The 12-Lead EKG Chapter 12

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The 12-Lead EKGChapter 12

• Robert J. Huszar, MD
• Instructor
• Patricia L. Thomas, MBA, RCIS

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Outline

• Electrical Axes and Vectors
• The electrical current, vectors, and the lead
  axis
• The hexaxial reference figure
• The QRS axis
• Determination of Axis

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Electrical Axes Vectors

• Instantaneous cardiac vector-the electrical
  current generated by the depolarization or
  repolarization of the atria or ventricles at any
  given moment
• Graphically as an arrow that has magnitude,
  direction, and polarity
• Length of the shaft of the arrow represents the
  magnitude of the electrical current
• Orientation or position of the arrow indicates
  the direction of flow of the electrical current
• The tip of the arrow represents the positive pole
  of the electrical current
• The tail is the negative pole

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Magnitude, Direction, and Polarity
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Normal DepolarizationFigure 12-2

• Interventricular Septum 1 2
• Right Ventricle 3
• Left Ventricle (thick lateral portion) 3
• Left Ventricular (lateral/posterior/base) 4

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The Hexaxial Reference Figure

• Composite of the two triaxial reference figures
  (limb leads augmented leads)
• Purpose is to determine the direction of the QRS
  axis
• Each - pole is assigned a degree number 00 to
  1800
• The top are given numbers
• The bottom are given numbers
• NOT TO BE CONFUSED WITH the - poles of the
  lead axes

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Electrical Axis Calculation

• Plotting of the net direction or the electrical
  axis
• QRS axis is most important it is correlates with
  the anatomy of the heart
• Pathophysiology causes a shift in the axis

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Calculate the Axis

• 1. Calculate the algebraic sum of QRS in Leads I
  III
• 2. Plot the vector results on a triaxial
  reference system
• 3. Draw the perpendicular until Lead I III
  cross.
• 4. Draw a separate line from the center to the
  intersection of the two perpendiculars gives the
  electrical axis.
• 5. Determine location (normal, RAD or LAD)

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Practical Method EA
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THE ENDOFCHAPTER 12

• Hauszar Robert, Basic Dysrhythmias,
  Interpretation Management, Third Edition,
  Mosby, Inc. 2002, pp. 171-187.
• Scheidt, Stephen M.D., Basic Electrocardiography,C
  IBA-GEIGY Pharmaceuticals, 1986
• pp. 3,18,19