Beatrice Lang
The electrical axis of the heart is a crucial aspect of ECG interpretation, reflecting the average direction of ventricular depolarization during contraction. While the electrical axis typically remains stable, pointing towards the left ventricle, deviations can occur due to right ventricular hypertrophy or intraventricular conduction blocks. These deviations provide valuable insights into potential underlying diseases and help guide differential diagnoses. The determination of the electrical axis is essential for identifying certain life-threatening arrhythmias and their subsequent management. This complex topic involves understanding vectors, which represent the magnitude and direction of action potentials generated by cardiac myocytes.
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What You'll Learn
- The mean electrical axis is the principal vector of ventricular depolarization
- The hexaxial lead system is used to visualise the directions in which the depolarisation wave may travel
- The normal mean electrical axis in humans falls between -30° and +90°
- The mean electrical axis normally points toward the left ventricle
- The mean electrical axis is a simple and rapid diagnostic procedure
The mean electrical axis is the principal vector of ventricular depolarization
The mean electrical axis is a fundamental aspect of ECG interpretation, reflecting the average direction of ventricular depolarization during ventricular contraction. It is the principal vector of ventricular depolarization, representing the sum of all the waves of depolarization occurring simultaneously. This is important as it provides insight into the underlying disease states and helps guide the differential diagnosis towards or away from certain conditions.
In the context of the heart, a vector represents both the magnitude and direction of the action potential generated by an individual myocyte. The mean electrical axis is the average of all the instantaneous mean electrical vectors that occur sequentially during ventricular depolarization. These vectors can be visualised using arrows, with the arrowhead pointing towards the net direction of the current flow. The length of the arrow corresponds to the voltage.
The mean electrical axis typically lies between −30° and +90°. A shift towards the left, known as a left axis deviation, occurs when the axis is less than −30°. On the other hand, a shift to the right, or right axis deviation, is observed when the axis exceeds +90°. These deviations can be indicative of various conditions, such as ventricular hypertrophy, conduction defects, or infarcted tissue.
The process of determining the mean electrical axis involves several steps. Firstly, the lead axis that exhibits equal biphasic properties, with no net deflection, is identified. Subsequently, the lead axis that is perpendicular to the biphasic lead and demonstrates a positive net deflection is located. This process aids in the determination of the mean electrical axis and facilitates the interpretation of the electrocardiogram.
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The hexaxial lead system is used to visualise the directions in which the depolarisation wave may travel
The hexaxial reference system can be used to visualise the directions in which the depolarisation wave may travel. The axis is described in degrees, with 0° indicating a horizontal direction towards the heart's left side for a vector. For a vector below 0° in a clockwise direction, the axis turns positive, and if a vector is straight downwards, it is +90°. If the axis extends from the left to the right side of the heart in a clockwise direction, it reaches +180°. If the vector extends straight upward, it is at -90°.
The lead with the most positive deflection is most aligned with the direction the heart's electrical activity is travelling. If the R wave is greater than the S wave, it suggests depolarisation is moving towards that lead. If the S wave is greater than the R wave, it suggests depolarisation is moving away from that lead. If the R and S waves are of equal size, it means depolarisation is travelling at exactly 90° to that lead.
The normal cardiac axis typically runs from the base of the heart to the apex. The normal axis is between -30° and +90°. If the axis is more positive than +90°, it is referred to as right axis deviation. If the axis is more negative than -30°, it is referred to as left axis deviation.
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The normal mean electrical axis in humans falls between -30° and +90°
The mean electrical axis is determined by evaluating the vectors produced under the electrodes and interpreting the electrical signal (QRS complex) recorded at each electrode. The QRS axis typically positions at 59° in a normally situated heart, pointing downward and slightly towards the left. This indicates that the apex is mostly positive compared to the heart's base.
