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How to study ecg

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how to study ECG electrocardiogram systematic approach beginners

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ECG waveform P wave QRS complex T wave labeled diagram

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I now have excellent content from Harrison's and Kaplan & Sadock's. Let me compile a comprehensive guide.

How to Study ECG

Why a Systematic Approach Matters

The most common errors in ECG interpretation are errors of omission — missing something because you didn't look for it. Every expert source, from Harrison's to Kaplan & Sadock's, agrees: always follow a fixed sequence, every single time.

Step 1 — Understand the Basics First

Before reading any ECG, you need to know what the waveforms represent:
ComponentMeaning
P waveAtrial depolarization
QRS complexVentricular depolarization
T waveVentricular repolarization
U waveAfter-repolarization (origin debated)
PR intervalAV conduction time (P start → QRS start)
QRS intervalVentricular conduction time
QT intervalTotal ventricular electrical activity
ST segmentBetween QRS end and T wave start
Paper basics:
  • Small box = 0.04 sec (horizontal) / 0.1 mV (vertical)
  • Large box = 0.2 sec (5 small boxes)
  • Standard ECG = 10 seconds long
ECG waveform diagram with P, QRS, ST, T, U labeled

Step 2 — The 14-Parameter Systematic Checklist

From Harrison's Principles of Internal Medicine (22nd ed.), analyze every ECG in this order:
  1. Standardization & technical features — Is it calibrated? Any lead misplacement? Artifacts?
  2. Rhythm — Regular or irregular? P before every QRS?
  3. Heart rate — Calculate using one of two methods (see below)
  4. PR interval / AV conduction — Normal: 0.12–0.20 sec (3–5 small boxes)
  5. QRS interval — Normal: <0.12 sec (<3 small boxes)
  6. QT/QTc interval — Prolonged QTc >440 ms (men), >460 ms (women)
  7. Mean QRS electrical axis — Use Leads I and aVF
  8. P waves — Morphology, presence, regularity
  9. QRS voltages — High (hypertrophy) or low (effusion, obesity, emphysema)
  10. Precordial R-wave progression — Should grow V1 → V5
  11. Abnormal Q waves — Suggests prior infarction
  12. ST segments — Elevation or depression
  13. T waves — Inversion, flattening, peaking
  14. U waves — Prominent in hypokalemia, bradycardia
Always compare with previous ECGs — this is invaluable. — Harrison's Principles of Internal Medicine, p. 1919

Step 3 — Calculating Heart Rate

Method 1 — "Rate by squares" (regular rhythms): Count large boxes between two R waves → divide 300 by that number.
  • Example: 4 large boxes between R waves → 300 ÷ 4 = 75 bpm
Method 2 — "QRS count" (irregular rhythms like AF): Count all QRS complexes on the 10-second strip → multiply by 6.
  • Example: 10 complexes × 6 = 60 bpm

Step 4 — Determine the Axis

  • Normal axis: −30° to +90°
  • Use Lead I (0°) and aVF (90°) — they are perpendicular to each other
  • Both upright → normal axis
  • Left axis deviation (LAD): Lead I up, aVF down — seen in LVH (from chronic hypertension)
  • Right axis deviation (RAD): Lead I down, aVF up — seen in RVH, pulmonary HTN, congenital disease

Step 5 — Classify the Rhythm

Approach rhythms by two questions:
  1. Narrow or wide QRS?
    • Narrow (<0.12 sec) = supraventricular origin (follows normal conduction)
    • Wide (≥0.12 sec) = ventricular origin OR aberrant conduction
  2. Regular or irregular?
    • Regularly irregular → recurring pattern (e.g., Wenckebach)
    • Irregularly irregular → completely disorganized (e.g., atrial fibrillation — no clear P waves)

Step 6 — Key ST/T Wave Patterns to Know

FindingThink of
ST elevationSTEMI, pericarditis, early repolarization, LV aneurysm
ST depressionSubendocardial ischemia, digoxin effect
T-wave inversionIschemia, PE, subarachnoid hemorrhage, myocarditis
Peaked T wavesHyperkalemia (or hyperacute STEMI)
Prolonged QTDrugs, hypokalemia, hypomagnesemia, congenital syndromes
Wide QRSBundle branch block, ventricular rhythm, hyperkalemia
Repolarization (ST-T) is an active, ATP-dependent process — making it especially susceptible to ischemia. — Kaplan & Sadock's Comprehensive Textbook of Psychiatry, p. 6825

How to Build Proficiency — Practical Tips

  1. Read ECGs daily — volume is everything. Free resources include ECG Wave-Maven (Harvard's self-assessment library) and Lifeinthefastlane.com.
  2. Use a fixed sequence every time — don't skip steps, even on "obvious" tracings.
  3. Start with normal — read 50+ normal ECGs before focusing on abnormal ones.
  4. Learn by pattern recognition — LBBB, RBBB, AF, STEMI, AV blocks each have a visual gestalt.
  5. Correlate with clinical context — age, sex, symptoms, and medications change interpretation dramatically.
  6. Reference textbooks:
    • Goldberger's Clinical Electrocardiography: A Simplified Approach (10th ed., 2024) — the classic beginner's book
    • Harrison's Principles of Internal Medicine (22nd ed.) — Chapter 247 for ECG basics
    • Braunwald's Heart Disease — for advanced/cardiology-level detail

Sources

  • Harrison's Principles of Internal Medicine, 22nd ed., Chapter 247, p. 1919
  • Kaplan & Sadock's Comprehensive Textbook of Psychiatry, pp. 6824–6826
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