The electrical axis of the heart is the mean direction of the action potentials traveling through the ventricles during ventricular activation (depolarization). The QRS complex, which represents ventricular depolarization, is used for the determination of the electrical heart axis.
The term, electrical heart axis, usually refers to the electrical axis in the frontal plane as measured by the limb leads.
Normally, the vector representing the heart axis originates from the AV node, where ventricular activation is initiated, and points towards the left ventricle. This is because the massive left ventricle dominates the process of ventricular depolarization.
Many factors may alter the electrical heart axis including:
- Anatomic Factors:
- Abnormal anatomic position of the heart in the thoracic cavity (such as in dextrocardia)
- Abnormal thoracic anatomy
- Abnormal position of the diaphragm (such as in obesity, pregnancy, ascites)
- Cardiopulmonary Pathology:
- Prior myocardial infarction
- Recent ischemia
- Pulmonary embolism
- Pulmonary obstructive disease
- Myocardial hypertrophy
- Dilated cardiomyopathy
- Conduction abnormalities
- Others
The normal electrical axis of the heart is situated between -30 degrees and +90 degrees (positive 90 degrees) with respect to the horizontal line. Variations in electrical heart axis can be classified as left axis deviation, right axis deviation or extreme axis deviation.
Left axis deviation: the electrical heart axis is between -30 degrees (negative 30 degrees) and -90 degrees (negative 90 degrees) with respect to the horizontal line.
- Right axis deviation: the electrical heart axis is between +90 degrees (positive 90 degrees) and 180 degrees (positive 180 degrees) with respect to the horizontal line.
- Extreme axis deviation (also known as northwest axis or no man’s land): the electrical heart axis is between +180 degrees (positive 180 degrees) and -90 degrees (negative 90 degrees) with respect to the horizontal line.
- Causes of left axis deviation include:Left ventricular hypertrophy (the left ventricle is enlarged and generates more electrical activity, so the heart axis is “pulled” to the left)
- Obesity (in obese persons, the diaphragm is often positioned high pushing the heart upwards and rotating the heart and its electrical axis to the left)
- Left bundle branch block or LBBB (Note that in patients with LBB, left axis deviation, normal axis, and to a lesser extent right axis deviation can also be encountered)
- Left anterior fascicular block (LAFB)
- Causes of right axis deviation include:
- Right ventricular hypertrophy (the right ventricle is enlarged and generates more electrical activity, so the heart axis is “pulled” to the right)
- Loss of tissue of the left ventricle (e.g. as caused by myocardial infarction)
- Pulmonary embolism (Due to the obstruction in the pulmonary artery the right ventricle has to generate higher pressures to pump blood into the pulmonary artery. This results in greater electrical activity of the right ventricle and right axis deviation.)
In determining the electrical heart axis in the frontal plane, analysis of two or more limb leads is required.
Vector analysis for the determination of the electrical heart axis:
In the determination of the electrical heart axis using vector 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 the charges between the activated (or depolarized) cardiac cells and the resting cardiac cells while the direction of the arrow represents the mean direction of the depolarization vectors with reference to the frontal leads (or the limb leads). By definition, ventricular depolarization propagates from a negatively charged area towards a positively charged area. Thus, the mean depolarization vector points to the positively charged area.
Determination of the electrical heart axis in the frontal plane by analysis of the limb leads:
- Case 1 – QRS complex predominantly positively deflected in lead I
Conclusion: A positively deflected QRS complex in lead I implies that the electrical heart axis is situated between –90 degrees and + 90 degrees with reference to the horizontal line. Explanation: Lead I has its positive pole at 0 degrees from the horizontal line. The line perpendicular to lead I intersects the horizontal at –90 degrees on one end and at +90 degrees on the other end. As the QRS complex in lead I is predominantly positively deflected, the electrical heart axis has to be situated between –90 degrees and +90 degrees (in clockwise rotation). Thus, the possibility of right axis deviation or extreme axis deviation is excluded in this case. Move the arrow in the animation to visualize this concept. - Case 2 – QRS complex predominantly positively deflected in lead IIConclusion: A positively deflected QRS complex in lead II implies that the electrical heart axis is situated between –30 degrees and +150 degrees with reference to the horizontal line. Explanation: Lead II has its positive pole at +60 degrees from the horizontal line. The line perpendicular to lead II intersects the horizontal line at –30 degrees on one end and at +150 degrees on the other end. As the QRS complex in lead II is predominantly positively deflected, the electrical heart axis has to be situated between –30 degrees and +150 degrees (in clockwise rotation). Thus, the possibility of left axis deviation or extreme axis deviation is excluded in this case. Move the arrow in the animation to visualize this concept.
