Molly Ayala
The mean electrical axis is the principal vector of ventricular depolarization, representing the sum of all the waves of depolarization occurring simultaneously. It is important to determine the heart's electrical axis to gain insight into underlying disease states and to steer diagnosis. The mean electrical axis typically points toward the left ventricle, as this is the larger of the two ventricles. Severe hypertrophy of the right ventricle will cause the electrical activity of the right heart to dominate, resulting in a right axis shift. The hexaxial lead system is arranged around the heart in a short-axis plane, referred to as the frontal plane, with the left ventricular apex oriented generally toward the positive pole of lead II. The QRS axis is at 90° to the isoelectric lead, pointing in the direction of the positive leads. Lead II is isoelectric and perpendicular to aVL, and is predominantly positive, resulting in a normal mean electrical axis at +60 degrees.
| Characteristics | Values |
|---|---|
| Mean electrical axis | The principal vector of ventricular depolarization |
| Normal direction of mean electrical axis | Towards the left ventricle |
| Reason | The left ventricle is the largest mass of myocardium |
| Lead II | Placed between right arm (-ve) and left leg (+ve), +60° |
| Lead II is positive | Axis will be around +60 degrees |
| Normal axis | Between 0° and +90° |
| RAD | Between +90° and +180° |
| LAD | Between 0° and -90° |
| Extreme axis | Between -90° and -180° |
| Normal mean QRS axis in adults | Between about −30° and +100° |
| RAD | QRS axis of +100° or more positive |
| LAD | QRS axis of –30° or more negative |
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What You'll Learn
- The mean electrical axis is the principal vector of ventricular depolarization
- The QRS axis is at 90° to the isoelectric lead
- The mean electrical axis points towards the left ventricle
- Severe hypertrophy of the right ventricle will cause a right axis shift
- The hexaxial lead system is arranged around the heart in a short-axis plane
The mean electrical axis is the principal vector of ventricular depolarization
The mean electrical axis is important in the field of medicine as it helps in diagnosing conduction abnormalities, detecting ventricular hypertrophy, and differentiating the origin of arrhythmias. To determine the mean electrical axis, one must first find the lead axis that is equally biphasic, followed by finding the lead axis that is perpendicular to the biphasic lead and has a positive net deflection. The mean electrical axis is typically determined using a multilead ECG, which measures the net electrical activity of the myocardium.
In a hexagonal system used for vertical axis determination, the normal directions of the lead vectors are as follows:
- Lead I: Placed between the right arm (negative) and left arm (positive) horizontally, 0°
- Lead II: Placed between the right arm (negative) and left leg (positive), +60°
- Lead III: Placed between the left arm (negative) and left leg (positive), +120°
- Lead aVR: Unipolar lead with a single positive electrode at the right arm, +210°
- Lead aVL: Unipolar lead with a single positive electrode at the left arm, -30°
- Lead aVF: Unipolar lead with a single positive electrode at the left foot, +90°
The QRS axis, which indicates ventricular depolarization, can be determined by examining the QRS complex. If the QRS is positive in a given lead, the axis points in roughly the same direction as this lead. Conversely, if the QRS is negative, the axis points in the opposite direction. If the QRS is isoelectric, the axis is at 90° to the lead.
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The QRS axis is at 90° to the isoelectric lead
The QRS axis is an important aspect of electrocardiogram (ECG) interpretation, providing valuable insights into the heart's electrical axis and underlying health conditions. When the QRS axis is at 90° to the isoelectric lead, it points in the direction of the positive leads, and this concept can be better understood through examples and calculations.
In ECG analysis, the isoelectric lead is crucial for determining the QRS axis. The isoelectric lead is characterised by zero net amplitude, which can be observed as a biphasic QRS with equal R wave height and S wave depth or as a flat-line QRS with no discernible features. Lead aVL is an example of an isoelectric lead, exhibiting similar-sized positive and negative deflections.
Let's consider an example to illustrate the concept. Lead aVL, being isoelectric, is located at -30° on the axis diagram. Since the QRS axis must be ± 90° from lead aVL, it can be determined to be either at +60° or -120°. Now, let's examine the positive leads, which are leads I (+60°), II (+60°), and aVF (+90°). With this information, we can conclude that the QRS axis falls within the quadrant between 0° and +90°, resulting in a normal axis.
The relationship between the QRS axis and the isoelectric lead is fundamental to ECG interpretation. If the QRS complex is positive in a given lead, the axis aligns with the direction of that lead. Conversely, if the QRS complex is negative, the axis points in the opposite direction. When the QRS complex is isoelectric, the axis is at 90° to that particular lead. This principle is essential for determining the QRS axis and understanding the overall electrical axis of the heart.
Additionally, the mean electrical axis, which represents the principal vector of ventricular depolarization, typically points towards the left ventricle due to its larger mass. To identify the mean electrical axis, an isoelectric lead, such as lead aVL, can be used. In this case, lead II, which is perpendicular to aVL and predominantly positive, indicates a normal mean electrical axis at +60 degrees.
