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Here is a comprehensive guide to basic ECG interpretation, drawn primarily from Harrison's Principles of Internal Medicine, 22nd Ed. (2025) and Braunwald's Heart Disease.
ECG Basic Interpretation
What Is an ECG?
The electrocardiogram (ECG) records the electrical activity of the heart from body surface electrodes. A standard 12-lead ECG uses:
- 6 limb leads: I, II, III (bipolar) + aVR, aVL, aVF (unipolar augmented)
- 6 precordial (chest) leads: V1–V6
Each lead "looks" at the heart from a different angle. Leads II, III, aVF face the inferior wall; I, aVL face the lateral wall; V1–V4 face the anterior wall.
The ECG Grid
- Small box = 0.04 s (horizontal) / 0.1 mV (vertical)
- Large box = 0.20 s (horizontal) / 0.5 mV (vertical)
- Standard paper speed: 25 mm/s
Normal Waveforms
Normal sinus rhythm. Regular QRS complexes, narrow morphology, isoelectric ST segments, upright T waves in I, II, V2–V6.
| Waveform | Represents | Normal |
|---|---|---|
| P wave | Atrial depolarization (SA node → AV node) | Duration < 0.12 s; amplitude < 2.5 mm; upright in I, II, aVF |
| PR interval | AV conduction time | 0.12–0.20 s (3–5 small boxes) |
| QRS complex | Ventricular depolarization | Duration < 0.10–0.12 s; axis −30° to +100° |
| ST segment | Plateau of ventricular action potential | Isoelectric (no elevation or depression) |
| T wave | Ventricular repolarization | Upright in I, II, V2–V6; inverted in aVR (normal) |
| QT interval | Electrical systole | QTc ≤ 440 ms (men) / ≤ 460 ms (women) |
| U wave | Slow repolarization of His–Purkinje | Small, follows T wave (best seen V2–V3) |
The Systematic Approach (Harrison's 14-Step Method)
A systematic approach prevents errors of omission. Analyze every ECG in this order:
- Standardization / calibration — confirm 10 mm/mV calibration mark; check paper speed; identify artifacts and lead placement errors
- Rhythm — regular vs. irregular; identify P waves; assess P–QRS relationship
- Heart rate — count R–R intervals; 300 ÷ (# large boxes between R waves) for regular rhythms; or count complexes in 6 seconds × 10
- PR interval — normal 0.12–0.20 s; prolonged = AV block; short = pre-excitation (WPW) or junctional rhythm
- QRS interval — > 0.12 s = bundle branch block or ventricular rhythm
- QT / QTc interval — prolonged QTc risks torsades de pointes
- Mean QRS electrical axis — identify which lead has the most isoelectric (equiphasic) QRS; the axis is perpendicular to it
- P waves — morphology (peaked = right atrial enlargement; notched/broad = left atrial enlargement)
- QRS voltages — low voltage (< 5 mm in all limb leads) or high voltage (LVH criteria)
- Precordial R-wave progression — R wave grows from V1 → V6; poor progression suggests anterior MI or LBBB
- Abnormal Q waves — pathological Q wave = > 0.04 s wide and/or > 25% depth of following R wave; suggests prior MI
- ST segments — elevation (injury/STEMI) vs. depression (ischemia/NSTEMI/reciprocal)
- T waves — inversion (ischemia, strain, PE); hyperacute T waves (early STEMI); peaked/tall T waves (hyperkalemia)
- U waves — prominent U waves suggest hypokalemia or drug effect (quinidine, amiodarone)
Always compare with any prior ECG — this is the single most valuable step.
Heart Rate Quick Calculation
For regular rhythms — count large boxes between two consecutive R waves:
| Large boxes (R–R) | Rate (bpm) |
|---|---|
| 1 | 300 |
| 2 | 150 |
| 3 | 100 |
| 4 | 75 |
| 5 | 60 |
| 6 | 50 |
Mnemonic: 300 – 150 – 100 – 75 – 60 – 50
QRS Axis
The frontal plane axis is assessed using limb leads:
| Axis Range | Name | Causes |
|---|---|---|
| −30° to +100° | Normal | — |
| More negative than −30° | Left axis deviation (LAD) | Left anterior fascicular block, inferior MI, LBBB |
| More positive than +100° | Right axis deviation (RAD) | RVH, left posterior fascicular block, RBBB, PE |
| −90° to ±180° | Extreme axis | Ventricular tachycardia, lead reversal |
Bedside trick: If lead I is positive (pointing left) and aVF is positive (pointing down) → normal axis. If I is positive and aVF is negative → suspect LAD; check if < −30° using lead II.
Ventricular Depolarization & R-Wave Progression
Ventricular depolarization has two major phases:
- Septal depolarization — left → right → small r in V1, small q in V6
- Free wall depolarization — dominated by LV mass → large S in V1, large R in V6
Normal R-wave progression: R amplitude increases progressively from V1 to V5/V6. The transition zone (R ≈ S) is normally V3 or V4.
- Poor R-wave progression (small r waves persisting to V4–V5): suggests anterior MI, LBBB, or lead misplacement.
Key Intervals Summary
| Interval | Normal | Abnormal |
|---|---|---|
| PR | 0.12–0.20 s | Short: WPW, junctional; Long: 1st–3rd degree AV block |
| QRS | < 0.12 s | > 0.12 s: LBBB, RBBB, ventricular pacing/rhythm |
| QTc | ≤ 440 ms ♂ / ≤ 460 ms ♀ | Prolonged: drugs, electrolytes, congenital; Short: hypercalcemia |
Normal ECG Example (12-lead)
Regular sinus rhythm at ~60–75 bpm, narrow QRS, isoelectric ST segments, normal R-wave progression V1→V6, upright T waves.
Common Pitfalls
- Lead reversal (RA–LA swap) mimics dextrocardia: global negativity in lead I, inverted P in I
- Artifact can mimic arrhythmias (tremor → AF; movement → VT)
- Left bundle branch block invalidates ST interpretation — always a "diagnostic challenge" per Harrison's
- Computer interpretation has major limitations — clinician review is mandatory
Sources: Harrison's Principles of Internal Medicine, 22nd Ed. (2025), Chapter 247; Braunwald's Heart Disease, 12th Ed., Chapter 61.
Can I get altogether as a pdf file for future reference of opd
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Your OPD reference PDF is ready! Here's the download link:
ECG Basic Interpretation OPD Reference
PDF Document · PDF
What's included in the PDF:
- The ECG Grid — time & voltage values per box
- The 12 Leads — territories, walls, and culprit arteries
- Normal Waveforms & Intervals — P, PR, QRS, ST, T, QT, U with normal values
- Harrison's 14-Step Systematic Approach — full table
- Heart Rate Quick Reference — 300 rule mnemonic table
- QRS Axis — normal/LAD/RAD with bedside I+aVF trick
- Ventricular Depolarization & R-Wave Progression — with diagram
- Key Intervals — PR, QRS, QTc with short/prolonged differentials
- Common Pitfalls — lead reversal, artifact, LBBB, posterior MI
- Rhythm Quick Guide — NSR through complete heart block
- STEMI Localization — all territories with reciprocal leads
- Bundle Branch Blocks — RBBB vs LBBB comparison
- Electrolyte Effects — K, Ca, Mg on ECG
Includes 4 actual ECG images from Harrison's and clinical databases. Sourced from Harrison's 22nd Ed. (2025) and Braunwald's Heart Disease.
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