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ECG (Electrocardiogram) - A Complete Beginner's Guide
Source: Harrison's Principles of Internal Medicine, 22nd Edition (2025)
What Is an ECG?
An electrocardiogram records the electrical activity of the heart from electrodes placed on the skin. Each lead is like a different camera angle looking at the same events - atrial and ventricular depolarization and repolarization - from different spatial orientations.
The 12 Leads
A standard ECG has 12 leads:
- 6 limb leads (I, II, III, aVR, aVL, aVF) - record potentials in the frontal plane
- 6 precordial (chest) leads (V1-V6) - record potentials in the horizontal plane
Precordial Electrode Placement
| Lead | Position |
|---|---|
| V1 | 4th intercostal space, right sternal border |
| V2 | 4th intercostal space, left sternal border |
| V3 | Between V2 and V4 |
| V4 | 5th intercostal space, midclavicular line |
| V5 | Anterior axillary line (same level as V4) |
| V6 | Midaxillary line (same level as V4) |
The Normal ECG Waveforms
Each cardiac cycle on the ECG consists of:
| Wave/Interval | Represents | Normal Duration |
|---|---|---|
| P wave | Atrial depolarization (SA node fires, atria contract) | <120 ms (<3 small squares) |
| PR interval | Time from atrial to ventricular depolarization (AV node delay) | 120-200 ms (3-5 small squares) |
| QRS complex | Ventricular depolarization (ventricles contract) | <120 ms (<3 small squares) |
| ST segment | Early ventricular repolarization (isoelectric) | Should be at baseline |
| T wave | Ventricular repolarization | Upright in most leads |
| QT interval | Total ventricular electrical activity (QRS + ST + T) | <440 ms (men), <460 ms (women) |
| U wave | Thought to reflect Purkinje fiber repolarization | Small, follows T wave |
ECG Paper Speed and Grid
- Each small square = 0.04 seconds (horizontal) and 0.1 mV (vertical)
- Each large square = 0.20 seconds (5 small squares)
The P Wave - Atrial Depolarization
The normal atrial depolarization vector is oriented downward and to the left (from the sinus node toward the AV node). This means:
- P wave is positive in lead II and negative in lead aVR - this confirms sinus rhythm
- In V1, the P wave may be biphasic: positive component (right atrium) then small negative component (left atrium)
An ectopic atrial pacemaker in the lower atrium or AV junction produces retrograde P waves (negative in II, positive in aVR).
The QRS Complex - Ventricular Depolarization
Ventricular depolarization has two main phases:
- Phase 1 - Septal depolarization: Left to right, produces a small r wave in V1 and small septal q wave in V6
- Phase 2 - Ventricular depolarization: The dominant left ventricle pulls the vector leftward and posteriorly, producing a large S wave in V1 and tall R wave in V6
This explains normal R-wave progression across the chest leads - R waves get taller from V1 to V5, and S waves decrease. The transition zone (where R=S) is normally at V3-V4.
QRS Axis
The mean QRS axis describes the overall direction of ventricular depolarization in the frontal plane:
| Axis | Range | Common Causes |
|---|---|---|
| Normal | -30° to +90°/+100° | Normal variant |
| Left axis deviation (LAD) | More negative than -30° | Left ventricular hypertrophy, left anterior fascicular block |
| Right axis deviation (RAD) | More positive than +90°/+100° | Right ventricular hypertrophy, left posterior fascicular block, lateral MI |
| Extreme | -90° to ±180° | Severe disease |
Quick trick: If lead I is positive and aVF is positive → normal axis. If I is positive and aVF is negative → LAD.
Systematic 14-Step ECG Interpretation
Harrison's recommends analyzing every ECG in this order:
- Standardization/calibration - check paper speed and gain (1 mV = 10 mm is standard), artifacts, correct lead placement
- Rhythm - regular or irregular? Sinus vs. ectopic?
- Heart rate - regular rhythm: divide 300 by the number of large squares between R waves; irregular: count QRS complexes in 10 seconds × 6
- PR interval - normal 120-200 ms; prolonged = AV block; shortened = pre-excitation
- QRS interval - normal <120 ms; wide = bundle branch block or ventricular origin
- QT/QTc interval - prolonged QT is a risk factor for torsades de pointes
- Mean QRS axis - use hexaxial diagram above
- P waves - morphology, axis, one P per QRS?
- QRS voltages - high = hypertrophy; low = pericardial effusion, obesity, COPD
- Precordial R-wave progression - loss = anterior MI or LBBB
- Abnormal Q waves - pathologic Q waves (>40 ms wide or >25% of QRS height) = old infarction
- ST segments - elevation = STEMI or pericarditis; depression = ischemia, NSTEMI
- T waves - inversion = ischemia, ventricular hypertrophy, BBB; peaked = hyperkalemia
- U waves - prominent U waves = hypokalemia, bradycardia, certain drugs
Always compare with previous ECGs - this is invaluable.
Calculating Heart Rate (Quick Methods)
Regular rhythm:
- Memorize: 300, 150, 100, 75, 60, 50 for 1, 2, 3, 4, 5, 6 large squares between R waves
- Formula: 300 ÷ (number of large squares between consecutive R peaks)
Irregular rhythm (e.g., atrial fibrillation):
- Count all QRS complexes in a 10-second rhythm strip and multiply by 6
Key Pathological Patterns (Quick Reference)
| ECG Finding | Think of |
|---|---|
| ST elevation in contiguous leads | STEMI (in the territory of the affected vessel) |
| ST depression + T inversion | NSTEMI / unstable angina |
| Wide QRS with RBBB pattern (rSR' in V1) | Right bundle branch block |
| Wide QRS with LBBB pattern (broad R in V5/V6) | Left bundle branch block |
| Absent P waves, irregularly irregular QRS | Atrial fibrillation |
| Sawtooth baseline, regular QRS | Atrial flutter (~300 bpm atrial rate) |
| Short PR + delta wave | Wolff-Parkinson-White (WPW) pre-excitation |
| Prolonged QT | Drug toxicity, electrolyte disturbance, congenital LQTS |
| Peaked T waves | Hyperkalemia |
| Pathologic Q waves | Old myocardial infarction |
ECG Limitations
Computerized ECG analysis has major limitations - it should never be accepted without careful clinician review. ECG interpretation requires correlation with the patient's age, sex, and clinical status. Many errors are errors of omission, which is why a systematic approach is essential.
- Harrison's Principles of Internal Medicine, 22E (2025), Chapter 247 - ECG Leads, Genesis of the Normal ECG, Clinical Interpretation of the ECG
Want me to go deeper on any specific part - arrhythmias, bundle branch blocks, axis, STEMI patterns, or anything else?
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