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ECG Reading from Basics

What is an ECG?

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1. The ECG Paper

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2. The 12 Leads

• I, II, III — standard bipolar leads
• aVR, aVL, aVF — augmented unipolar leads

• V1, V2, V3, V4, V5, V6 — chest leads

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3. The Waves, Intervals & Segments

          R
          |
     P    |      T
    /\    |     /\
---/  \---+----/  \---  baseline
          |QS
          |
   [P-R]  [QRS] [ST][T]

Waves

QRS Nomenclature

• Q = first negative deflection before R
• R = first positive deflection
• S = negative wave following R
• R' = second positive deflection (e.g., RBBB)
• Capital letters (Q, R, S) = large amplitude; lowercase (q, r, s) = small amplitude

Intervals & Segments

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4. The 14-Parameter Systematic Approach (Harrison's)

1. Standardization & calibration — Is the gain set to 1 mV/10 mm? Any artifacts?
2. Rhythm — Regular or irregular? Identify the pacemaker
3. Heart rate
4. PR interval / AV conduction
5. QRS interval
6. QT / QTc interval
7. Mean QRS electrical axis
8. P waves
9. QRS voltages
10. Precordial R-wave progression
11. Abnormal Q waves
12. ST segments
13. T waves
14. U waves

Always compare with previous ECGs if available.

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5. Heart Rate

Method 1 — Rate by Squares (regular rhythm)

Method 2 — Count QRS complexes (irregular rhythm)

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6. Rhythm

1. Where is the pacemaker? (SA node → normal; AV node → junctional; ventricle → ventricular)
2. What is the conduction path? (Normal: SA node → AV node → His-Purkinje → ventricles)
3. Is it regular and at the correct rate?

Normal Sinus Rhythm (NSR) criteria:

• P wave before every QRS ✓
• QRS after every P wave ✓
• P waves upright in leads I and II ✓
• Regular R-R intervals ✓
• Rate 60–100 bpm ✓

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7. Axis

• LAD: Left ventricular hypertrophy, left bundle branch block, inferior MI
• RAD: Right ventricular hypertrophy, right bundle branch block, lateral MI, COPD

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8. Intervals in Detail

PR Interval

• Normal: 0.12–0.20 s
• Short PR (< 0.12 s): Pre-excitation (WPW syndrome), junctional rhythm
• Long PR (> 0.20 s): First-degree AV block

QRS Duration

• Normal: < 0.12 s (< 3 small boxes)
• Wide QRS (≥ 0.12 s): Bundle branch block, ventricular rhythm, hyperkalemia, drug toxicity

QT Interval

• Rate-dependent — use corrected QTc (Bazett formula: QTc = QT ÷ √R-R)
• Normal QTc: < 440 ms in men, < 460 ms in women
• Prolonged QT: Risk of Torsades de Pointes; caused by drugs (antipsychotics, antibiotics), electrolyte disturbances, congenital long QT syndrome

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9. P Waves

• Normal: rounded, upright in I and II, inverted in aVR
• Tall & peaked P (> 2.5 mm in II): Right atrial enlargement ("P pulmonale")
• Broad & notched P (> 0.12 s in II): Left atrial enlargement ("P mitrale")
• Absent P waves: Atrial fibrillation (irregular baseline), junctional rhythm

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10. QRS Complex — Key Abnormalities

Bundle Branch Blocks

R-Wave Progression (precordial leads)

• Normal: R wave grows from V1 → V5 (transition zone V3–V4)
• Poor R-wave progression: Anterior MI, LBBB, COPD

Pathological Q waves

• Significant Q wave: width ≥ 0.04 s OR depth ≥ 1/4 of R wave height
• Indicates prior myocardial infarction (dead tissue produces no electrical activity)

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11. ST Segment & T Wave

ST Segment

T Waves

• Normal: upright in I, II, V3–V6; inverted in aVR; variable in III and V1
• Hyperacute T waves (tall, peaked): Earliest sign of acute STEMI; also hyperkalemia
• Inverted T waves: Ischemia, ventricular strain, bundle branch blocks, pulmonary embolism (V1–V4)
• Flattened T waves: Electrolyte disturbance, ischemia

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12. Summary: Quick Reading Sequence

RRAQQST — a simple memory aid:

1. Rate
2. Rhythm
3. Axis
4. QRS width + morphology
5. QT interval
6. ST segment
7. T waves (+ P waves + PR interval)

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Example: Normal ECG

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Key Normal Values — Quick Reference

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