Wiktor Wise
The electrical axis of the heart is a crucial component of electrocardiogram (ECG) interpretation, providing valuable insights into the underlying health of the heart and helping to guide diagnoses. Changes in body position, such as transitioning from sitting to standing, can cause variations in the electrical axis, QRS amplitude, ST segment, and T wave. These shifts are attributed to alterations in the anatomical orientation of the heart within the chest cavity, changes in lung volume, and variations in electrode-skin contact. During exercise, the heart rate increases linearly with the intensity of the physical activity, and this relationship between heart rate and workload can be plotted to observe the heart's chronotropic response. While a rapid heart rate increase during or after exercise may suggest the development of tachyarrhythmia, a sudden decrease in heart rate along with low blood pressure may indicate presyncope or syncope associated with a vasovagal response. Furthermore, abnormal heart rate responses during recovery are strong predictors of all-cause mortality in clinical populations. Therefore, understanding the behaviour of the mean electrical axis during exercise is essential for interpreting ECG readings, making accurate diagnoses, and providing effective patient care.
| Characteristics | Values |
|---|---|
| Normal range of mean electrical axis | Between 0 to +160 degrees in cats and +40 to +100 degrees in dogs |
| Normal range of QRS axis | Between -30° and +90° |
| Abnormal axis deviation | Left Axis Deviation = QRS axis less than -30° |
| Right Axis Deviation = QRS axis greater than +90° | |
| Extreme Axis Deviation = QRS axis between -90° and 180° | |
| Changes in ECG during exercise | Changes in the electrical axis, QRS amplitude, ST segment, and T wave |
| Changes in ECG during rest | Combined lead-I patterns (camel-hump or ST-segment prolongation) |
| Changes in ECG during exercise stress tests | Supine and standing positions |
| Indicators to stop exercise during stress testing | Ventricular fibrillation (VF), sustained ventricular tachycardia (VT), and ST-segment elevation (≥1 mm) in leads without diagnostic Q waves |
| Excessive ST-segment depression (>2 mm of horizontal or downsloping ST-segment depression) or marked axis shift | |
| Arrhythmias other than VT including multifocal PVCs, triplets, supraventricular tachycardia (SVT), heart block, and bradyarrhythmias | |
| Development of bundle branch block | |
| Heart rate response during exercise | Heart rate increases linearly with exercise intensity up to the maximum heart rate |
| Heart rate response post-exercise | A rapid fall in heart rate may precipitate presyncope or syncope associated with a vasovagal response |
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What You'll Learn
- Detecting changes in the mean electrical axis requires a multilead ECG
- The hexaxial lead system is divided into four equal quadrants
- A positive QRS in Lead I indicates a normal mean electrical axis
- Right ventricular hypertrophy and right bundle branch block cause a right axis shift
- Changes in body position cause shifts in the electrical axis
Detecting changes in the mean electrical axis requires a multilead ECG
Detecting changes in the mean electrical axis is crucial for understanding the heart's electrical axis and identifying any underlying abnormalities. To achieve this, a multilead ECG is necessary, as it provides a comprehensive view of the heart's electrical activity from different angles.
The electrical axis of the heart is the sum of all depolarization vectors, and determining it is a key step in interpreting an ECG. Myocardial cells are polarized during the resting state, and during depolarization, ion shifts cause an orderly wavefront across the myocardium. This results in the familiar ECG tracing, with the wave of depolarization moving toward or away from the positive lead, creating positive or negative deflections, respectively.
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). In human electrocardiography, the same leads form the basis of the hexaxial reference system used to calculate the heart's electrical axis in the frontal plane.
To detect changes in the mean electrical axis, a multilead ECG setup is required. A single lead II ECG tracing is usually sufficient for diagnosing arrhythmias, but for identifying changes in the mean electrical axis, additional leads are needed. This helps rule out artifacts as the cause of abnormalities seen in a single-lead tracing and aids in diagnosing conduction abnormalities, detecting ventricular hypertrophy, and determining the origin of arrhythmias.
The process of determining the mean electrical axis involves obtaining a six-lead ECG from a patient positioned in right lateral recumbency, with their limbs perpendicular to their long body axis. One method for estimating the mean electrical axis is to find 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. If the QRS complex is predominantly positive in this perpendicular lead, the mean electrical axis is directed toward the positive pole; if it's negative, the axis is directed away from the positive pole.
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The hexaxial lead system is divided into four equal quadrants
The hexaxial lead system is a standard multilead system used in veterinary medicine, formed by six limb leads. Leads I, II, and III are bipolar leads, meaning they have a positive and negative pole. Leads aVR, aVL, and aVF are unipolar leads, meaning they only have a positive pole. The hexaxial lead system is arranged around the heart in a short-axis plane, referred to as the frontal plane, which is labelled in degrees (0 to +180 and 0 to -180).
