Calculate QTc. Prolonged if > 450 ms (male) or > 460 ms (female)
• Always compare with a previous ECG if available
• Put findings into clinical context - chest pain patient + ST changes = act urgently

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Part 7 - Key Arrhythmias at a Glance

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Part 8 - Advanced Patterns (Professional-Level Pearls)

• ST elevation ≥ 1 mm concordant with QRS (same direction): 5 points
• ST depression ≥ 1 mm in V1-V3 (concordant): 3 points
• ST elevation ≥ 5 mm discordant (opposite to QRS): 2 points

1. Peaked, narrow T waves (earliest sign)
2. Widening PR, flattening P wave
3. Widening QRS
4. Sine wave pattern
5. VF / asystole

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Part 9 - Systematic Practice Approach

1. Never pattern-match first - always complete all 8 steps before concluding
2. State your findings out loud (or in writing) before looking at the automatic computer interpretation
3. Always compare to prior ECG - acute changes are often more diagnostic than the findings themselves
4. Rate + Rhythm → Ischemia → Conduction is the ED triage order

• ECGwaves.com - structured systematic guide with cases
• ECG Made Simple - free videos and interactive examples
• SCST 2024 ECG Reporting Guidelines - professional reporting standards (UK)
• Practice on Litfl.com - extensive library with annotated cases
• For books: Dubin's Rapid Interpretation of EKG (classic beginner-to-intermediate), Hampton's ECG Made Easy, Chou's Electrocardiography in Clinical Practice (advanced)

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