Beatrice Lang
The mean electrical axis is the average of all the instantaneous mean electrical vectors occurring during ventricular depolarization. The orientation of the mean electrical axis is influenced by various factors, including the anatomical position of the heart, the direction of electrical action potential, and the presence of underlying pathologies. Interpreting the electrocardiogram (ECG) is crucial in determining the heart's electrical axis, which can provide insights into potential diseases. The cardiac axis represents the sum of depolarization vectors generated by individual cardiac myocytes, and deviations from the normal axis can indicate abnormal conditions. Factors such as increased cardiac muscle mass, changes in ventricular activation sequences, and ventricular ectopy can contribute to axis deviations. Additionally, physiological traits like height and weight can influence the orientation of the mean electrical axis.
| Characteristics | Values |
|---|---|
| Normal cardiac axis | QRS axis between -30° and +90° |
| 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° |
| Normal orientation of the apex of the heart | Towards the left |
| Factors causing right axis deviation | Increased thickness of the right ventricle, left-sided pneumothorax, lung hyperinflation, dextrocardia, ventricular ectopy, right ventricular hypertrophy, reduced muscle mass of the left ventricle, altered conduction pathways, change in the position of the heart in the chest |
| Factors causing left axis deviation | Increased cardiac muscle mass, changes in the sequence of ventricular activation, ventricular regions incapable of being activated |
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What You'll Learn
- The mean electrical axis is the average of all the instantaneous mean electrical vectors during ventricular depolarization
- The normal mean electrical axis lies between -30° and +90°
- A more vertical heart orientation shifts the axis to the right
- The left ventricle generates the most electrical force visible on the ECG
- The electrical axis can be estimated using the quadrant or degree method
The mean electrical axis is the average of all the instantaneous mean electrical vectors during ventricular depolarization
The mean electrical axis is a crucial concept in understanding the electrical activity of the heart. It is defined as the average of all the instantaneous mean electrical vectors that occur during ventricular depolarization. This process involves the spread of electrical activation from the sinus node, located in the right atrium, to the rest of the heart.
Ventricular depolarization refers to the sequential activation of the ventricles, which are the two large, lower chambers of the heart. During this process, electrical impulses follow a specific pathway, travelling through the atrioventricular node, the bundle of His, and the left and right bundle branches along the interventricular septum. This sequential activation results in a series of mean electrical vectors, each with its own magnitude and direction.
The mean electrical axis is then calculated as the average of these individual vectors. It provides a comprehensive representation of the overall electrical activity of the heart during ventricular depolarization. This calculation takes into account the various perspectives of the limb leads, as each lead records the sequence of depolarization vectors from a unique angle.
The normal range for the mean electrical axis is between -30° and +90°. Deviations from this range can provide important clinical insights. For example, a left axis deviation is indicated by a value less than -30°, while a right axis deviation occurs when the value exceeds +90°. These deviations can be caused by various factors, including increased cardiac muscle mass, changes in the sequence of ventricular activation, or the presence of infarcted tissue.
In summary, the mean electrical axis is a powerful tool for interpreting electrocardiograms (ECGs) and gaining insights into the underlying health of the heart. By considering the average of all instantaneous mean electrical vectors during ventricular depolarization, clinicians can identify deviations from the normal range and make informed diagnoses and treatment decisions.
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The normal mean electrical axis lies between -30° and +90°
The mean electrical axis is the average of all the instantaneous electrical vectors occurring during ventricular depolarization. The electrical axis is an important factor in interpreting the electrocardiogram (ECG) and understanding the heart's electrical axis. The normal mean electrical axis for the heart typically lies between -30° and +90°.
The cardiac axis is the sum of all depolarization vectors of the heart, and vector analysis is used to determine the direction of the net flow of current through the heart. The normal cardiac vector usually runs from the base of the heart to the apex, with the apex of the heart oriented towards the left. The left ventricle, which makes up most of the heart muscle, generates the most electrical force visible on the ECG.
The mean electrical axis can be estimated from the ECG by using the quadrant method or the degree method. The quadrant method involves observing Leads I and II, while the degree method involves identifying the lead with the smallest QRS complex and locating its axis on the hexaxial reference system.
A vector below 0° in a clockwise direction results in a positive axis, and if a vector is straight downwards, it is +90°. A left axis deviation is observed when the QRS axis is less than -30°, while a right axis deviation occurs when the QRS axis is greater than +90°. Right axis deviation can be caused by various factors, including increased thickness of the right ventricle, pre-excitation syndromes, and congenital conditions such as dextrocardia.
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A more vertical heart orientation shifts the axis to the right
The electrical axis of the heart is the net direction in which the wave of depolarization travels. It is measured using an electrocardiogram (ECG). Normally, this begins at the sinoatrial node (SA node), and from here, the wave of depolarization travels down to the apex of the heart. The apex of the heart is normally oriented towards the left. However, a more vertical orientation of the heart shifts the axis to the right.
