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Overview
  • P wave 
    • represents atrial depolarization
  • PR Interval
    • interval from initial depolarization of atria to initial depolarization of ventricles
    • interval varies with conduction velocity through AV node
      • ↑ conduction velocity through AV node, ↓ PR interval
        • ↑ conduction velocity through AV node via sympathetic nervous system
      • ↓ conduction velocity through AV node, ↑ PR interval
        • ↓ conduction velocity through AV node via parasympathetic nervous system, heart block
    • interval < 200 msec normally
  • QRS complex
    • represents ventricular depolarization
    • hides atrial repolarization
    • complex < 120 msec normally
    • hyperkalemia → widened QRS 
  • T wave 
    • represents ventricular repolarization
    • inversion indicates recent myocardial infarction (MI)
    • hyperkalemia → peaked T waves
    • hypokalemia → flattened T waves
  • QT interval
    • interval from first ventricular depolarization to last ventricular repolarization
    • represents mechanical contraction of ventricles 
  • ST segment
    • isoelectric portion of QT interval
    • represents period when ventricles are depolarized
    • correlates with phase 2 (plateau) of ventricular action potential 
  • U wave
    • a sign of hypokalemia and bradycardia
  • R-R interval
    • represents cycle length
    • heart rate (HR) = 1 / cycle length
 

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