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ECG normal electrocardiogram waveform PQRST labeled diagram
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Here is a comprehensive guide to ECG basics and interpretation, sourced from Harrison's Principles of Internal Medicine 22E and Kaplan & Sadock's Comprehensive Textbook of Psychiatry.
What is an ECG?
An ECG (electrocardiogram) is a graphical record of the heart's electrical activity, detected by metallic electrodes on the limbs and chest wall. It is noninvasive, inexpensive, and detects arrhythmias, conduction disturbances, myocardial ischemia/infarction, electrolyte disturbances, and drug toxicities. — Harrison's, Ch. 247
The ECG Waveform
Each wave reflects a specific electrical event:
| Wave / Complex | Electrical Event |
|---|---|
| P wave | Atrial depolarization |
| QRS complex | Ventricular depolarization |
| ST segment | Ventricular plateau (isoelectric) |
| T wave | Ventricular repolarization |
| U wave | Thought to reflect repolarization of Purkinje fibers (small, often absent) |
| J point | Junction between end of QRS and start of ST segment |
Atrial repolarization is not visible — it is buried within the QRS complex.
Key Intervals & Normal Values
| Interval | What It Measures | Normal Range |
|---|---|---|
| PR interval | Atrial → ventricular conduction (includes AV node delay) | 120–200 ms (3–5 small boxes) |
| QRS duration | Ventricular depolarization time | ≤100–110 ms (<2.5 small boxes) |
| QT interval | Total ventricular depolarization + repolarization | Rate-dependent; QTc ≤450 ms (men), ≤460 ms (women) |
| RR interval | Time between consecutive beats | Used to calculate heart rate |
Heart rate calculation:
- Divide 300 by the number of large boxes between two consecutive R waves
- Or count QRS complexes in a 10-second strip × 6
The 12 Leads
The 12 ECG leads view the heart from different angles:
- 6 limb leads (I, II, III, aVR, aVL, aVF) — frontal plane
- 6 chest leads (V1–V6) — horizontal plane
Each lead records a positive deflection when depolarization moves toward its positive pole, and a negative deflection when it moves away.
Systematic Approach to ECG Interpretation
Harrison's recommends analyzing 14 parameters in every ECG:
- Standardization / calibration — Is the paper speed 25 mm/s? Is 1 mV = 10 mm?
- Rhythm — Sinus vs. non-sinus; regular vs. irregular
- Heart rate — Bradycardia (<60 bpm) or tachycardia (>100 bpm)?
- PR interval — Prolonged (heart block) or short (pre-excitation)?
- QRS duration — Wide (bundle branch block, ventricular origin) or narrow?
- QT/QTc interval — Prolonged = risk of torsades de pointes
- Mean QRS electrical axis — Normal, left axis deviation, right axis deviation?
- P waves — Present before every QRS? Normal morphology?
- QRS voltages — Low voltage or high voltage (hypertrophy)?
- Precordial R-wave progression — R grows V1→V6?
- Abnormal Q waves — Pathological Q waves = prior infarction?
- ST segments — Elevation or depression?
- T waves — Inversion, flattening, or peaked?
- U waves — Prominent U wave = hypokalemia?
Rhythm Assessment
| Feature | Significance |
|---|---|
| P wave before every QRS | Normal sinus rhythm |
| No P waves | Atrial fibrillation or junctional rhythm |
| Wide QRS (>100 ms) | Bundle branch block or ventricular origin |
| Narrow complex tachycardia | Supraventricular origin |
| Wide complex tachycardia | Ventricular tachycardia until proven otherwise |
Key ST/T Wave Changes
| Finding | Common Causes |
|---|---|
| ST elevation | STEMI, pericarditis, myocarditis, early repolarization, LV aneurysm, hyperkalemia |
| ST depression | Ischemia, digoxin effect |
| T-wave inversion | Ischemia, pericarditis, PE, subarachnoid hemorrhage, electrolyte disturbances |
| Peaked T waves | Hyperkalemia (earliest ECG sign), hyperacute MI |
| QT prolongation | Hypokalemia, hypomagnesemia, drugs (antipsychotics, TCAs), hypothyroidism |
ECG Grid Basics
- Small box = 40 ms (0.04 s) horizontally; 0.1 mV vertically
- Large box = 200 ms (0.20 s) horizontally; 0.5 mV vertically
- Standard speed = 25 mm/s; standard calibration = 1 mV = 10 mm
Sources: Harrison's Principles of Internal Medicine, 22E, Ch. 247 (Goldberger); Kaplan & Sadock's Comprehensive Textbook of Psychiatry, Ch. 27.2
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