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I now have comprehensive ECG content and images. Let me compile everything into a thorough, well-structured answer.
ECG Basics & Interpretation
Source: Harrison's Principles of Internal Medicine, 22E (2025) - Chapter 247 Electrocardiography (Ary L. Goldberger)
What is an ECG?
An electrocardiogram (ECG or EKG) is a graphical recording of the electrical activity generated by the heart, detected through metallic electrodes attached to the limbs and chest wall. It is noninvasive, inexpensive, and immediately available. Beyond arrhythmia detection, it reveals conduction disturbances, myocardial ischemia/infarction, metabolic disturbances (e.g., hyperkalemia), drug toxicities, and susceptibility to sudden cardiac arrest.
1. Electrophysiologic Basis
The normal heartbeat begins at the sinoatrial (SA) node, which has spontaneous automaticity. The impulse spreads through the atria, then passes through the AV node and His bundle (together = AV junction), bifurcates into the right and left bundle branches, and reaches the ventricular myocardium via Purkinje fibers.
The cardiac conduction system. The left bundle branch further divides into left anterior and left posterior fascicles.
2. ECG Waveforms & Intervals
The basic ECG waveform. The J point marks the junction between QRS end and ST segment onset.
| Waveform/Interval | Represents | Normal Value |
|---|---|---|
| P wave | Atrial depolarization | Duration <120 ms; amplitude <2.5 mm |
| PR interval | AV conduction time (SA node to ventricular muscle) | 120-200 ms (3-5 small boxes) |
| QRS complex | Ventricular depolarization | <100 ms (up to 110 ms in some) |
| ST segment | Ventricular plateau phase (isoelectric) | Isoelectric (at baseline) |
| T wave | Ventricular repolarization (active recovery) | Positive in most leads |
| QT interval | Total ventricular depolarization + repolarization | Corrected QTc <440 ms (men), <460 ms (women) |
| U wave | Possibly late ventricular repolarization / Purkinje repolarization | Small, follows T wave |
| J point | QRS-ST junction | At isoelectric line |
Paper Speed & Calibration
- Standard sweep speed: 25 mm/s
- Smallest box (1 mm) = 40 ms (0.04 s)
- Larger box (5 mm) = 200 ms (0.20 s)
- Vertical amplitude: 1 mV = 10 mm at standard calibration
Calculating Heart Rate
- Count the large boxes between two R waves: HR = 300 ÷ (number of large boxes)
- Or count small boxes: HR = 1500 ÷ (number of small boxes)
3. The 12 Leads
A standard ECG has 12 leads - each providing a different "view" of the heart's electrical activity.
Limb Leads (Frontal Plane)
| Lead | View |
|---|---|
| I | Left lateral |
| II | Inferior (most used for rhythm strips) |
| III | Inferior |
| aVR | Right shoulder (right atrium) - normally negative |
| aVL | Left lateral |
| aVF | Inferior (foot) |
Precordial (Chest) Leads (Horizontal Plane)
Chest lead positions. V1: 4th ICS right sternal border. V2: 4th ICS left sternal border. V3: between V2 & V4. V4: 5th ICS midclavicular line. V5: anterior axillary line. V6: midaxillary line.
| Lead | View |
|---|---|
| V1-V2 | Right ventricle / septal |
| V3-V4 | Anterior LV |
| V5-V6 | Lateral LV |
4. QRS Complex & R-Wave Progression
Ventricular depolarization occurs in two phases: (1) septal depolarization left-to-right (producing small r in V1, small q in V6), and (2) free-wall depolarization dominated by the larger LV, directed leftward/posteriorly (producing S in V1, tall R in V6).
Normal R-wave progression: R waves increase in amplitude from V1 through V4/V5. The transition zone (where R and S are equal) is usually at V3 or V4.
- Poor R-wave progression (small R waves persisting through V4) suggests anterior infarction or other pathology.
5. QRS Axis
The mean QRS axis describes the overall direction of ventricular depolarization in the frontal plane:
| Axis | Degrees | Significance |
|---|---|---|
| Normal | -30° to +100° | Normal |
| Left axis deviation (LAD) | More negative than -30° | LBBB, left anterior fascicular block, inferior MI, LVH |
| Right axis deviation (RAD) | More positive than +90 to +100° | RVH, RBBB, left posterior fascicular block, lateral MI, dextrocardia |
Quick rule: If QRS is positive in lead I and positive in lead II → normal axis. Positive I, negative II → LAD. Negative I, positive II → RAD.
