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Determining the Cardiac Axis of Heart

 

Objectives:

  • The frontal plane and the Hexaxial Reference System
  • Basic considerations
  • Method 1: Figuring Out the Electrical Axis when an Equiphasic Complex is Present
  • Method 2: Figuring Out the Electrical Axis when an Equiphasic Complex is NOT Present

The Frontal Plane and the Hexaxial Reference System

  • The frontal plane is represented by the six limb leads. The position of the limb leads and the location of the different quadrants in the frontal plane are shown. Note that the leads are 30° apart. The normal axis in the adult extends from  -30° to +90°, thus  -1° to -30 ° is considered normal axis.


Basic Considerations

  • Before attempting to determine the axis of any deflection in the ECG, the location of all the six leads in the frontal plane as well as the location of the positive and negative terminals of each lead should be mastered
  • The ECG deflection is maximally upright if the flow of current is directed toward the positive side of the lead and is maximally inverted if the flow of current is directed toward the negative side.
  • Thus, if the flow of current is parallel to lead I (0° to 180°), lead I will record the tallest deflection if the flow of current is directed toward 0° and the deepest deflection if the flow of current is directed toward 180° (Fig: A, B).
  • The lead perpendicular to lead I will record an isoelectric complex. Isoelectric or equiphasic implies that the deflection above and below the baseline are about equal. Since lead aVF is perpendicular to lead I, lead aVF will record an isoelectric deflection (Fig:C,D)


Method 1: Figuring Out the Electrical Axis when an Equiphasic Complex is Present

  • When an isoelectric QRS complex is present in any lead in the frontal plane, the axis of the QRS complex is perpendicular to the lead with the isoelectric complex.
    • Lead I is perpendicular to lead aVF
      • When an equiphasic QRS complex is recorded in lead I (0°), the axis of the QRS complex is +90° or -90°. 
        • lead aVF will register the tallest deflection if the current is directed toward the positive side of aVF at +90° (A) and the deepest deflection if the current is directed toward –90° , away from the positive side of aVF (B)
      • Similarly, when an equiphasic QRS complex is recorded in lead aVF (+90°), the axis of the QRS complex is 0° or 180° .
    • Lead II is perpendicular to lead aVL
    • Lead III is perpendicular to lead aVR



Some Examples:

  • Isoelectric Deflection in Lead I
    • Because lead I is perpendicular to aVF, and lead aVF has a tall complex, the axis is +90°
  • Isoelectric Deflection in aVL
    • Because lead aVL is perpendicular to lead II, and lead II shows the tallest deflection, the axis is +60°
  • Isoelectric Deflection in aVR
    • Because aVR is perpendicular to lead III, and lead III has the deepest complex, the axis is -60° . Note that tall R waves are present in aVL (-30), which is the negative side of lead III





Method 2: Figuring Out the Electrical Axis when an Equiphasic Complex is NOT Present

  • Not all ECGs, however, will have an isoelectric complex. If an isoelectric complex is not present, the mean QRS axis can be estimated just as rapidly by the following method.
  • Select the smallest QRS complex: 
    • The axis is obtained using the same method as calculating the axis when an isoelectric complex is present.
      • (See figure below) Lead aVL is selected because the complex is the smallest and is almost isoelectric. Lead aVL is perpendicular to lead II. Because lead II shows the tallest complex, the axis is approximately 60°. Adjustment has to be made to correct for the actual axis because the complex in lead aVL is not actually isoelectric. 
      • Because aVL is negative (R <S), the axis is adjusted further away from 60°, thus the axis is approximately 70° rather than 60° .
      • Had aVL been positive (R >S), the axis is adjusted closer to 50° rather than 70° .



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