Umair Gaines
The mean electrical axis is the average of all the instantaneous mean electrical vectors occurring sequentially during ventricular depolarization. It is important to understand the mean electrical axis as it helps diagnose conduction abnormalities, ventricular hypertrophy, and the origin of arrhythmias. To determine the mean electrical axis, a multilead ECG is required, with the patient's limbs perpendicular to their long axis. Four methods exist for estimating the mean electrical axis, with the first two 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 perpendicular to the isoelectric lead identifies the mean electrical axis. The second method involves choosing the lead with the smallest net deflection and extrapolating to determine the approximate mean electrical axis. The third method involves using a hexaxial lead system, where the mean electrical axis is determined by the predominant orientation of the QRS complex. The fourth method is the quadrant method, which uses the relative magnitudes of the R waves in leads I and III to determine the mean electrical axis.
| Characteristics | Values |
|---|---|
| Definition | The mean electrical axis is the average of all the instantaneous mean electrical vectors occurring sequentially during depolarization of the ventricles. |
| Detection | An electrocardiogram (ECG) is required to identify the exact nature of an arrhythmia. |
| Types of ECG | A single lead II ECG tracing is generally used to diagnose arrhythmias, but a multilead ECG is necessary to detect changes in the mean electrical axis. |
| Normal Range | In cats, the normal mean electrical axis is between 0 and +160 degrees. In dogs, it is between +40 and +100 degrees. |
| Right Axis Shift | In dogs, a right axis shift is between +100 and -90 degrees. In cats, it is between +160 and -90 degrees. |
| Left Axis Shift | In dogs, a left axis shift is between +40 and -90 degrees. In cats, it is between 0 and -90 degrees. |
| Hexaxial Lead System | The hexaxial lead system includes leads I, II, III, avR, avL, and avF, which are positioned at different angles around the heart. |
| Positive and Negative Poles | Each lead has a positive and negative pole. A wave of depolarization traveling toward the positive pole results in a positive deflection, while a wave traveling away from the positive pole results in a negative deflection. |
| Amplitude | The amplitude (height) of an ECG R wave is proportional to how parallel the wave front of depolarization is to the lead. |
| Calculation Methods | Four basic methods exist for estimating the mean electrical axis. The first two methods are considered more accurate, while the latter two are reasonable approximations. |
| Example Calculation | In the example of six limb leads, aVL is biphasic at -30 degrees. The positive perpendicular axis to aVL is +60 degrees, resulting in a mean electrical axis of approximately +60 degrees. |
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What You'll Learn
The mean electrical axis in dogs and cats
The mean electrical axis is the average of all the instantaneous mean electrical vectors occurring during the ventricular depolarization of the ventricles. To detect changes in the mean electrical axis, a multilead ECG is required. This is a useful diagnostic tool as it helps to diagnose conduction abnormalities, detect ventricular hypertrophy, and differentiate the origin of arrhythmias.
In dogs, the normal range of the mean electrical axis is between +40 and +100 degrees. A right axis shift is defined as a mean electrical axis between +100 and -90 degrees, while a left axis shift occurs when the mean electrical axis is between +40 and -90 degrees.
In cats, the normal range of the mean electrical axis is between 0 and +160 degrees. A right axis shift occurs when the mean electrical axis is between +160 and -90 degrees, and a left axis shift occurs when it is between 0 and -90 degrees.
To determine the mean electrical axis in both dogs and cats, a six-lead ECG is required, with the patient positioned in right lateral recumbency and 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, which are reasonable approximations. The first method involves finding an isoelectric lead by identifying 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 will identify the mean electrical axis. The second method involves choosing a lead with the smallest net deflection and extrapolating to determine the approximate mean electrical axis.
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How to obtain a six-lead ECG
To obtain a six-lead ECG, you will need to attach electrodes to the chest and/or limbs of the patient. These electrodes record small voltage changes as potential differences, which are then transposed into a visual tracing.
The six electrodes are placed on the chest wall, with each lead offering unique information that cannot be derived mathematically from the other leads. The placement of the electrodes is as follows:
- V1: fourth intercostal space, to the right of the sternum
- V2: fourth intercostal space, to the left of the sternum
- V3: diagonally between V2 and V4
- V4: between ribs 5 and 6 in the midclavicular line
- V5: on the same level as V4, but in the anterior axillary line
- V6: on the same level as V4 and V5, but in the midaxillary line
It is recommended that the hair on the chest wall is shaved before the placement of electrodes to improve the quality of the registration.
