Determining The Electrical Axis Of The Heart

I. Principles To Remember:

• Principle # 1 : When a positive sensing electrode sees an electrical signal coming straight toward it, the EKG machine will write the highest amplitude deflection on the EKG paper
• Principle # 2 : When the positive sensing electrode sees an electrical signal crossing it on a perpendicular path, the EKG machine will write the smallest amplitude deflection on the EKG paper.

II. Quadrants:

• If the electrical axis of the heart is between +0 to +90 degrees, the heart's electrical vector or axis is said to be in the normal quadrant
• If the electrical axis of the heart is between -0 to -90 degrees, the heart's electrical vector or axis is said to be in left axis deviation
• If the electrical axis of the heart is between +90 to +180 degrees, the heart's electrical vector or axis is said to be in right axis deviation
• If the electrical axis of the heart is between -180 to -90 degrees, the heart's electrical vector or axis is said to be in extreme right axis deviation

III. How Do We Determine The Heart's Electrical Axis ?

Look at Lead I and aVF

• Step # 1 : Determine if the overall QRS complex deflection is grossly positive or grossly negative in Lead I and in Lead aVF
• Step # 2 :
  • If the QRS complexes are both positively deflected (up,up), then the electrical axis for the heart is in the normal quadrant
  • If the QRS complex for Lead I is positively deflected but the QRS complex for aVF is negatively deflected (up,down), then the electrical axis for the heart is said to be left axis deviated
  • If the QRS complex in Lead I is negatively deflected but the QRS complex for aVF is positively deflected (down,up), then the electrical axis for the heart is said to be right axis deviated
  • If the QRS complex in lead I and in aVF are both negatively deflected (down,down), then the electrical axis for the heart is said to be extreme right axis deviated

It is not enough to merely determine the gross overall quadrant for the heart's electrical vector. It is better to be able to determine where in the quadrant the axis is located.

Here is how you "fine tune" axis determination

• Determine the gross quadrant
• Find the smallest QRS deflection in the six limb leads
• Rotate 90 degrees from the smallest electrode into the quadrant that Lead I and aVF said the axis would be located
• Determine if the QRS complex of the smallest deflected lead is grossly positive or grossly negative
• If the R wave to S wave difference is 0 - 1 mm, make no fine adjustments in the axis degree setting.
• If the R wave to S wave difference is between 2-3 mm, then adjust the axis 10 degrees towards the + or - pole of the smallest QRS

• If the smallest QRS deflection is grossly negative, the fine adjustment of 10 degrees is swung toward the negative pole of that lead
• If the smallest QRS deflection is grossly positive, the fine adjustment of 10 degrees is swung toward the positive pole of that lead
• If the R wave to S wave difference is between 4-5 mm, then adjust the axis 15 degrees towards the + or - pole of the smallest QRS