Approach to EKG

Sinoatrial (SA) node

• 1 mm cell collection in upper right atrium with intrinsic depolarization rate at 60-100 beats/min
• Initiates atrial systole and generates P wave

• Located near inferior portion of interatrial septum
• Provides delay for atria to send blood to ventricles before initiating ventricular systole
• Protects ventricles from excessive stimulation from atria
• PR interval indicates how well AV node is functioning

• Left bundle
  • Divides into left anterior and posterior fascicles that differentiate into Purkinje fibers.
  • Supplied by left anterior descending (LAD) artery
• Right bundle
  • Innervates right ventricle to depolarize myocardium
• Bundles and Purkinje fibers allow for smooth depolarization of ventricles and correspond to QRS complex

• P wave 
  • Indicates impulse traveled from SA node to AV node
  • Symbolizes atrial depolarization
• QRS complex 
  • Indicates ventricular depolarization
  • Impulse has traveled from atria through AV node and into His bundles system
• T wave 
  • Indicates ventricular repolarization
  • Impulse has traveled ventricles and back to atria
• U wave 
  • Delayed repolarization abnormality
  • Usually seen in hypokalemia
• Vector of impulse
• Impulse traveling towards a lead creates upward (positive) deflection
• Impulse traveling away from lead creates downward (negative) deflection

• Paper speed
  • Typically at 25 mm/sec
• Small boxes
  • 1 mm representing 40 msec
• Large boxes
  • 5 small boxes (5 mm) representing 0.2 sec
• Vertical boxes
  • Every 10 mm represents 0.1 mV of electrical potential

• Limb leads (I, II, III, aVF, aVR, and aVL) 
  • Look at heart from frontal plane
• Precordial leads (V1-V6) 
  • Look at heart from vertical plane