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Determining the Electrical Axis of the Heart

Determination of the heart axis

Positive and Negative ECG Deflections

• The cardiac electrical vector must
• always be projected onto the electrical axis of the ECG lead
  
  
• Positive ECG deflection occurs
• when the vector points toward the surface electrode (+)
• Negative ECG deflection occurs
• when the vector points away from the surface electrode (+)

1. Determining the Quadrant (Leads I, aVF)

• Limb Leads (I, aVF) are perpendicular to each other
• They divide the heart into 4 quadrants in the hexaxial system

Main Cardiac Vector and the 4 Quadrants

• Main vector points from the electrical center to the limb leads
• Based on the positive or negative deflection in the leads (I, aVF)
• It is possible to determine which quadrant the main vector points to
• Left Lower Quadrant (yellow)
• Leads I and aVF are positive
• Because the main vector points toward lead I and toward lead aVF
• Left Upper Quadrant (red)
• Lead I is positive and aVF is negative
• Because the main vector points toward lead I and away from lead aVF
• Right Lower Quadrant (green)
• Lead I is negative and aVF is positive
• Because the main vector points away from lead I and toward lead aVF
• Right Upper Quadrant (blue)
• Lead I is negative and aVF is negative
• Because the main vector points away from both lead I and lead aVF

Biphasic QRS Complex

• Finally, identify the equiphasic (isoelectric) QRS complex in a certain lead
• This lead is then perpendicular to the electrical vector

Biphasic QRS Complex

• If a single vector is perpendicular to the ECG lead
• Then no deflection will be seen on the ECG
• However, during ventricular depolarization, many depolarization vectors are generated
• The ventricular vector rotates around the central axis
• We see 3 main summation vectors on the ECG as a QRS complex
• Biphasic QRS Complex
• Occurs in a lead that is almost perpendicular to the main vector
• The main vector rotates in 3D space during ventricular depolarization
• During the rotation, it points "a little" TOWARD and AWAY FROM the perpendicular lead
• Equiphasic (Isoelectric) QRS
• The QRS complex has exactly equal positive and negative deflections
• The lead where the equiphasic QRS complex is seen is exactly perpendicular to the main vector
  
  
• Electrical Axis (vector) can be determined with an accuracy of 10°
• If the vector points exactly between leads (e.g., at exactly 45°)
• then the leads aVL and III will be "almost equiphasic" (they will be more positive)

Lead II and Axis Deviation

• If the vector points to the upper left quadrant (negative aVF and positive lead I)
• Then lead II is used to determine the cardiac axis

Cardiac Axis in the Upper Left Quadrant

• (A) Equiphasic QRS in Lead II
• The positive and negative deflections are equal
• The vector is exactly perpendicular to Lead II (Lead II has +60°)
• This is a very rare situation
• The cardiac axis is exactly -30°
• (B) Normal Cardiac Axis
• In Lead II, the biphasic QRS is more positive
• Because the vector partially points TOWARD Lead II
• (C) Left Axis Deviation
• In Lead II, the biphasic QRS is more negative
• Because the vector partially points AWAY FROM Lead II

Normal Cardiac Axis

Normal Cardiac Axis

• The vector points to the lower left quadrant
• Leads I and aVF are positive
• Equiphasic QRS in aVL
• The vector is perpendicular to lead aVL
• Highest R wave in Lead II
• The vector's axis is +60°

Right Axis Deviation

Right Axis Deviation

• The vector points to the lower right quadrant
• Lead I is negative and aVF is positive
• Lead aVR is negative
• The vector points away from lead aVR
  (i.e., somewhere between leads III and aVF)
• R wave III > aVF
• The vector is closer to lead III
• The vector's axis is +110°