Domain 8.3EKG and Cardiovascular Testing

EKG Interpretation Basics

Last updated June 16, 2026

Overview

While Medical Assistants do not diagnose, you must recognize a normal tracing, calculate heart rates, and identify life-threatening emergencies to ensure patient safety.

1. The Waveforms: The Alphabet of EKG

Feature Represents Normal
P Wave Atrial Depolarization (Contraction) Upright and rounded.
QRS Complex Ventricular Depolarization (Contraction) Tall spike. < 0.12 sec width.
T Wave Ventricular Repolarization (Relaxation) Upright, follows QRS.
PR Interval Time from Atria to Ventricles 0.12 - 0.20 sec.
ST Segment Pause between Depolarization/Repolarization Flat line. Elevation/Depression = Ischemia/Infarct.

2. The Grid and Rate Calculation

The Grid (at 25 mm/s)

  • Small Box: 0.04 seconds.
  • Large Box: 0.20 seconds (5 small boxes).

Calculating Heart Rate

  1. The 1500 Method (Precise):

    • Use for Regular Rhythms.
    • Count small boxes between two R-waves.
    • Formula: 1500 ÷ Number of Small Boxes.
    • Example: 20 small boxes. 1500 ÷ 20 = 75 bpm.

  2. The 6-Second Method (Estimating):

    • Use for Irregular Rhythms.
    • Count the number of QRS complexes in a 6-second strip (30 large boxes).
    • Formula: Number of QRS x 10.
    • Example: 8 QRS complexes x 10 = 80 bpm.

3. Rhythms to Recognize

Sinus Rhythms (Normal P-waves present)

  • Normal Sinus Rhythm: 60–100 bpm. Regular.
  • Sinus Bradycardia: < 60 bpm. Regular.
  • Sinus Tachycardia: > 100 bpm. Regular.

Common Arrhythmias

  • Atrial Fibrillation (A-Fib): Irregularly irregular. No distinct P-waves. Baseline is chaotic/quivering.
  • Atrial Flutter: Sawtooth pattern (F-waves).
  • Premature Ventricular Contraction (PVC): A single, wide and bizarre QRS beat that interrupts the rhythm. No P-wave precedes it.

4. Medical Emergencies

Action: If you see these, STOP and alert the provider IMMEDIATELY.

  1. Ventricular Tachycardia (V-Tach): Rapid, wide, regular QRS complexes. "Tombstone" appearance.
  2. Ventricular Fibrillation (V-Fib): Chaotic, disorganized quivering. No identifiable waves. Cardiac Arrest.
  3. Asystole: Flat line. No electrical activity.

5. Patient Safety During Testing

  • Role: Alert & Await. Recognize distress, stop the test, get the provider.
  • Stop Signs: If a patient reports Chest Pain, Dizziness, or Shortness of Breath during an EKG or Stress Test, stop immediately. Chest pain means NO gain.