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The mean electrical axis points towards the left ventricle
The mean electrical axis is the principal vector of ventricular depolarization. It represents the sum of all the waves of depolarization that are occurring simultaneously. The mean electrical axis typically points towards the left ventricle, which is the larger of the two ventricles and comprises most of the heart muscle. This means that the left ventricle generates the most electrical force visible on the ECG.
The mean electrical axis is determined by evaluating the vectors produced under the electrodes. A positive QRS complex in a lead has a ventricular axis in the same direction as the lead, whereas a negative QRS complex in a lead has a ventricular axis in the opposite direction. If the QRS complex is isoelectric in a lead, the ventricular axis is perpendicular (90°) to that lead. The normal adult QRS axis is between -30° and +90°, directed down and to the left.
The mean electrical axis is important in the interpretation of an electrocardiogram (ECG). It helps to identify the exact nature of an arrhythmia and can provide insight into underlying disease states. A single lead II ECG tracing is generally sufficient to diagnose arrhythmias, but a multilead ECG is necessary to detect changes in the mean electrical axis.
The left ventricle is oriented towards the positive pole of lead II. Lead II is placed between the right arm (negative) and the left leg (positive) at +60°. The QRS axis must be ± 90° from lead II, at either +150° or -30°. The mean electrical axis is approximately +60°, which is normal.
In summary, the mean electrical axis typically points towards the left ventricle because it is the largest ventricle and generates the most electrical force. The left ventricle is oriented towards the positive pole of lead II, which has a normal axis of +60°.
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Severe hypertrophy of the right ventricle will cause a right axis shift
The mean electrical axis is the principal vector of ventricular depolarization, representing the sum of all the waves of depolarization occurring simultaneously. It typically points towards the left ventricle, as it is the larger of the two ventricles and produces the most substantial electrical forces.
However, severe hypertrophy of the right ventricle can cause a right axis shift. This is because the right ventricle undergoes an abnormal enlargement or pathologic increase in ventricular muscle mass, resulting in a maladaptive response to chronic pressure overload. This condition, known as right ventricular hypertrophy (RVH), is commonly associated with chronic, severe lung disease. The enlarged right ventricle fills the retrosternal space, which can be observed through chest radiographs.
The presence of RVH leads to right axis deviation (RAD), causing a shift in the mean electrical axis. The QRS axis, which is used to determine the electrical axis, shifts to between +90° and +180° in cases of RAD, indicating a deviation from the normal range of -30° to +90°. This shift can be identified through an electrocardiogram (ECG), which measures the heart's electrical impulses.
The ECG plays a crucial role in evaluating RVH, as it helps detect abnormalities in the mean electrical axis and provides insights into underlying disease states. It is particularly useful when the right side of the heart appears larger than usual, as a larger heart has a harder time conducting electrical impulses effectively. By analyzing the QRS complexes in specific leads, clinicians can determine the ventricular axis and identify any deviations caused by RVH.
In summary, severe hypertrophy of the right ventricle can cause a right axis shift due to the abnormal enlargement of the right ventricle, resulting in electrical forces that dominate over those of the left ventricle. This shift can be detected and evaluated through ECG measurements, which provide valuable information for diagnosis and treatment.
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The hexaxial lead system is arranged around the heart in a short-axis plane
The hexaxial lead system is used to determine the mean electrical axis, which is the principal vector of ventricular depolarization. The mean electrical axis typically points towards the left ventricle, as this is the larger of the two ventricles and has the largest mass of myocardium. Therefore, the surface ECG predominantly records its activity. In the hexaxial lead system, the left ventricular apex is oriented towards the positive pole of lead II.
The mean electrical axis can be determined by finding the most isoelectric lead in the frontal plane and then looking for the perpendicular lead on the hexaxial reference system. Lead II is perpendicular to lead aVL and is predominantly positive. This means that the mean electrical axis is normal at +60 degrees.
The hexaxial lead system is also used to determine the QRS axis, which is the average direction of all cardiac vectors. The QRS axis is at 90 degrees to the isoelectric lead, pointing in the direction of the positive leads. For example, if lead I and lead aVF are both positive, the QRS axis will be between 0 and +90 degrees, which is a normal axis.
The hexaxial lead system is a valuable tool for interpreting ECG data and determining the electrical axis of the heart. It helps to identify arrhythmias and diagnose underlying disease states, as well as to detect ventricular hypertrophy and differentiate the origin of arrhythmias.
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Frequently asked questions
The mean electrical axis is the principal vector of ventricular depolarization. It represents the sum of all the waves of depolarization occurring simultaneously.
The mean electrical axis points towards the left ventricle because it is the largest mass of myocardium, so the surface ECG predominantly records its activity.
The normal mean electrical axis in adults lies between about -30° and +100°.
Lead II is placed between the right arm (negative) and left leg (positive) and is positioned at +60°. The mean electrical axis is normal at +60° when lead II is predominantly positive.
Deviations of the mean electrical axis result from either right ventricular hypertrophy or a block in the intraventricular conduction system.
