Condensed Overview for Rapid Review
1. Core Principle
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EKG = graph of electrical activity relative to electrode orientation.
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Direction of depolarization + lead axis → determines deflection direction.
2. Waveform Origins (Lead II Reference)
| Wave | Electrical Event | Deflection & Reason |
|---|---|---|
| P | Atrial depolarization (SA → AV) | Upright (toward + electrode) |
| PR Segment | AV nodal delay | Isoelectric |
| Q | Septal depolarization (L→R) | Small negative |
| R | Main ventricular depolarization | Large positive |
| S | Basal depolarization | Negative |
| ST Segment | Fully depolarized ventricles | Isoelectric |
| T | Ventricular repolarization | Upright (negative charges → negative electrode) |
3. Lead Group Overview
| Group | Leads | Plane | Heart Regions Viewed |
|---|---|---|---|
| Limb (I, II, III) | RA, LA, LL | Frontal | Lateral (I), Inferior (II, III) |
| Augmented (aVR, aVL, aVF) | Derived from limb leads | Frontal | aVR: RV, aVL: high lateral LV, aVF: inferior |
| Precordial (V1–V6) | Chest | Horizontal | V1–V2: RV/septum, V3–V4: anterior, V5–V6: lateral LV |
4. Wave Progression
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R wave amplitude ↑ from V1 → V6.
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S wave amplitude ↓ from V1 → V6.
➡️ Indicates transition from right → left ventricular dominance.
5. EKG Paper Metrics
| Box Type | Time | Voltage | Notes |
|---|---|---|---|
| Small (1 mm) | 0.04 sec | 0.1 mV | |
| Large (5 mm) | 0.20 sec | 0.5 mV | |
| Intervals: |
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PR < 0.20 s
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QRS < 0.12 s
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QTc < 430 ms (♂), < 460 ms (♀)
6. High-Yield Clinical Concepts
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Lead II: best for rhythm (matches mean cardiac vector).
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Pathologic Q wave → possible infarct.
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Prolonged QT → risk of torsades.
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aVR inverted relative to others.
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Isoelectric ST → normal; deviation → ischemia/injury.
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