Condensed Overview for Rapid Review

Condensed Overview for Rapid Review

1. Core Principle

• EKG = graph of electrical activity relative to electrode orientation.

• 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

• R wave amplitude ↑ from V1 → V6.

• 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:

• PR < 0.20 s

• QRS < 0.12 s

• QTc < 430 ms (♂), < 460 ms (♀)

6. High-Yield Clinical Concepts

• Lead II: best for rhythm (matches mean cardiac vector).

• Pathologic Q wave → possible infarct.

• Prolonged QT → risk of torsades.

• aVR inverted relative to others.

• Isoelectric ST → normal; deviation → ischemia/injury.