The normal range of the mean electrical axis is essential for identifying deviations that may indicate underlying health issues. For example, a left axis deviation (LAD) is indicated by a QRS axis less than -30°, while a right axis deviation (RAD) occurs when the QRS axis exceeds +90°. Extreme axis deviation, which can be normal in newborns, is characterised by a QRS axis between -90° and 180°.
To determine the mean electrical axis, a six-lead ECG is obtained with the patient positioned in right lateral recumbency, ensuring the limbs are perpendicular to their body's long axis. The lead with the tallest R wave indicates that the mean electrical axis is within 30 degrees of the positive pole of that lead. Additionally, the quadrant graphing method can be employed, where the hexaxial lead system is divided into four equal quadrants by leads I and aVF.
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The mean electrical axis normally points toward the left ventricle
The mean electrical axis is the principal vector of ventricular depolarization. It represents the sum of all the waves of depolarization that are occurring simultaneously. The mean electrical axis typically points towards the left ventricle, as this is the larger of the two ventricles. The left ventricle comprises most of the heart muscle and generates the most electrical force visible on an ECG.
The direction of the depolarization, and thus the electrical axis, is generally alongside the heart's longitudinal axis, to the left and downwards. The normal heart axis is between -30° and +90°. If the axis is more positive than +90°, it is referred to as a right axis deviation. If the axis is more negative than -30°, it is referred to as a left axis deviation. The axis is described in degrees, with 0° indicating a horizontal direction towards the heart's left side for a vector. For a vector below 0° in a clockwise direction, the axis turns positive, and if a vector is straight downwards, it is +90°.
The mean QRS vector, representing the average direction of all cardiac vectors, typically positions at 59° in a normally situated heart. This vector points downward and slightly towards the left, indicating the apex is mostly positive compared to the heart's base. The heart's vertical axis is determined practically using 3 unipolar and 3 bipolar ECG leads, while the horizontal axis is calculated using chest leads.
The electrical axis reflects the average direction of ventricular depolarization during ventricular contraction. The cardiac vector suggests the direction of the net flow of current through the heart. The normal cardiac vector typically runs from the base of the heart to the apex.
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The mean electrical axis is a simple and rapid diagnostic procedure
The direction of depolarization, and thus the electrical axis, is generally alongside the heart's longitudinal axis, to the left and downwards. The axis is described in degrees, with 0° indicating a horizontal direction towards the heart's left side for a vector. For a vector below 0° in a clockwise direction, the axis turns positive, and if a vector is straight downwards, it is +90°. If the axis extends from the left to the right side of the heart in a clockwise direction, it reaches +180°.
Determining the electrical axis can provide insight into underlying disease states and help steer the differential diagnosis towards or away from certain diagnoses. For example, an abnormal axis deviation, indicating underlying pathology, is demonstrated by a left axis deviation of less than -30° or a right axis deviation of greater than +90°. Extreme axis deviation, from -90° to 180°, can indicate conditions such as right ventricular hypertrophy, acute cor pulmonale, or left bundle branch block.
Additionally, shifts in the mean electrical axis, in conjunction with radiographic changes, can help diagnose cardiac chamber enlargement and conduction defects. However, it is important to remember that cardiac chamber enlargement can occur without a concurrent shift in the mean electrical axis, and changes in the mean electrical axis can occasionally occur without overt cardiac disease.
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Frequently asked questions
The mean electrical axis of the heart typically points toward the left ventricle, which is the largest mass of myocardium. This is reflected in the ECG, which predominantly records the activity of the left ventricle.
The mean electrical axis of the heart reflects the average direction of ventricular depolarization during ventricular contraction. It is an integral part of ECG interpretation and can provide insights into underlying diseases, helping to diagnose conduction abnormalities and ventricular hypertrophy.
The mean electrical axis of the heart is determined by the sum of all individual vectors generated by depolarization waves. It is described in degrees, with 0° indicating a horizontal direction towards the heart's left side. The QRS axis, which is the most important to determine, can be used to identify if the axis is normal, leftward (left axis deviation), rightward (right axis deviation), or indeterminate (northwest axis).
