Trystan Casey
The mean electrical axis of the heart is a fundamental concept in cardiac electrophysiology and is used to detect cardiac disease. It involves measuring the electrical activity of the heart to identify cardiac conduction disturbances, hypertrophy, and the origin of arrhythmias. To calculate the mean electrical axis, a multilead ECG is required, and the QRS axis must be determined. The QRS axis is found by first identifying the isoelectric lead, which is the frontal lead with zero net amplitude, and then locating the positive leads with the tallest R waves. The QRS axis is positioned at 90° to the isoelectric lead and points towards the positive leads. This calculation is essential for diagnosing conduction abnormalities, ventricular hypertrophy, and arrhythmia origins.
| Characteristics | Values |
|---|---|
| What is the mean electrical axis? | The direction of the net current is termed the "mean electrical axis of the heart". |
| How is it calculated? | The mean electrical axis can be calculated by the Novosel formula using the net QRS voltages in leads I and aVF. |
| What is the normal range? | The normal range of the mean electrical axis in dogs is +40 to +100 degrees and in cats is 0 to +160 degrees. |
| How to determine the mean electrical axis? | To determine the mean electrical axis, you must 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. |
| What does it help diagnose? | Shifts in the mean electrical axis, in conjunction with radiographic changes, can help diagnose cardiac chamber enlargement and conduction defects, ventricular hypertrophy, and differentiate the origin of arrhythmias. |
| What is the easiest axis to calculate? | The easiest axis to calculate is the vertical axis, which is at right angles to lead I and in the direction of aVF, making it exactly +90°. |
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What You'll Learn
The normal range of the mean electrical axis in dogs and cats
The mean electrical axis (MEA) is a crucial component of electrocardiography (ECG) that helps determine the heart's electrical axis, which is the sum of all depolarisation vectors of the heart. It provides valuable insights into underlying diseases and aids in diagnosing certain life-threatening arrhythmias. The normal range of the MEA varies for dogs and cats and is influenced by factors such as patient position and the shape of the thorax.
For dogs, the normal range of the MEA is between +40 degrees and +100 degrees. A right axis shift in dogs is defined as an MEA between +100 degrees and -90 degrees, while a left axis shift occurs when the MEA falls between +40 degrees and -90 degrees. The MEA values for dogs can vary depending on the breed, with some studies indicating that healthy Doberman Pinschers may have a left axis deviation compared to the standard reference values for the general canine population.
On the other hand, the normal range of the MEA in cats is between 0 degrees and +160 degrees. A right axis shift in cats occurs when the MEA is between +160 degrees and -90 degrees, while a left axis shift is observed when the MEA is between 0 degrees and -90 degrees. Left anterior fascicular block, a common conduction abnormality in cats, results in a marked left axis deviation of the MEA and is often associated with feline cardiomyopathy.
To determine the MEA accurately, a multilead ECG is necessary. The patient should be positioned in right lateral recumbency, with their limbs perpendicular to the long axis of their body. Interpreting the electrical signals recorded at each electrode as positive, negative, or isoelectric, and considering their relationships, helps determine the ventricular (QRS) axis. The amplitude (height) of an ECG R wave is also significant, as it is proportional to how parallel the wave front of depolarisation is to the lead.
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How to determine the mean electrical axis
The mean electrical axis of the heart is a mandatory topic in almost any undergraduate course on cardiac electrophysiology. It is a clinically important aspect of ECG diagnostics that can be used to detect cardiac disease. The direction of the net current during cardiac contraction is termed the "mean electrical axis of the heart".
To determine the mean electrical axis, you must obtain a multilead ECG from a patient positioned in right lateral recumbency with the limbs perpendicular to the long axis of the patient's body. The amplitude (height) of an ECG R wave is proportional to how parallel the wave front of depolarization is to that lead. Each ECG lead records the electrical activity of the heart relative to that lead. A wave of depolarization that is travelling toward the positive pole of a lead will result in a positive deflection in that lead. Conversely, a wave of depolarization that is travelling away from a positive lead will result in a negative deflection in the lead.
The mean electrical axis can be calculated by the Novosel formula using the net QRS voltages in leads I and aVF. The QRS axis is at 90° to the isoelectric lead, pointing in the direction of the positive leads. The normal range of the mean electrical axis in dogs is +40 to +100 degrees, and in cats, it is 0 to +160 degrees.
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Using a multilead ECG to detect changes in the mean electrical axis
To detect changes in the mean electrical axis, a multilead ECG is necessary. A multilead ECG will help rule out an artifact as the cause of electrocardiographic abnormalities seen in a single-lead tracing. It will also help in diagnosing conduction abnormalities, detecting ventricular hypertrophy, and differentiating the origin of arrhythmias.
