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Updated: Jul 4 2019

Electrocardiogram (ECG)

  • Introduction
    • The electrocardiogram (ECG) records the flow of electrical impulses throughout the heart
    • Electrical currents flow from negatively charged areas to positively charged areas
      • depolarization currents flowing towards a positive electrode will lead to an upward deflection seen in the ECG reading
      • repolarization currents flowing away from the positive electrode will lead to a downward deflection seen in the ECG reading
      • the ECG records the sum of the electrical forces that flow through the heart
        • the ECG deflections depend on how these electrical forces align to a specific ECG lead
          • e.g., lead aVR, a right-arm electrode, will demonstrate a positive deflection when electrical activity points towards the right arm
  • C1: Snapshot: Y C2: Content/grammar: Y C3: Images/Videos: N C4: Questions: Y C5: References: N ECG Lead Reference
    • There are 6 limb leads and 6 chest lead, which comprise the 12-lead ECG
      • limb leads
        • aVR, aVL, and aVF
        • I, II, III
        • the limb leads read electrical forces in the frontal view
      • chest (precordial) leads
        • V1-V6
        • the precordial leads read electrical forces in the perpendicular plane
  • Conduction Pathway
    • Depolarization normally begins at the sinoatrial (SA) node
      • the SA node is located at the junction of the superior vena cava and right atrium
    • Depolarization from the SA node spreads to the right and left atria
    • After the right and left atria, the wave of depolarization reaches the atrioventricular (AV) node
      • there is an expected delay in the transmission of depolarization to the ventricles at the AV node
    • The impulse then goes to the bundle of His and then the right and left Purkinje fibers
    • Pacemaker rates
      • SA > AV > bundle of His/Purkinje/ventricles
    • Speed of transmission
      • bundle of His = Purkinje fibers > atria > ventricles > AV node
  • ECG And Conduction Pathway
      • ECG Basics
      • ECG Finding
      • Information
      • P wave
      • Represents atrial depolarization
      • PR interval
      • Represents the time from the start of atrial depolarization to the start of ventricular depolarization
      • Normally < 200 msec
      • QRS complex
      • Represents ventricular depolarization
      • Normally < 120 msec
      • ST segment
      • Represents an isoelectric point in ventricular depolarization
      • Correlates with phase 2 (plateau) of ventricular action potential
      • T wave
      • Represents ventricular repolarization
      • Becomes peaked in hyperkalemia and flattened in hypokalemia
      • U wave
      • Believed to represent ventricular repolarization but during the late phaseBecomes prominent in hypokalemia
      • J point
      • The point between the end of the QRS complex and the beginning of the ST segment
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Private Note

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