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EKG Basics and Lead Placement

Jul 10, 2025

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Overview

This lecture covers the foundational physics and physiology behind EKGs, explains how to interpret basic EKG waveforms, and introduces the 12-lead EKG system, lead placement, and measurement basics.

Basic Principles of EKGs

  • EKGs measure electrical activity in the heart using electrodes.
  • Positive charge moving toward a positive electrode creates an upward (positive) deflection.
  • Positive charge moving away from a positive electrode creates a downward (negative) deflection.
  • No net electrical movement or perpendicular movement to the lead axis produces an isoelectric (flat) line.

EKG Waveform Components

  • P wave: Atrial depolarization (main vector from SA node to AV node).
  • PR segment: Delay at AV node, no net charge movement (isoelectric line).
  • PR interval: From start of P wave to end of PR segment.
  • Q wave: Initial ventricular septal depolarization (left to right, away from lead II).
  • R wave: Main ventricular depolarization (towards lead II, left ventricle dominates).
  • S wave: Final depolarization of ventricles at the base (away from lead II).
  • ST segment: Ventricles fully depolarized, no net movement (isoelectric).
  • T wave: Ventricular repolarization (negative charge moving to negative electrode, upward deflection).

12-Lead EKG Overview

  • 3 limb leads: I, II, III (Einthoven’s triangle—right arm, left arm, left leg).
  • 3 augmented unipolar limb leads: aVR, aVL, aVF.
    • aVR: Opposite of lead II; views right ventricle and basal septum.
    • aVL: Lateral view, high lateral wall of left ventricle.
    • aVF: Inferior view, inferior wall of heart.
  • 6 precordial (chest) leads: V1–V6 placed on chest, assess horizontal/transverse plane.

Precordial Lead Placement & Interpretation

  • V1: Right 4th intercostal space, parasternal; V2: Left 4th intercostal, parasternal.
  • V3: Between V2 and V4; V4: Left 5th intercostal, midclavicular.
  • V5: Left 5th intercostal, anterior axillary; V6: Left 5th intercostal, midaxillary.
  • R wave increases from V1 to V6; S wave decreases.
  • V1–V3: Right ventricle; V2–V3: Basal septum; V2–V4: Anterior wall; V5–V6: Lateral wall.

EKG Strip Measurement Basics

  • Large box: 5mm = 0.20 sec (width), 0.5 mV (height).
  • Small box: 1mm = 0.04 sec (width), 0.1 mV (height).
  • 1 large box = 5 small boxes.
  • PR interval: <0.20 sec (1 large box).
  • QRS complex: <0.12 sec (3 small boxes).
  • QT interval: <430 ms (men), <460 ms (women); concern if near 500 ms.

Key Terms & Definitions

  • Depolarization — Change in cell charge to positive, spreading electrical activity.
  • Repolarization — Return of cell charge to negative after depolarization.
  • Isoelectric Line — Flat EKG phase with no net electrical movement.
  • Vector — Direction of mean electrical activity.
  • Lead — Electrode pairing giving a specific heart view.
  • QRS Complex — Ventricular depolarization on EKG.

Action Items / Next Steps

  • Memorize EKG waveform components and what each represents.
  • Learn the 12-lead placements and which parts of the heart they view.
  • Practice measuring EKG intervals using box widths.
  • Prepare for deeper EKG case interpretation in future lessons.

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