Understanding and interpreting electrocardiograms (EKGs) is an essential skill for AEMT and Paramedic students, as it plays a critical role in diagnosing and managing cardiac conditions. We’re going to simplify the process by breaking down the key concepts and providing practical tips for accurate electrode placement and EKG reading. Let’s get started!

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Introduction to EKG Waves

An EKG (electrocardiogram) represents the electrical activity of the heart as it contracts and relaxes. The different waves on an EKG correspond to specific phases of the cardiac cycle, and understanding these can provide crucial insights into a patient’s heart health.

P Wave

  • What it is: The first positive deflection on the EKG.
  • What it represents: Atrial depolarization, which is the electrical activity causing the atria to contract.

Isoelectric Line

  • What it is: The flat baseline from which all EKG waves originate and return.
  • Significance: It serves as the reference point for measuring wave deflections.

Q Wave

  • What it is: The first negative deflection in the QRS complex.
  • Significance: Part of the ventricular depolarization process. Note that not everyone has a Q wave, so sometimes the QRS complex may consist only of the RS waves.

R Wave

  • What it is: The first positive deflection following the Q wave (if present) in the QRS complex.
  • Significance: A major component of the ventricular depolarization process.

S Wave

  • What it is: The first negative deflection following the R wave in the QRS complex.
  • Significance: Completes the QRS complex, representing the final phase of ventricular depolarization.

QRS Complex

  • What it represents: Ventricular depolarization, which is the electrical activity causing the ventricles to contract.

T Wave

  • What it is: The positive deflection following the QRS complex.
  • What it represents: Ventricular repolarization, the process of the ventricles relaxing and preparing for the next contraction.Note: Atrial repolarization does not have a visible wave because it is hidden within the QRS complex.

Introduction to EKG Intervals

Intervals on an EKG represent the time periods between specific wave points. These intervals are crucial for understanding the timing and coordination of the heart’s electrical activity.

PR Interval

  • Where to measure: From the beginning of the P wave to the start of the QRS complex.
  • Normal range: 0.12 to 0.20 seconds.
  • Significance: Indicates the time it takes for the electrical impulse to travel from the atria to the ventricles, reflecting AV node function.

ST Segment

  • Where to measure: From the end of the S wave to the beginning of the T wave.
  • Significance: Important for identifying myocardial infarction (heart attack). Look for elevation or depression in this segment, which can indicate ischemia or injury.

QT Interval

  • Where to measure: From the beginning of the Q wave to the end of the T wave.
  • Normal range: Males: 0.35 to 0.45 seconds / Females: 0.36 to 0.46 seconds.
  • Significance: Represents the total time for ventricular depolarization and repolarization. Abnormal QT intervals can indicate a risk of arrhythmias and other cardiac conditions.

By understanding these fundamental components and intervals, you’ll be better equipped to read EKGs and respond effectively to cardiac events in the field. Keep practicing, and soon identifying these waves and intervals will become second nature.

EKG Timing Basics

  • Small Box: Each small box represents 0.04 seconds.
  • Large Box (5 Small Boxes): Each large box represents 0.20 seconds.

Electrode Placement Mnemonic

To easily remember where to place the electrodes, use this mnemonic:

  • White (Snow): Place on the right arm (“Snow over grass”).
  • Green (Grass): Place on the right leg (“Snow over grass”).
  • Black (Smoke): Place on the left arm (“Smoke over fire”).
  • Red (Fire): Place on the left leg (“Smoke over fire”).

Electrode Placement Tips


Position the electrodes on the outer aspect of the humeral heads (upper arm bones). This helps reduce muscle artifact and ensures a more stable signal, avoiding interference from chest muscles or other upper body movements.


Place the electrodes near the ankles. This minimizes motion artifacts from leg movements and provides a stable base for the ECG recording. Ensure the electrodes are positioned similarly on both legs for consistent and accurate readings.

Practical Tips for Students

  1. Skin Preparation: Clean and dry the skin before placing the electrodes. Shave excessive hair and use alcohol wipes to enhance electrode adhesion and signal quality.
  2. Consistent Placement: Always place the electrodes in the same positions for consistency in readings, which is crucial for monitoring changes over time.
  3. Check Connections: Ensure all electrodes are properly connected to the leads and have no loose connections that could affect signal quality.
  4. Monitor Patient Movement: Excessive movement can create artifacts in the ECG reading. Encourage the patient to remain still during the procedure for the most accurate results.

By following these guidelines and using the mnemonics, you can efficiently and accurately place ECG electrodes, leading to better patient care and more reliable ECG interpretations.