Electrocardiogram Study Notes

Electrocardiogram Overview

Definition and Function

  • Electrocardiogram (ECG): A recording of the heart's electrical activity over a period of time, visualizing both the rhythm and the conduction pathways of the heart.
  • Key Functions of ECG:
      - Spontaneous, rapid activation of the myocardium
      - Sequential activation of the myocardium
      - Interpretation of pacemaker activity, the sequence of cardiac activations, the timing of the spread of signals, and the travel path of signals through cardiac muscle.

Cardiac Action Potential

Automaticity and Sequence of Activation

  • Automatic Generation: The action potential begins at the SA node, the heart's natural pacemaker.
  • Sequential Activation:
      - Activation proceeds from atria to ventricles sequentially.
  • Timing: The spread of electrical activity is highly coordinated.

Path of Action Potential

  • Differences in musculature exist between the atria and ventricles, affecting how action potentials travel.
  • Pathway sequence includes:
      1. SA Node
      2. Intranodal Pathways
      3. AV Node
      4. Bundle of His
      5. Left and Right Bundle Branches
      6. Purkinje Fibers

Recording Electrical Activity

Electrocardiograph and Electrodes

  • Electrocardiograph: Device that records electrical activity within the heart.
  • Electrodes: Conductive pads placed on the body surface.
      - Calculate potential differences by placing two electrodes in space (these two form a lead).
      - Leads give different angles of viewing the heart.
      - In any lead, one electrode is defined as exploring (positive) and the other as reference (negative).

ECG Lead Systems

12-Lead ECG System

  • The 12-Lead ECG captures heart electrical activity from multiple angles:
      - Can visualize activity in both horizontal and frontal planes.
      - Leads: 12 leads are derived from 10 electrodes.
      - Limb Leads (6 in total): Includes I, II, III, aVF, aVR, and aVL, exploring electrodes positioned in the frontal plane.
      - Chest Leads: (V1-V6) placed on the anterior chest wall with the reference point in the chest.

Limb Leads

  • Bipolar Limb Leads:
      - Leads I, II, III: Measure potential difference between two electrodes on various limbs.
  • Unipolar Limb Leads: aVR, aVL, aVF register potential difference between a theoretical point (center of Einthoven’s triangle) and the electrode of each limb.

Chest Leads

  • There are six precordial leads signified by V followed by numbers 1 through 6.

Basic Principles of ECG

Understanding Electrical Vectors

  • Electrical Vectors: Movement of charged particles during the cardiac action potential creates electrical vectors.
  • Mean Electrical Vector: The average of all electrical vectors at a specific time point.

Lead Mechanics

  • Exploring Electrode Function: The electrode that