Molly Ayala
The electrical axis in an ECG represents the major direction of the overall electrical activity of the heart. The QRS axis is the most important to determine, but the P wave or T wave axis can also be measured. The normal cardiac axis is directed downward and slightly to the left, with the QRS axis between -30° and +90°. To calculate the electrical axis, the mean QRS axis is oriented towards the lead with the greatest net QRS deflection, which is calculated by adding the number of small squares corresponding to the height of the R wave and subtracting the squares corresponding to the Q and S waves. The standard multilead system used in veterinary medicine is the hexaxial lead system, formed by six limb leads.
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What You'll Learn
The normal axis range
To calculate the electrical axis, the mean QRS axis is identified by locating the lead with the greatest net QRS deflection. This involves adding the small squares corresponding to the height of the positive deflection (R wave) and subtracting the squares corresponding to the negative deflection (Q and S waves). This calculation helps determine the overall electrical activity of the heart, which can be normal or exhibit deviations like left axis deviation (LAD), right axis deviation (RAD), or an indeterminate northwest axis.
In veterinary medicine, the normal mean electrical axis varies by species. For dogs, the normal range is between +40° and +100°, while in cats, it spans from 0° to +160°. These ranges are determined using the hexaxial lead system, which consists of six limb leads, including bipolar leads I, II, and III, and unipolar leads aVR, aVL, and aVF.
It is important to note that the normal axis range in an ECG can vary depending on the specific context and the age of the patient. For example, in paediatric ECG interpretation, the cardiac axis lies between +30° and +190° at birth and gradually moves leftward as the child ages. Therefore, it is always essential to refer to specific guidelines and interpretations for the particular situation when interpreting ECG results.
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How to calculate the net QRS deflection
To calculate the net QRS deflection, you need to identify the QRS complex. This is a collection of waveforms, including the QRS complex and the ECG complex. The QRS complex represents ventricular depolarization, and the repolarization of the atria is also part of the QRS.
The QRS complex can be classified as net positive or net negative, depending on its net direction. A normal Q wave is narrow and small in amplitude, while an R wave is the first positive deflection of the QRS and an S wave is the first wave after the R wave, dipping below the baseline.
To calculate the net deflection, you add the number of small squares that correspond to the height of the R wave (positive deflection) and subtract the number of small squares corresponding to the height of the Q and S waves (negative deflection). This calculation can often be done by simply eye-balling the ECG.
For example, take QRS complex 'B'. If the R wave is 5 mm in height and the S wave is 7 mm in depth, the net deflection would be +5-7 or -2. So, the QRS complex 'B' has a negative net deflection of -2.
The QRS axis must be at ± 90° from aVL at either +60° or -120°. If the QRS is isoelectric, the axis is at 90° to this lead.
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The difference between left axis deviation and right axis deviation
The electrical axis of the heart can be determined using an electrocardiogram (ECG). This is a key step in interpreting an ECG, as it can provide insights into underlying disease states and help diagnose certain life-threatening arrhythmias. The mean QRS axis is oriented toward the lead with the greatest net QRS deflection, which is calculated by adding the number of small squares corresponding to the height of the R wave (positive deflection) and subtracting the number of squares corresponding to the height of the Q and S waves (negative deflection).
The normal cardiac axis, or normal axis, is directed downward and slightly to the left, with a QRS axis between -30° and +90°. Any deviation from this range indicates an abnormal axis and underlying pathology. Left axis deviation (LAD) occurs when the QRS axis is less than -30°, while right axis deviation (RAD) occurs when the QRS axis is greater than +90°. In adults, RAD is typically between +90° and 180°, or beyond +100°.
Left axis deviation indicates that the QRS vector is directed upward and to the left. This can be further classified as borderline LAD, which occurs due to inferior MI and results in a QRS axis of -30°, or physiological LAD, where lead II is isoelectric (neither positive nor negative) and the QRS axis is between 0° and -90°.
Right axis deviation indicates that the QRS vector is directed downward and to the right. This can be caused by right ventricular hypertrophy, acute right ventricular strain (e.g. due to pulmonary embolism), chronic lung disease (e.g. COPD), or sodium-channel blockade (e.g. TCA poisoning). RAD can also be classified as vertical axis, which is seen in patients with emphysema and a vertically oriented heart, with a QRS axis of exactly +90°.
In addition to LAD and RAD, there is also extreme axis deviation, which occurs when the QRS axis is between -90° and 180° (or "Northwest Axis"), and indeterminate axis, where the QRS complex is isoelectric or equiphasic in all leads with no dominant QRS deflection.