The six-lead ECG will provide a detailed view of the patient's heart, allowing for the detection of common arrhythmias. It is important to note that a multilead ECG is necessary to detect changes in the mean electrical axis, which can help diagnose conduction abnormalities, detect ventricular hypertrophy, and differentiate the origin of arrhythmias.
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The hexaxial lead system
To calculate the mean electrical axis using the hexaxial lead system, there are four basic methods, with the first two considered more accurate:
- Identify an isoelectric lead, where the positive and negative deflections of the QRS complex are equal. The lead perpendicular to the isoelectric lead will identify the mean electrical axis.
- Examine the QRS complex in leads I and avF to determine if it is positive, isoelectric, or negative. Combining the coloured areas and identifying the quadrant of overlap determines the axis. For example, if both leads I and avF are positive, the axis is between 0° and +90°, which is considered normal.
- If no single lead is equally biphasic, choose the lead with the smallest net deflection and extrapolate to determine the approximate mean electrical axis.
- By visual inspection, if the QRS complex in either lead I or II does not have a net positive deflection, the mean electrical axis is abnormal. Conversely, when both leads exhibit a net positive deflection, the mean electrical axis falls within the normal range of -30° to +90°.
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How to identify the mean electrical axis
The mean electrical axis is the average of all the instantaneous mean electrical vectors occurring sequentially during the depolarization of the ventricles. To identify the mean electrical axis, a multilead electrocardiogram (ECG) is required. The patient must be positioned in right lateral recumbency with the limbs perpendicular to the long axis of their 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. The first method involves finding an isoelectric lead by identifying 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 will identify the mean electrical axis.
The second method involves the use of a hexaxial lead system, which combines the axes of the three augmented limb leads (aVR, aVL, and aVF) to form a triaxial lead diagram. These diagrams can be combined to produce a hexaxial lead diagram, which is used to determine the mean QRS axis and describe axis deviation. Each lead in the hexaxial diagram has a positive and negative pole, and the angular designation of the leads ranges from +90° to +160°. By determining the polarity and angular value of each lead, the mean electrical axis can be calculated.
The third method is based on the quadrant method, which involves examining the QRS complex in leads I and II. If both leads have a net positive deflection, the mean electrical axis falls between -30° and +90°, which is considered normal. If the QRS complex in either lead I or II does not have a net positive deflection, it indicates an abnormal mean electrical axis.
The fourth method is a reasonable approximation and is used when no single lead is equally biphasic. In this case, the lead with the smallest net deflection is chosen, and the approximate mean electrical axis is extrapolated.
It is important to note that the mean electrical axis values differ between species. For example, in dogs, a normal mean electrical axis falls between +40° and +100°, while in cats, it is between 0° and +160°.
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How to determine the mean QRS axis
To determine the mean QRS axis, a multilead ECG is necessary. The mean QRS axis is the average of all the instantaneous mean electrical vectors occurring sequentially during ventricular depolarization. The QRS complex is the mean direction of the electrical activity of the heart. Each ECG lead records the electrical activity of the heart relative to that lead.
The hexaxial lead system (I, II, III, avR, avL, avF) is used to determine the mean QRS axis. Each lead has a positive and negative pole, and the angular designation of the leads ranges from 0° to +160°. The angular designation of lead I is 0°, with the leads above it having negative values and the leads below having positive values. Leads above lead I include aVL and aVR, and leads below include II, III, and avF.
The mean QRS axis can be determined by finding the general direction in which the QRS complex is predominantly oriented. If the QRS complex in either lead I or II does not have a net positive deflection, then the mean electrical axis is abnormal. When both leads have a net positive deflection, then the mean electrical axis falls between -30 and +90 degrees, which is normal.
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, which is when 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.
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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.
The mean electrical axis can be calculated by looking at lead II, which shows a biphasic RS complex. Therefore, the axis must be at right angles to lead II. Because lead II is at +60° on the hexaxial scale, the axis must be either –30° or +150°.
In cats, the normal mean electrical axis is between +160 and -90 degrees. In dogs, the normal mean electrical axis is between +40 and +100 degrees.