The standard multilead system used in veterinary medicine is the hexaxial lead system formed by six limb leads (I, II, III, aVR, aVL, and aVF). Leads I, II, and III are bipolar leads, meaning that they have a positive and negative pole. Leads aVR (augmented voltage right arm), aVL (augmented voltage left arm), and aVF (augmented voltage left foot) are unipolar leads, meaning they only have a positive pole. The activity of the positive pole is compared with the average of the other two leads.
To determine the mean electrical axis, you must 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. The normal range of the mean electrical axis in dogs is +40 to +100 degrees, and in cats, it is 0 to +160 degrees. Each ECG lead records the electrical activity of the heart relative to that lead. A wave of depolarization traveling toward the positive pole of a lead will result in a positive deflection in that lead. Conversely, a wave of depolarization traveling away from a positive lead will result in a negative deflection in the lead.
ECG leads are positioned around the heart at different angles, which is why the waveform recorded by each lead appears different. The amplitude (height) of an ECG R wave is proportional to how parallel the wave front of depolarization is to that lead. When a wave of depolarization is moving toward the positive pole, the more directly parallel the wave front is to the lead, the taller the R wave will be. The more perpendicular the wave of depolarization is, the smaller the resulting R wave will be.
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The mathematical method for determining the mean electrical axis
The mean electrical axis of the heart can 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˚, with any clockwise rotation representing a positive angle and any counter-clockwise rotation representing a negative angle. The limb leads are shown as position vectors with a well-defined angle to the x-axis. According to the concept of the mean electrical axis, the centre of the heart is assumed to be located at the centre of the "circle of axes", and the triangles formed by the bipolar and augmented unipolar limb leads are assumed to be equilateral.
The mean electrical axis can be calculated using the Novosel formula and the net QRS voltages in leads I and aVF. For example, for an ECG with left axis deviation, we find VI = 0.5 mV – 0.1 mV = 0.4 mV and VₐVF = 0.4 mV – 1.0 mV = –0.6 mV. By inserting these values into the formula, we get: tan(θ) = –1.73. This equation has two solutions in the interval from 0˚ to 360˚, which are θ₁ = –60˚ and θ₂ = 120˚. The mean electrical axis always has the same sign as the net QRS voltage in aVF, so the result is θ = –60˚.
Additionally, to detect changes in the mean electrical axis, a multilead ECG is necessary. The QRS axis can be calculated by first finding the isoelectric lead, which is the frontal lead with zero net amplitude. This can be a biphasic QRS where the R wave height is equal to Q or S wave depth, or a flat-line QRS with no discernible features. The next step is to find the positive leads, which are the leads with the tallest R waves or largest R/S ratios. Finally, the QRS axis is calculated, which is at 90° to the isoelectric lead, pointing in the direction of the positive leads.
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The QRS axis calculation
Step 1: Understanding the Basics
Before delving into the calculation, it's important to grasp the underlying principles. The QRS axis reflects the overall direction of ventricular depolarisation, which is the spread of electrical activity in the heart. This axis is typically oriented downward and slightly to the left, as the left ventricle constitutes most of the heart muscle.
Step 2: Identifying the Normal Range
The normal range for the QRS axis in humans is between -30 degrees and +90 degrees. This range indicates a normal cardiac axis. Deviations from this range can signify underlying issues.
Step 3: Determining the Isoelectric Lead
To calculate the QRS axis, you must first identify the isoelectric lead. An isoelectric lead is one where the positive deflection is equal to the negative deflection, resulting in zero net amplitude. This can be visualised as a biphasic QRS with an R wave height equal to the Q or S wave depth or a flat-line QRS with no noticeable features.
Step 4: Locating the Positive Leads
The next step is to identify the leads with the tallest R waves or the largest R/S ratios. These leads are considered positive, and their presence is essential for QRS axis calculation.
Step 5: Calculation of the QRS Axis
Finally, the QRS axis can be determined using the following formula:
> The QRS axis is at 90° to the isoelectric lead, pointing in the direction of the positive leads.
This calculation provides the angle of the QRS axis relative to the isoelectric lead, giving you a precise measurement for analysis.
Additional Considerations:
It's worth noting that the QRS axis calculation can be refined by using multiple leads. For instance, examining leads I and aVF or leads I, II, and III can provide a more comprehensive understanding of the axis. Additionally, in certain cases, you may need to adjust the amplitude of specific leads for greater accuracy.
In conclusion, the QRS axis calculation is a fundamental aspect of ECG interpretation, aiding in the diagnosis of conduction abnormalities, ventricular hypertrophy, and arrhythmias. By following the steps outlined above, healthcare professionals can effectively determine the QRS axis and gain valuable insights into cardiac health.
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Frequently asked questions
The mean electrical axis is the direction of the net current of the heart, also known as the QRS axis.
Calculating the mean electrical axis is important because shifts in the axis can help diagnose cardiac chamber enlargement and conduction defects.
The mean electrical axis can be calculated using the Novosel formula with the net QRS voltages in leads I and aVF.
The normal range of the mean electrical axis in dogs is +40 to +100 degrees, and in cats, it is 0 to +160 degrees.
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