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How to determine the QRS axis
The QRS axis is a crucial component of ECG axis interpretation, which is essential for understanding cardiac function and identifying potential abnormalities. To determine the QRS axis, several methods can be employed, including the quadrant method and the use of specific leads. Here is a step-by-step guide on how to determine the QRS axis:
Understanding the Basics
The QRS axis represents the direction of ventricular depolarisation and is an essential aspect of ECG interpretation. It is important to note that the normal cardiac axis, which represents the sum of depolarisation vectors, is directed downward and slightly to the left due to the left ventricle's dominance in the heart muscle. This normal axis is reflected by a QRS axis between -30° and +90°. Any deviation from this range indicates an abnormality, such as left axis deviation (QRS axis less than -30°) or right axis deviation (QRS axis greater than +90°).
The Multilead Approach
To accurately determine the QRS axis, a multilead ECG is necessary. The standard multilead system used in veterinary medicine is the hexaxial lead system, which consists of six limb leads: Leads I, II, and III (bipolar leads) and leads aVR, aVL, and aVF (unipolar leads). These leads are positioned around the heart at different angles, resulting in varying waveforms recorded by each lead.
The Role of Lead I and aVF
The fastest and most efficient way to estimate the QRS axis is by examining Leads I and aVF. If both Leads I and aVF show a positive QRS complex, indicating that the electrical vector is heading towards the positive lead, the axis will be between 0° and +90°, which is considered a normal axis. Conversely, if Lead I is positive and Lead aVF is negative, the axis will be between 0° and -90°, which may indicate left axis deviation.
Combining Leads for Accuracy
For a more precise estimation of the QRS axis, combining Leads I, II, and aVF is beneficial. If the QRS complex is positive in a given lead, the axis points in roughly the same direction. Conversely, if the QRS complex is negative, the axis points in the opposite direction. Leads I, II, and aVF can help differentiate between pathological left axis deviation and a normal axis or physiological left axis deviation.
Calculating the Electrical Axis
To calculate the electrical axis, the mean QRS axis is oriented towards the lead with the greatest net QRS deflection. This is calculated by adding the number of small squares corresponding to the height of the R wave (positive deflection) and subtracting the number of squares corresponding to the height of the Q and S waves (negative deflection). This calculation helps determine the exact degree measurement of the QRS axis.
Identifying Abnormalities
Abnormalities in the QRS axis can indicate conduction abnormalities, ventricular hypertrophy, or the origin of arrhythmias. For example, an extreme axis deviation, with a QRS axis between -90° and 180°, is referred to as a "Northwest Axis" and can be associated with ventricular tachycardia or other conditions such as pulmonary embolism or chronic lung disease.
In summary, determining the QRS axis involves a combination of lead interpretation, calculation of net QRS deflection, and an understanding of normal and abnormal ranges. This process is vital for ECG interpretation and can provide valuable insights into cardiac health and potential pathologies.
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How to calculate the electrical axis using multiple leads
The electrical axis of the heart is the sum of all depolarization vectors of the heart, and determining it is a key step in interpreting an ECG. It provides insight into underlying disease states and helps narrow the differential diagnosis. To calculate the electrical axis using multiple leads, you can follow these steps:
Firstly, obtain a six-lead ECG from a patient positioned in right lateral recumbency, ensuring the limbs are perpendicular to the patient's long body axis. This setup will allow you to capture the necessary data for calculating the electrical axis.
Next, identify an isoelectric lead by examining the QRS complex. An isoelectric lead is one in which the positive and negative deflections of the QRS complex are equal. The lead that is perpendicular to the isoelectric lead will help identify the mean electrical axis. If the QRS complex is predominantly positive in this perpendicular lead, the mean electrical axis is directed toward the positive pole. Conversely, if the QRS complex is negative, the mean electrical axis veers away from the positive pole.
Additionally, you can utilise the amplitude and direction of Electrocardiographic deflections in any two leads that are perpendicular to each other. The equation for this method is based on the trigonometric tangent of an angle. In this equation, G represents the net deflection in the lead with the greater deflection, and s is the net deflection in the lead with the smaller deflection. The angle calculated from this equation is then added or subtracted algebraically from the lead with the greater deflection, depending on the angle's location relative to 0°.
By employing these methods, you can calculate the electrical axis using multiple leads from an ECG. It is important to note that an interprofessional team of clinicians, nurses, and technicians trained in interpreting ECGs is crucial for accurate diagnosis and patient care.
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Frequently asked questions
The electrical axis of an ECG is the major direction of the overall electrical activity of the heart.
The normal range of the QRS axis is between -30° and +90°.
To calculate the electrical axis, you need to determine the heart rate, measure important intervals, and then calculate the net QRS deflection by adding the number of small squares corresponding to the height of the R wave and subtracting the squares corresponding to the Q and S waves.
There are three types of axis deviations: Left Axis Deviation (LAD), Right Axis Deviation (RAD), and Extreme Axis Deviation ("Northwest Axis"). LAD is indicated by a QRS axis less than -30°, RAD by a QRS axis greater than +90°, and Extreme Axis Deviation by a QRS axis between -90° and 180°.
The standard multilead system used in veterinary medicine is the hexaxial lead system, which consists of six limb leads: Leads I, II, III, aVR, aVL, and aVF.