- Case 3 – QRS complex predominantly positively deflected in lead IIIConclusion: A positively deflected QRS complex in lead III implies that the electrical heart axis is situated between +30 degrees and +210 degrees (or –150 degrees) with reference to the horizontal line. Explanation: Lead III has its positive pole at +120 degrees. The line perpendicular to lead III intersects the horizontal line at +30 degrees on one end and at +210 degrees (or –150 degrees) on the other end. As the QRS complex in lead III is predominantly positively deflected, the electrical heart axis has to be situated between +30 degrees and +210 degrees (in clockwise rotation). Thus, the possibility of left axis deviation is excluded in this case. Move the arrow in the animation to visualize this concept.
- Case 4 – QRS complex predominantly positively deflected in lead aVF
Conclusion: A positively deflected QRS complex in lead aVF implies that the electrical heart axis is situated between 0 degrees and 180 degrees with reference to the horizontal line. Explanation: Lead aVF has its positive pole at +90 degrees. The line perpendicular to lead aVF coincides with the horizontal line and is at 0 degrees from the horizontal line on one end and at 180 degrees on the other end. As the QRS complex in lead aVF is predominantly positively deflected, the electrical heart axis has to be situated between 0 degrees and 180 degrees (in clockwise rotation). Thus, the possibility of left axis deviation or extreme axis deviation is excluded. Move the arrow in the animation to visualize this concept. - Case 5 – QRS complex predominantly positively deflected in lead aVLConclusion: A positively deflected QRS complex in lead aVL implies that the electrical heart axis is situated between –120 degrees (or +240 degrees) and +60 degrees with reference to the horizontal line. Explanation: Lead aVL has its positive pole at +90 degrees. The line perpendicular to lead aVL intersects the horizontal line at –120 degrees on one end and at +60 degrees on the other end. As the QRS complex in lead aVL is predominantly positively deflected, the electrical heart axis has to be situated between –120 degrees and +60 degrees (in clockwise rotation). Thus, the possibility of right axis deviation is excluded. Move the arrow in the animation to visualize this concept.
- Case 6 – QRS complex predominantly positively deflected in lead aVRConclusion: A positively deflected QRS complex in lead aVR implies that the electrical heart axis is situated between -60 degrees (or +300 degrees) and +120 degrees with reference to the horizontal line. Explanation: Lead aVR has its positive pole at +210 degrees (or -150 degrees). The line perpendicular to lead aVR intersects the horizontal line at -60 degrees at one end and +120 degrees on the other end. As the QRS complex in lead aVR is predominantly positively deflected, the electrical heart axis has to be situated between -60 degrees and +120 degrees (in clockwise rotation). Thus, the possibility of a normal electrical heart axis is excluded. Move the arrow in the animation to visualize this concept.
Using a combination of different limb leads for the determination of the electric heart axis
When integrating the information obtained from a combination of different limb leads, the electrical heart axis can be determined by excluding all “impossible scenarios”.
Examples:
- When the QRS complexes in leads I and II both are predominantly positively deflected, left, right, and extreme axis deviations are all excluded. Thus, the electrical heart axis falls within the normal range.
- When the QRS complexes in leads I and aVF both are predominantly positively deflected, left, right, and extreme axis deviations are all excluded. Thus, the electrical heart axis falls within the normal range.
- When the QRS complexes in lead I are predominantly positively deflected and lead aVF is predominantly negatively deflected, right axis and extreme axis deviations are all excluded. Check lead II to assess if the electrical heart axis is normal or deviated to the left. If the QRS complexes in lead II are predominantly positively deflected, the axis is normal. If the QRS complexes in lead II are predominantly negatively deflected, the axis is deviated to the left.