The electrical axis of the heart is the average direction of the action potentials that travel through the ventricles during ventricular activation.
The QRS complex, which represents ventricular depolarization, is used for the determination of the Cardiac Axis. This complex refers to three vectors that describe the following:
- Wave Q: It has negative values and is the first wave that appears on the electrocardiograms .
- Wave R: It is the classic image of the electrocardiogram, it has positive polarity and it is the wave after the Q.
- Wave S: Possesses negative polarity and follows R.
The term, Electric Cardiac Axis, usually refers to the electrical axis in the frontal plane measured by the wires of the limb.
Normally, the vector that represents the Cardiac Axis originates in the AV node (Auriculoventricular), where the ventricular activation begins and points towards the left ventricle.
This is because the massive left ventricle dominates the process of ventricular depolarization.
Many factors can alter the Cardiac Axis, which include:
- Abnormal anatomical position of the heart in the thoracic cavity (as in Dextrocardia ).
- Abnormal thoracic anatomy.
- Abnormal position of the diaphragm (as in obesity, pregnancy, ascites).
- Cardiopulmonary pathology.
- Previous myocardial infarction.
- Recent ischemia.
- Pulmonary embolism.
- Obstructive pulmonary disease.
- Myocardial hypertrophy.
- Dilated cardiomyopathy.
- Driving abnormalities.
The normal electrical axis of the heart is between -30 degrees and +90 degrees (90 degrees positive), with respect to the horizontal line. The variations in the Electric Cardiac Axis can be classified as deviation of the left axis, deviation of the right axis or extreme deviation of the axis.
Deviation of the left axis
The Electric Cardiac Axis is between -30 degrees (negative 30 degrees) and -90 degrees (negative 90 degrees) with respect to the horizontal line.
The causes of the deviation of the left axis include:
- Left ventricular hypertrophy (the left ventricle enlarges and generates more electrical activity, so the Cardiac Axis “moves” to the left).
- Obesity (in obese people, the diaphragm is often placed high by pushing the heart up and turning the heart and its electrical axis to the left).
- Left branch block (Note that in patients with LBB, deviation from the left axis, normal axis and, to a lesser extent, deviation from the right axis can also be found).
- Left anterior fascicular block.
Deviation of the right axis
The Electric Cardiac Axis is between +90 degrees (90 degrees positive) and 180 degrees (180 degrees positive) with respect to the horizontal line.
The causes of the deviation of the right axis include:
- Right ventricular hypertrophy (the right ventricle enlarges and generates more electrical activity, so the Cardiac Axis “moves” to the right).
- Loss of tissue from the left ventricle (for example, caused by a myocardial infarction).
- Pulmonary embolism (Due to obstruction in the pulmonary artery, the right ventricle has to generate higher pressures to pump blood to the pulmonary artery, which results in increased electrical activity of the right ventricle and a deviation of the right axis).
Vector analysis for the determination of the Cardiac Axis
In the determination of the Electric Cardiac Axis by means of vectorial analysis, ventricular depolarization is represented as a mean depolarization vector with an arrow pointing to a specific direction.
The length of the vector represents the magnitude of the potential created by the difference in charges between the activated (or depolarized) cardiac cells and the resting cardiac cells, while the direction of the arrow represents the average direction of the depolarization vectors with reference to the front cables.
By definition, ventricular depolarization spreads from a negatively charged area to a positively charged area. Therefore, the mean depolarization vector points to the positively charged area.
By integrating the information obtained from a combination of different limb leads, the Electric Cardiac Axis can be determined by excluding all “impossible scenarios”.
That is, when the QRS complexes in leads I and II deviate predominantly positively, all deviations from the left, right and end axes are excluded. Therefore, the Electric Cardiac Axis is within the normal range.
When the QRS complexes in leads I and VF deviate predominantly positively, all deviations from the left, right and end axes are excluded. Therefore, the Electric Cardiac Axis is within the normal range.