QRS axis

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«Step 3: Conduction (PQ, QRS, QT, QTc) Step 5: P wave morphology»


Author(s) I.A.C. van der Bilt, MD
Moderator T.T. Keller
Supervisor
some notes about authorship


Click and drag the arrow in the above animation to change the heart axis and see how the ECG changes.


The electrical heart axis is an average of all depolarizations in the heart. The depolarization wave begins in the right atrium and proceeds to the left and right ventricle. Because the left ventricle wall is thicker than the right wall, the arrow indicating the direction of the depolarization wave is directed to the left.


For a lot of people, this is a difficult concept. The theoretical part seems difficult but by doing it a lot, the reader will find it becoming clear.


How do you determine the electrical heart axis

ECG lead angulation.png
The heart axis indicates the average direction of the depolarization wave. A normal heart axis, the picture shows an example, is between -30 and +90 degrees. In this example, the heart axis is +45 degrees.
Hart axis.png

When you average all electrical signals from the heart, you can indicate the direction of the average electrical depolarization with an arrow (vector). This is the heart axis. Especially a change of the heart axis or an extreme deviation can be an indication for pathology.

  • Positive (the average of the QRS surface above the baseline) QRS deflection in lead I: the electrical activity is directed to the left (of the patient)
  • Positive QRS deflection in lead AVF: the electrical activity is directed down.

This indicates a normal heart axis. Usually, these two leads are enough to diagnose a normal heart axis!

The largest vector in the heart is from the AV-node in the direction of the ventricular depolarization. Under normal circumstances, this is directed left and down.(towards leads I and AVF). The position of the QRS vector is given in degrees. See the figure, the middle of the figure is the AV-node. A horizontal line towards the left arm is defined as 0 degrees.

A normal heart axis is between -30 and +90 degrees.

Rule: biggest QRS deflection in I and II is an intermediate = normal heart axis. So positive deflections in I and II indicates a normal heart axis.

Interpretation

The interpretation of the electrical heartaxis has a few rules of thumb:

  • First, when a positive depolarization wave moves towards a positive electrode, a positive, upwards deflection is registered on the ECG.
  • Second, there are 4 quadrants where the QRS-vector can point to:
    • left upper quadrant --> left axis deviation (between -30º and -90º)
    • left lower quadrant --> normal (between -30º and 90º)
    • right below and right --> right axis deviation (between 90º and -150º)
    • right upper quadrant --> extreme axis (between -90º and -150º)

Example:

The QRS in lead I, will have a negative deflection in a right axis deviation. The vector is not directed towards the electrode. However, lead AVF will be positive, the vector is directed towards the electrode.

Iso-electrical
When the depolarization is perpendicular on the lead, this is called iso-electrical. The QRS is neither positive nor negative.
Undetermined axis
When all extremity leads are biphasic, the axis is directed to the front or back, in a transverse plane. The axis is than undetermined.


Abnormal heart axis

The direction of the vector can changes under different circumstances:

  1. When the heart itself is rotated (right ventricular overload), obviously the axis turns with it.
  2. In case of ventricular hypertrophy, the axis will deviate by the bigger electrical activity and the vector will turn towards the hypertrophied tissue.
  3. Infarcted tissue is electrically dead. No electrical activity is registered and the QRS vector turns away from the infarcted tissue
  4. In conduction problems, the axis deviates too. When the right ventricle depolarizes later than the left ventricle, the axis will turn to the right (RBBB). This is because the right ventricle will begin the contraction later and therefore will also finish later. In a normal situation the vector is influenced by the left ventricle but now only by the right ventricle.


Left axis deviation

Left heartaxis
Left anterior hemi block

Causes of left axis deviation include:


Right axis deviation

Right heartaxis

Causes of right axis deviation include: