Trystan Casey
The mean electrical axis of the heart, also known as the cardiac axis, is a fundamental concept in cardiac electrophysiology and ECG diagnostics. It represents the average direction of ventricular depolarisation during the cardiac cycle, providing valuable insights into the electrical activity of the heart. By understanding the mean electrical axis, healthcare professionals can detect cardiac abnormalities, such as hypertrophy, conduction disturbances, and arrhythmias. This axis is typically determined through electrocardiography, a technique that measures the heart's electrical activity to facilitate the diagnosis of cardiac diseases. While calculating the mean electrical axis, it is essential to consider the combination of bipolar and unipolar leads, as deviations from the normal range can indicate underlying pathologies.
| Characteristics | Values |
|---|---|
| What does the mean electrical axis represent? | The average direction of the ventricular depolarisation wave in the frontal plane. |
| What is the procedure for determining the mean electrical axis? | Obtain a six-lead ECG from a patient positioned in right lateral recumbency with the limbs perpendicular to the long axis of the patient's body. |
| How many methods exist for estimating the mean electrical axis? | Four basic methods, with the first two considered more accurate. |
| What is the first method? | Find an isoelectric lead by identifying a lead where the positive and negative deflections of the QRS complex are equal. The lead perpendicular to the isoelectric lead identifies the mean electrical axis. |
| What is the second method? | Not specified in sources. |
| What is the third method? | Not specified in sources. |
| What is the fourth method? | Not specified in sources. |
| What is the normal mean electrical axis range for humans? | Between 0° and 90°. |
| What is the normal mean electrical axis range for cats? | 0 to +160 degrees. |
| What is the normal mean electrical axis range for dogs? | +40 to +100 degrees. |
| What is the normal mean electrical axis range for newborns? | +30 to +190 degrees, moving leftward with age. |
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What You'll Learn
- The mean electrical axis is a simple and rapid diagnostic procedure
- It can be determined graphically using the circle of axes
- The cardiac axis represents the sum of ventricular depolarisation vectors
- The mean electrical axis is the average direction of the ventricular depolarisation wave
- The calculation requires a six-lead ECG from a patient in right lateral recumbency
The mean electrical axis is a simple and rapid diagnostic procedure
There are four basic methods for estimating the mean electrical axis, with the first two being considered more accurate. The first method involves finding an isoelectric lead, where the positive and negative deflections of the QRS complex are equal. The lead that is perpendicular to the isoelectric lead identifies the mean electrical axis. If the QRS complex is predominantly positive in the perpendicular lead, the mean electrical axis is directed toward the positive pole. If the QRS complex is negative, the mean electrical axis is directed away from the positive pole.
The second method involves using the "circle of axes" to graphically determine the mean electrical axis. 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 these leads 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 of these lines to form the cardiac vector.
The third and fourth methods are reasonable approximations. They involve using a combination of bipolar and unipolar leads, such as leads I, II, III, aVR, aVL, and aVF. However, it is important to note that using a combination of these leads can produce incorrect results due to the different strengths of the unipolar and bipolar leads.
Shifts in the mean electrical axis, along with radiographic changes, can help diagnose cardiac chamber enlargement and conduction defects. This is especially useful in practices without access to echocardiography.
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It can be determined graphically using the circle of axes
The mean electrical axis of the heart, θ, can be determined graphically using the "circle of axes". This method was first introduced by Willem Einthoven in 1913 and remains clinically important for ECG diagnostics.
The "circle of axes" is a graphical representation of the electrical axis of the heart on a Cartesian coordinate system in the frontal plane. To begin, a horizontal line is drawn from the centre of the circle towards the left arm, which is defined as 0˚. Any rotation clockwise from this line represents a positive angle, while a counter-clockwise rotation represents a negative angle.
The cardiac vector, E, is then 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 these leads is equal to the net QRS voltage. For example, if the net QRS voltage is 0.8 mV in lead I and 1.2 mV in lead II, then θ = 49˚.
To graphically derive the mean electrical axis, a line perpendicular to each lead axis is drawn through the mark indicating the net QRS voltage. These lines intersect, and the centre of the circle of axes is connected to this intersection point, forming the cardiac vector.
While the "circle of axes" method provides valuable information on the state of intraventricular conduction and ventricular muscle mass, it is important to note that other methods, such as the Novosel formula, may offer quicker and easier alternatives for determining the mean electrical axis in a clinical setting.
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The cardiac axis represents the sum of ventricular depolarisation vectors
The cardiac axis, or the mean electrical axis, is the average of all the instantaneous mean electrical vectors occurring during the depolarisation of the ventricles. It represents the sum of depolarisation vectors generated by individual cardiac myocytes. In electrophysiology, a vector represents both the magnitude and direction of the action potential generated by an individual myocyte.
