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I now have comprehensive information from Goldman-Cecil Medicine, Medical Physiology, and Harriet Lane Handbook. Here is a thorough guide to ECG basics and interpretation:
ECG Basics & Interpretation
1. What Is an ECG?
An electrocardiogram (ECG/EKG) is a graphic recording of the electrical activity of the heart over time. It is recorded on paper (or digitally) at a standard speed of 25 mm/sec, with voltage on the vertical axis (10 mm = 1 mV) and time on the horizontal axis.
Grid reference:
- 1 small box = 1 mm = 0.04 sec (40 msec)
- 1 large box = 5 mm = 0.2 sec (200 msec)
- A standard ECG records 10 seconds of cardiac activity
2. The Cardiac Conduction System
The electrical impulse follows this pathway:
SA node → Atrial muscle (+ Bachmann bundle to left atrium) → AV node (physiologic delay) → Bundle of His → Right & Left bundle branches (left divides into anterior and posterior fascicles) → Purkinje fibers → Ventricular muscle
3. The ECG Waves — What Each Represents
| Wave / Segment | Electrical Event |
|---|---|
| P wave | Atrial muscular depolarization |
| PR interval | Conduction through atria → AV node → His-Purkinje |
| QRS complex | Ventricular muscular depolarization |
| ST segment | Early ventricular repolarization (isoelectric phase) |
| T wave | Ventricular repolarization |
| U wave | Small deflection after T wave (same polarity); exact origin debated |
| J point | Junction between end of QRS and beginning of ST segment |
Note: Atrial repolarization is buried within the QRS complex (too low amplitude to see).
4. Normal Intervals & Values
(Goldman-Cecil Medicine, Table 42-1)
| Parameter | Normal Range |
|---|---|
| Heart rate | 50–100 bpm |
| P wave duration | < 0.12 sec (120 msec) |
| PR interval | 0.09–0.20 sec (90–200 msec) |
| QRS duration | 0.075–0.11 sec (75–110 msec) |
| QTc (males) | 0.39–0.45 sec (390–450 msec) |
| QTc (females) | 0.39–0.46 sec (390–460 msec) |
| QRS axis | −30° to +90° |
5. Calculating Heart Rate
Two quick methods:
- Formula: HR = 60,000 ÷ RR interval (msec), or HR = 300 ÷ number of large boxes between R waves
- Shortcut (large boxes between R waves):
- 1 box → 300 bpm
- 2 boxes → 150 bpm
- 3 boxes → 100 bpm
- 4 boxes → 75 bpm
- 5 boxes → 60 bpm
6. Systematic Approach to Reading an ECG
Use this 7-step framework every time:
| Step | What to Assess |
|---|---|
| 1. Rate | Calculate from RR interval |
| 2. Rhythm | Regular vs. irregular? P before every QRS? |
| 3. P waves | Present, uniform, upright in I & aVF? |
| 4. PR interval | 0.09–0.20 sec? Constant? |
| 5. QRS complex | Duration < 0.12 sec? Morphology normal? |
| 6. ST segment & T wave | Elevation/depression? T wave inversion? |
| 7. QT interval | Calculate QTc; normal per gender |
Sinus rhythm criteria: Every QRS preceded by a P wave + upright P in leads I and aVF + normal, consistent PR interval.
7. The 12 Leads — What They "Look At"
A standard 12-lead ECG uses electrodes placed on limbs and chest to view the heart from 12 angles:
| Lead Group | Leads | View |
|---|---|---|
| Inferior | II, III, aVF | Inferior wall (RCA territory) |
| Lateral | I, aVL, V5, V6 | Lateral wall (LCx territory) |
| Anterior/Septal | V1–V4 | Anterior wall (LAD territory) |
| Right-sided | aVR | Cavity/right heart |
8. Key Intervals — Clinical Significance
- Long PR (> 0.20 sec): First-degree AV block (delayed AV node conduction)
- Short PR: Pre-excitation (Wolff-Parkinson-White), junctional rhythm
- Wide QRS (> 0.12 sec): Bundle branch block, paced rhythm, hyperkalemia, accessory pathway
- Long QTc: Risk of torsades de pointes (caused by drugs, electrolyte abnormalities, congenital LQTS)
- ST elevation: Myocardial infarction (STEMI), pericarditis, Brugada
- ST depression / T inversion: Ischemia, NSTEMI, strain pattern
9. QTc Calculation (Bazett's Formula)
$$QTc = \frac{QT}{\sqrt{RR}}$$
(QT and RR both measured in seconds)
Measure QT from the beginning of the QRS to the end of the T wave. Use leads II, V5, or V6 and average 3–5 cycles.
Quick Abnormality Summary
| Finding | Think of |
|---|---|
| Delta wave + short PR | WPW pre-excitation |
| Tall peaked T waves | Hyperkalemia |
| Flattened T + U wave | Hypokalemia |
| J wave (Osborn wave) | Hypothermia |
| Diffuse ST elevation, saddle-shaped | Pericarditis |
| ST elevation in specific territory | STEMI |
| Wide QRS + LBBB pattern | LV conduction delay, cardiomyopathy |
Sources: Goldman-Cecil Medicine 26th Ed., Chapter 42 (Electrocardiography); Medical Physiology (Boron & Boulpaep); Harriet Lane Handbook 23rd Ed., Chapter 7.
Would you like to go deeper into any specific topic — such as bundle branch blocks, axis deviation, ischemia/infarction patterns, or arrhythmia recognition?
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