A shift in the electrical axis of the heart can occur due to various factors. Physiologically, a more vertical heart orientation is seen in tall and thin individuals. Certain pathological conditions can also cause a rightward displacement of the heart, such as a left-sided pneumothorax and lung hyperinflation (e.g. COPD). Additionally, the congenital condition of dextrocardia results in a right axis deviation.
The electrical axis of the heart is typically determined by analysing the ventricular axis on an ECG. The cardiac vector, which represents the direction of the net flow of current through the heart, is an important concept in understanding the electrical axis. The cardiac vector normally runs from the base of the heart to the apex, with the left ventricle generating the most electrical force visible on the ECG.
The hexaxial reference system is used to visualise the directions in which the depolarization wave may travel. Leads I, II, and III are placed on the body in specific locations, and their polarities are important for quick axis determination. For example, Lead I is placed between the right arm (negative) and left arm (positive) horizontally, representing 0° on the hexaxial reference system. If the electrical axis falls between -30° and +90°, it is considered normal. Deviations from these values, such as right axis deviation (RAD), may indicate underlying pathologies.
In summary, a more vertical heart orientation shifts the electrical axis to the right due to the normal orientation of the apex of the heart being towards the left. This shift can be influenced by physiological factors, such as height and weight, or pathological conditions affecting the orientation of the heart. The electrical axis is important in diagnosing underlying diseases and guiding treatment decisions.
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The left ventricle generates the most electrical force visible on the ECG
The ECG, or electrocardiogram, is a tool used by cardiologists to record the heart's electrical activity graphically. It involves attaching 10 electrical cables to the body: one to each limb and six across the chest. The ECG reading reflects the net electrical activity at a given moment. The direction of the endpoint from the starting point represents the axis or predominant direction of electrical depolarization, which is determined primarily by the muscle mass of the left ventricle.
The interpretation of an ECG requires determining the relationship between the QRS axis and limb leads of the ECG. The QRS complex is generally the biggest wave and represents ventricular depolarization. The P wave is a small deflection wave that represents atrial depolarization, while the T wave corresponds to ventricular repolarization. The PR interval is the time between the first deflection of the P wave and the first deflection of the QRS complex.
The analysis of an ECG involves understanding the size of the electrical variations, the duration of the events, and detailed vector analysis. For example, an enlarged Q wave may indicate a myocardial infarction, while an enlarged suppressed or inverted Q wave suggests enlarged ventricles. T waves that appear flatter indicate insufficient oxygen delivery to the myocardium.
The electrical cardiac axis on an ECG represents the mean direction of the electrical action potential during ventricular depolarization. The cardiac electrical activation is generated from the sinus node located high in the right atrium and spreads throughout the atria to the atrioventricular node in the infero-posterior region of the interatrial septum. It then enters the base of the ventricle at the bundle of His, following the left and right bundle branches along the interventricular septum.
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The electrical axis can be estimated using the quadrant or degree method
The electrical axis of the heart is the mean direction of the electrical action potential during ventricular depolarisation. It is important to understand the electrical axis as it can provide insight into underlying disease states and help steer the differential diagnosis towards or away from certain diagnoses.
A more accurate approach than the simple quadrant method is the 3-lead method, which considers leads I, II, and aVF. If the net QRS deflection is positive in both leads I and II, the QRS axis is normal. If the net QRS deflection is positive in lead I but negative in lead II, there is LAD. If lead I is negative and lead aVF is positive, the axis is rightward, and if both leads are negative, there is an extreme axis.
Additionally, the ventricular (QRS) axis can be estimated by locating the most isoelectric limb lead along the frontal plane, which represents a net amplitude of 0 and the smallest overall amplitude. This method can be used to determine the frontal plane axis in a bundle branch block setting, by examining the initial 80 to 100 milliseconds of QRS deflection, reflecting left ventricular activation.
The degree method involves understanding the specific degree measurements of the QRS axis. The normal axis typically falls between -30° and +90°, with the mean QRS vector at 59° in a normally situated heart. Left axis deviation (LAD) is between -30° and -90°, right axis deviation (RAD) is between +90° and 180° (or +100° in the adult range), and extreme axis deviation is between -90° and 180°.
In paediatric ECG interpretation, the normal range at birth is between +30° and +190°, moving leftward with age to between 0° and +120° between 8 and 16 years.
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Frequently asked questions
The mean electrical axis is the average of all the instantaneous mean electrical vectors occurring sequentially during depolarization of the ventricles.
To determine the mean electrical axis, first find the lead axis that is equally biphasic (equally positive and negative QRS deflections – i.e., no net deflection).
The orientation of the mean electrical axis can be affected by increased cardiac muscle mass, changes in the sequence of ventricular activation, or ventricular regions being incapable of being activated. For example, right ventricular hypertrophy, reduced muscle mass of the left ventricle, altered conduction pathways, and a change in the position of the heart in the chest can all cause right axis deviation.
The mean electrical axis for the heart normally lies between -30 and +90°. Less than -30° is termed a left axis deviation, and greater than +90° is termed a right axis deviation.
