Molly Ayala
The electrical axis of the heart is a crucial aspect of ECG diagnostics, representing the average direction of ventricular depolarization during ventricular contraction. This axis is determined by the sum of all depolarization vectors generated by the heart's cardiac myocytes. Clinically, this is reflected by the ventricular axis, which is interpreted by assessing the relationship between the QRS axis and limb leads of the ECG. The normal cardiac axis, directed downward and slightly to the left, is indicated by a QRS axis between -30° and +90°. Deviations from this range can signify underlying pathologies, such as left or right axis deviation, or extreme axis deviation in cases of right ventricular hypertrophy or acute cor pulmonale.
| Characteristics | Values |
|---|---|
| Definition | The mean electrical axis of the heart is the average of all the instantaneous mean electrical vectors occurring sequentially during depolarization of the ventricles. |
| Direction | The mean electrical axis normally points towards the left ventricle, as this is the larger of the two ventricles. |
| Deviation | Abnormal axis deviation, indicating underlying pathology, is demonstrated by: Left Axis Deviation = QRS axis less than -30°; Right Axis Deviation = QRS axis greater than +90°; Extreme Axis Deviation = QRS axis between -90° and 180°. |
| Causes of Deviation | Deviations of the mean electrical axis result from either right ventricular hypertrophy or a block in the intraventricular conduction system. |
| Visualisation | The electrical axis of the heart may be visualised on a Cartesian coordinate system in the frontal plane, using the "circle of axes". |
| Determination | The mean electrical axis can be determined using trigonometry, vector analysis, or by evaluating the QRS complex in leads I and II of an ECG. |
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What You'll Learn
- The mean electrical axis is the average of all the instantaneous electrical vectors during ventricular depolarisation
- The axis is described in degrees, with 0° indicating a horizontal direction towards the heart's left side
- The normal axis range is between -30° and +90°
- A right axis deviation is indicated by an axis of more than 90°
- A left axis deviation is indicated by an axis of less than -30°
The mean electrical axis is the average of all the instantaneous electrical vectors during ventricular depolarisation
The mean electrical axis of the heart is a fundamental aspect of ECG diagnostics. It is the average of all the instantaneous electrical vectors during ventricular depolarisation. This average is calculated by determining the direction of the net flow of current through the heart, which is the sum of all depolarisation vectors generated by individual cardiac myocytes.
The mean electrical axis is also known as the principal vector of ventricular depolarisation. It is influenced by the fact that the left ventricle comprises most of the heart muscle and generates the most electrical force visible on the ECG. This is because the left ventricle is the larger of the two ventricles. Therefore, the mean electrical axis typically points towards the left ventricle.
The direction of the depolarisation, and thus the mean electrical axis, is generally alongside the heart's longitudinal axis, to the left and downwards. This is reflected by the ventricular axis, which is determined by the relationship between the QRS axis and limb leads of the ECG. The QRS axis represents the average direction of all cardiac vectors and typically positions at 59° in a normally situated heart.
The mean electrical axis can be determined using the "circle of axes", which involves plotting the net voltage of the QRS complex in two bipolar limb leads. The hexaxial lead system is also used to determine the vertical axis of the heart, with the horizontal axis calculated using chest leads.
Abnormal axis deviation, indicating underlying pathology, is demonstrated by a QRS axis of less than -30° (left axis deviation) or greater than +90° (right axis deviation).
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The axis is described in degrees, with 0° indicating a horizontal direction towards the heart's left side
The mean electrical axis of the heart is a clinically important aspect of ECG diagnostics. It is the average of all the instantaneous mean electrical vectors occurring during ventricular depolarization. The direction of the depolarization 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 left side of the heart. This is because the left ventricle makes up most of the heart muscle and generates the most electrical force visible on the ECG.
The mean electrical axis is also referred to as the principal vector of ventricular depolarization. It represents the sum of all the waves of depolarization that are occurring simultaneously. The mean electrical axis normally points towards the left ventricle, as this is the larger of the two ventricles. The hexaxial lead system is arranged around the heart in a short-axis plane, referred to as the frontal plane. In this plane, the left ventricular apex is oriented generally towards the positive pole of lead II.
The mean electrical axis can be determined graphically using the "circle of axes". The cardiac vector is reconstructed from the net QRS voltage in two bipolar limb leads, typically leads I and II. The length of the projection of the cardiac vector onto leads I and II equals the net QRS voltage. A line perpendicular to the respective lead axis is drawn through each mark, and the centre of the circle of axes is connected to the intersection point between these two lines to form the cardiac vector.