6. ST Segment & T Wave Abnormalities
ST Elevation - Key Causes
| Cause | Pattern |
|---|---|
| STEMI | Regional (follows coronary territory), convex/tombstone shape |
| Pericarditis | Diffuse, saddle-shaped; PR depression |
| Early repolarization | Benign variant, notch at J point, common in young males |
| Brugada syndrome | Coved ST elevation V1-V3, RBBB-like morphology |
| LVH/LBBB | Discordant ST changes (opposite to main QRS deflection) |
| Hyperkalemia | May cause V1-V3 elevation |
ST Depression - Key Causes
- Subendocardial ischemia (horizontal or downsloping)
- Digoxin effect (reverse tick / "sagging")
- Reciprocal changes in STEMI
- LVH strain pattern
T Wave Changes
- Inversion: Ischemia, ventricular hypertrophy, bundle branch block, pulmonary embolism (right heart strain), pericarditis
- Peaked/tall T waves: Hyperkalemia (early), posterior MI (prominent T in V1-V2)
- Flat T waves: Hypokalemia, hypothyroidism
QT Prolongation
Prolonged QTc (>500 ms) increases risk of Torsades de Pointes:
- Drugs: antiarrhythmics (amiodarone, sotalol), antipsychotics, fluoroquinolones, azithromycin
- Electrolytes: hypokalemia, hypomagnesemia, hypocalcemia
- Congenital long QT syndrome
7. Systematic 14-Step Interpretation (Harrison's Approach)
A systematic approach is mandatory to avoid errors of omission. Analyze every ECG for:
- Standardization & calibration (1 mV = 10 mm, paper speed)
- Lead placement & artifacts
- Rhythm (sinus vs. ectopic)
- Heart rate
- PR interval / AV conduction
- QRS interval
- QT/QTc intervals
- Mean QRS electrical axis
- P waves (morphology, axis)
- QRS voltages (hypertrophy criteria)
- Precordial R-wave progression
- Abnormal Q waves
- ST segments
- T waves & U waves
Compare with any previous ECGs - this is invaluable.
8. P Wave Abnormalities
| Finding | Meaning |
|---|---|
| Peaked P >2.5 mm in II ("P pulmonale") | Right atrial enlargement |
| Notched/broad P >120 ms in II ("P mitrale") | Left atrial enlargement |
| No P waves | Atrial fibrillation (irregularly irregular), junctional rhythm |
| Retrograde P (after QRS) | Junctional rhythm |
| P waves not related to QRS | Complete (3rd degree) AV block |
9. Ventricular Hypertrophy
LVH Voltage Criteria (Sokolow-Lyon)
- S in V1 + R in V5 or V6 >35 mm (age >35 years)
- R in aVL >11-13 mm
- Often accompanied by "strain pattern" (ST depression + T wave inversion in lateral leads I, aVL, V5-V6)
RVH
- Dominant R wave in V1 (R > S)
- Right axis deviation
- ST depression + T wave inversion in V1-V3 (strain)
10. Clinical Correlations at a Glance
| Condition | Key ECG Finding |
|---|---|
| Anterior STEMI | ST elevation V1-V4, Q waves |
| Inferior STEMI | ST elevation II, III, aVF |
| Lateral STEMI | ST elevation I, aVL, V5-V6 |
| Posterior MI | Tall R + ST depression V1-V2 (mirror image) |
| Atrial fibrillation | No P waves, irregularly irregular RR |
| Complete heart block | P-QRS dissociation, slow ventricular rate |
| LBBB | Broad QRS, notched "M" in V5-V6, QS in V1 |
| RBBB | Broad QRS, RSR' ("bunny ears") in V1, wide S in I and V6 |
| Hyperkalemia | Peaked T → wide QRS → sine wave |
| PE (S1Q3T3) | S wave in I, Q wave in III, T inversion in III |
| Pericarditis | Diffuse ST elevation + PR depression |
| Digoxin effect | Sagging ST depression ("reverse tick") |
Source: Harrison's Principles of Internal Medicine, 22E (2025), Chapter 247 - Electrocardiography (pp. 1911-1918), by Ary L. Goldberger. Also based on Goldberger's Clinical Electrocardiography: A Simplified Approach, 10th ed. (Elsevier, 2024).
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