Basics of EKGs

Introduction

• Purpose: Understand the basics of EKGs (Electrocardiograms) including physics, physiology, and reading a 12-lead EKG.
• Initial Concept: Idea to use muscle tissue with electrodes to study electrical activity.

Basic Concepts

Depolarization and Electrodes

• Depolarization: Positive ions like calcium and sodium flood into cells, causing them to flip from negative to positive.
• Electrodes: Placed on muscle tissue to read electrical signals.
  • Positive charge moving towards positive electrode = upward deflection on EKG.
  • Positive charge moving away from positive electrode = downward deflection on EKG.
  • Negative charge moving towards negative electrode = upward deflection on EKG.
  • Isoelectric Line: No net movement of electrical activity or electrical activity moving perpendicular to axes.

Lead Systems and Waveforms

Lead II Example

• Lead Placement:
  • Negative electrode on the right arm.
  • Positive electrode on the left leg.
• P Wave: Indicates atrial depolarization moving towards the AV node from the SA node.
  • PR Segment: Reflects delayed conduction through the AV node.
  • PR Interval: Begins at the start of the P wave to the start of the QRS complex.

QRS Complex

• Q Wave: Indicates septal depolarization moving away from the positive electrode.
• R Wave: Large upward deflection due to depolarization through the ventricles.
• S Wave: Downward deflection due to depolarization moving away from lead.

T Wave

• T Wave: Indicates ventricular repolarization.
  • Negative charges moving towards negative electrode produce upward deflection.

ST Segment

• ST Segment: Entire ventricular myocardium is depolarized; no net movement of charge, thus an isoelectric line.

Leads and Their Positioning

Limb Leads

• Lead 1: Measures activity from right arm to left arm.
• Lead 2: Measures from right arm to left leg.
• Lead 3: Left arm to left leg.

Augmented Unipolar Limb Leads

• aVR: Measures from right arm to the negative of the other leads.
• aVL: Measures from left arm.
• aVF: Measures from the foot.

Precordial Leads (Chest Leads)

• V1, V2, V3: Primarily show right ventricular and septal activity.
• V4, V5, V6: Show left ventricular activity.

Importance of Lead Positions

• Leads: Orientation and directions determine specific areas of the heart they represent.
• Examples: For example, leads looking towards the lateral wall give insights into that portion.

EKG Strip Components

Large Box

• Dimensions: 5 mm (width and height).
• Time Duration: 0.20 seconds per large box.
• Voltage: 0.5 millivolts per large box.

Small Box

• Dimensions: 1 mm (width and height).
• Time Duration: 0.04 seconds per small box.
• Voltage: 0.01 millivolts per small box.

Key Intervals

• PR Interval: Should be less than 0.20 seconds (1 large box).
• QRS Complex: Should be less than 0.12 seconds (3 small boxes).
• QT Interval: Should be less than 430 milliseconds for males and less than 460 milliseconds for females.