The mean electrical axis of the heart can also be visualised on a Cartesian coordinate system in the frontal plane. A horizontal line from the centre towards the left arm is defined as 0˚, and any rotation clockwise represents a positive angle, while a counter-clockwise rotation represents a negative angle. The bipolar (I, II, III) and augmented unipolar (aVF, aVR, aVL) limb leads are shown as position vectors with a well-defined angle to the x-axis.
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The normal axis range is between -30° and +90°
The normal electrical axis of the heart, also known as the QRS axis, is between -30° and +90°. This range is indicative of a normal cardiac axis, which represents the sum of depolarisation vectors generated by individual cardiac myocytes. The QRS axis is determined by evaluating the vectors produced under the electrodes, and the relationship between the QRS axis and the limb leads of the ECG.
The QRS axis is directed downward and slightly to the left, with the left ventricle comprising most of the heart muscle and generating the most electrical force visible on the ECG. This is reflected in the normal axis range, which is between -30° and +90°.
If the QRS axis falls outside this range, it can indicate an abnormal axis deviation. For example, a QRS axis less than -30° is considered a left axis deviation, while a QRS axis greater than +90° is considered a right axis deviation. Extreme axis deviation is indicated by a QRS axis between -90° and 180°.
It's important to note that the QRS axis can vary with age, particularly in paediatric ECG interpretation. At birth, the cardiac axis lies between +30 to +190 degrees and moves leftward with age.
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A right axis deviation is indicated by an axis of more than 90°
The electrical axis of the heart is an important aspect of ECG interpretation, reflecting the average direction of ventricular depolarization during ventricular contraction. The cardiac axis is the sum of all depolarization vectors of the heart, and vector analysis determines the direction of the net flow of current. The axis is described in degrees, with 0° indicating a horizontal direction towards the heart's left side. The normal QRS axis in adults should be between -30° and +90°, directed downward and slightly to the left.
A right axis deviation (RAD) is indicated when the QRS axis is greater than +90°. This means that the axis extends from the left to the right side of the heart in a clockwise direction. In the context of the hexagonal system used for vertical axis determination, this corresponds to certain lead polarities. For instance, Lead I is placed between the right arm (negative) and left arm (positive) horizontally, at 0°, while Lead II is placed between the right arm (negative) and left leg (positive) at +60°. The polarities of these leads are important for quick axis determination.
Right axis deviation is considered normal in infants and children, with the mean QRS axis during the first four weeks of life being +110° or more. After the first month, the average axis is typically less than +90°, although some children may still have a QRS axis of up to +110°. In adults, a QRS axis of more than +110° is uncommon and often suggests an abnormality. However, even a QRS axis within the range of +90° to +110° may indicate an abnormality in older patients, especially if other ECG abnormalities are present.
Right axis deviation can be associated with various conditions, such as RVH (right ventricular hypertrophy), pulmonary emphysema, a vertical heart, and extensive lateral wall myocardial infarction. In patients with COPD (chronic obstructive pulmonary disease), the frontal plane axis may fall within the range of +90° to +110° in the absence of pulmonary hypertension. Additionally, a rightward shift of the P axis in the orthogonal leads has been identified as a predictor of deteriorating pulmonary function in patients with COPD.
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A left axis deviation is indicated by an axis of less than -30°
The electrical axis of the heart, or cardiac axis, represents the sum of depolarization vectors generated by individual cardiac myocytes. In other words, it reflects the average direction of ventricular depolarization during ventricular contraction. Vector analysis is the basic concept of analysing an ECG, and adequate knowledge of it is important for all clinicians.
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°.
A left axis deviation (LAD) is indicated by an axis of less than -30°. The normal QRS axis should be between -30° and +90°. Left axis deviation is defined as the major QRS vector, falling between -30° and -90°. To further distinguish normal from left axis deviation, one can examine lead II. If lead II is downward (negative), then the axis is more towards -120, and left axis deviation is present. If the QRS complex in lead II is upright (positive), then the axis is more towards +60°, and the QRS axis is normal.
The fastest non-specific method to determine the QRS axis is to find the major direction of the QRS complex—positive or negative—in leads I and aVF. If the QRS complex is upright (positive) in both lead I and lead aVF, then the axis is normal. If the QRS is upright in lead I (positive) and downward in lead aVF (negative), then the axis is between 0° and -90°.
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Frequently asked questions
The mean electrical axis of the heart is the average of all the instantaneous mean electrical vectors occurring during ventricular depolarization. It is also known as the cardiac axis and is an integral part of ECG interpretation.
The mean electrical axis of the heart represents the sum of all the waves of depolarization that are occurring simultaneously. It reflects the average direction of ventricular depolarization during ventricular contraction. Vector analysis is used to determine the direction of the net flow of current.
The normal range for the mean electrical axis of the heart 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.