The ventricular axis is typically used in common clinical practice to determine the electrical axis. However, the atrial axis can also be useful in certain clinical situations. The axis is described in degrees, with 0° indicating a horizontal direction towards the heart's left side for a vector. The normal cardiac axis is directed downward and slightly to the left, with the QRS axis between -30° and +90°.
To determine the mean electrical axis, a six-lead ECG is obtained from a patient positioned in right lateral recumbency, with the limbs perpendicular to the long axis of the patient's body. There are four basic methods for estimating the mean electrical axis, with the first two considered more accurate than the latter two, which are reasonable approximations. One method involves finding an isoelectric lead, where the positive and negative deflections of the QRS complex are equal. The lead that is perpendicular to the isoelectric lead is the one that will identify the mean electrical axis.
The mean electrical axis is a simple and rapid diagnostic procedure that should be considered part of routine ECG analysis. It can help diagnose cardiac chamber enlargement and conduction defects, and is particularly useful in practices without access to echocardiography.
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The mean electrical axis is the average direction of the ventricular depolarisation wave
The mean electrical axis is a simple and rapid diagnostic procedure that should be considered part of routine ECG analysis. It represents the average direction of ventricular depolarization during ventricular contraction. The direction of the depolarization is generally alongside the heart's longitudinal axis, to the left and downwards.
The mean electrical axis is calculated to the nearest degree by the ECG machine. It can also be approximated manually by judging the net direction of the QRS complex in leads I and II. The QRS axis is at 90° to the isoelectric lead, pointing in the direction of the positive leads. The isoelectric lead is the frontal lead with zero net amplitude. This can be either a biphasic QRS where R wave height is equal to Q or S wave depth, or a flat-line QRS with no discernible features.
The mean electrical axis is the sum of all the mean electrical vectors occurring during ventricular depolarization. The QRS complex is different for each of the limb leads because each of the leads records the sequence of depolarization vectors from a different perspective. The mean electrical axis is depicted by a green arrow in the figure. The figure also shows the same instantaneous mean vectors (red arrows) superimposed upon the axial reference system.
In a six-limb lead example, aVL (-30°) is biphasic. The positive perpendicular axis to aVL is +60°. Therefore, the mean electrical axis is approximately +60°, which is normal.
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The calculation requires a six-lead ECG from a patient in right lateral recumbency
To calculate the mean electrical axis, a six-lead ECG is required from a patient in right lateral recumbency. This positioning involves the patient lying on their right side with their legs perpendicular to the body and parallel to each other. It is important to place the patient on a pad or thick blanket to minimise noise interference with the ECG signal.
The six-lead ECG will have electrodes attached to the patient's chest and/or limbs to record small voltage changes as potential differences, which are then transposed into a visual tracing. The electrodes should be placed on the chest wall equidistant from the heart rather than on specific limbs. The most useful lead is V4R, which is obtained by placing the V4 electrode in the 5th right intercostal space in the mid-clavicular line.
The six leads of the ECG will be the limb leads—I, II, and III—which are most often used to evaluate rhythm and mean electrical axis. Lead I is a dipole from the right to the left arm, lead II is from the right arm to the left leg, and lead III is from the left arm to the left leg. Augmented limb leads aVL, aVR, and aVF are also useful in calculating the mean electrical axis.
There are four basic methods for estimating the mean electrical axis, with the first two considered more accurate than the latter two. The first method involves finding an isoelectric lead, where the positive and negative deflections of the QRS complex are equal. The lead that is perpendicular to the isoelectric lead is the one that identifies the mean electrical axis. If the QRS complex is predominantly positive in the perpendicular lead, the mean electrical axis is directed toward the positive pole; if it is negative, the mean electrical axis is directed away from the positive pole.
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Frequently asked questions
The mean electrical axis is the average direction of the ventricular depolarisation wave in the frontal plane, that is, the total vector of all the depolarisations that occur throughout the ventricles during the cardiac cycle.
The mean electrical axis is a clinically important aspect of ECG diagnostics. It is used to detect hypertrophy, cardiac conduction disturbances, and the origin of arrhythmias.
To calculate the mean electrical axis, you must obtain a six-lead ECG from a patient positioned in right lateral recumbency with their limbs perpendicular to the long axis of their body. Four basic methods exist for estimating the mean electrical axis, with the first two considered more accurate than the latter two.
The first method is to find an isoelectric lead, which is a lead in which the positive and negative deflections of the QRS complex are equal. The lead that is perpendicular to the isoelectric lead is the one that identifies the mean electrical axis. The second method involves using the "circle of axes" to graphically derive the mean electrical axis of the heart by plotting the net voltage of the QRS complex in two bipolar limb leads. The third method is the Quadrant method, which involves using leads I and II to isolate the axis. The fourth method is calculating the electrical axis from leads I and aVF without correction.
The normal range of the mean electrical axis is between 0° and 90°.