The quadrant graphing method is one way to determine the mean electrical axis. In this method, the hexaxial lead system is divided by leads I and aVF into four equal quadrants. Arrows are drawn on leads I and aVF in the direction of the predominant QRS complex deflection. For example, if the QRS complex in lead I is predominantly negative, an arrow will be drawn toward the negative pole of lead I. The mean electrical axis will be closer to the larger of the two. The mean electrical axis can be placed in one of the four quadrants by examining the relative size of the QRS complexes in leads I and aVF.
Interpreting the electrocardiogram (ECG) is a key step in determining the heart's electrical axis. The cardiac axis is the sum of all depolarization vectors of the heart, and vector analysis determines the direction of the net flow of current through the heart. A sudden increase in heart rate during or post-exercise may indicate the development of a tachyarrhythmia. Certain rhythms indicate the need to stop exercise immediately during stress testing, including ventricular fibrillation (VF) and sustained ventricular tachycardia (VT).
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A positive QRS in Lead I indicates a normal mean electrical axis
The interpretation of an electrocardiogram (ECG) is a key step in determining the heart's electrical axis. The cardiac axis is the sum of all depolarization vectors of the heart, and vector analysis determines the direction of the net flow of current through the heart. The QRS axis is the most important to determine.
The most efficient way to estimate the axis is to look at Lead I and Lead aVF. The QRS complex in each lead is examined to determine if it is positive, isoelectric (equiphasic), or negative. A positive QRS in Lead I puts the axis in roughly the same direction as Lead I. A positive QRS in Lead aVF similarly aligns the axis with Lead aVF. Combining both coloured areas – the quadrant of overlap determines the axis. So, if Lead I and aVF are both positive, the axis is between 0° and +90° (i.e. normal axis).
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 mean electrical axis will be within 30 degrees of the positive pole of this lead.
The approximate location of the mean electrical axis can be found by identifying the lead with the tallest R wave. 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.
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Right ventricular hypertrophy and right bundle branch block cause a right axis shift
Interpreting an electrocardiogram (ECG) is a key step in determining the heart's electrical axis. The cardiac axis is the sum of all depolarization vectors of the heart, and vector analysis determines the direction of the net flow of current through the heart. Right ventricular hypertrophy and right bundle branch block (RBBB) can cause a right axis shift. The right bundle branch is part of the heart's electrical conduction system, and a block in this area can impact the normal flow of electrical impulses, leading to a change in the electrical axis.
Right ventricular hypertrophy refers to an enlargement of the right ventricle, which can be caused by various conditions such as pulmonary hypertension or congenital heart defects. This enlargement can alter the heart's electrical activity and result in a right axis deviation on an ECG. The right bundle branch block is a condition where the electrical impulse that controls heart rhythm slows or blocks as it moves through the right bundle branch. This can lead to a delay in ventricular contraction and changes in the ECG pattern, including a right axis shift.
In a healthy heart, the electrical impulse travels down the right and left bundle branches simultaneously, resulting in a normal ECG pattern. However, in the case of RBBB, there is a delay or block in the right bundle branch, causing the left bundle branch to activate first. This change in the normal conduction pattern can lead to a right axis deviation, as observed in an ECG. The degree of deviation can vary depending on the severity of the RBBB and the presence of other cardiac conditions.
The presence of right ventricular hypertrophy and RBBB can have important clinical implications. While an isolated RBBB may not require treatment, when combined with heart failure, it can indicate the need for cardiac resynchronization therapy. Additionally, the presence of RBBB can increase the risk of death in individuals with heart failure or a heart attack. Therefore, an understanding of the electrical axis and its deviations is crucial for clinicians to interpret ECG results accurately and make appropriate diagnoses and treatment decisions.
During exercise, the heart rate increases linearly with exercise intensity up to the maximum heart rate. This increase in heart rate can result in ECG changes, including alterations in the electrical axis. While a right axis shift during exercise may not always be cause for concern, especially in healthy individuals, it is important to monitor for other ECG changes that could indicate dangerous rhythms or underlying cardiac conditions. A sudden increase in heart rate during or after exercise, for example, could suggest the development of tachyarrhythmia, while a rapid fall in heart rate and a dramatic drop in blood pressure may indicate presyncope or syncope.
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Changes in body position cause shifts in the electrical axis
Changes in body position are known to cause shifts in the electrical axis of the heart. This is mainly due to changes in the anatomical orientation of the heart within the chest cavity, changes in lung volume, and changes in electrode contact with the skin.
The electrical axis of the heart is the sum of all depolarization vectors of the heart, and it provides insight into underlying disease states. The normal cardiac axis is directed downward and slightly to the left, with the QRS axis between -30° and +90°.
When the body is in a reclining or sitting position, the mean QRS axis can vary significantly compared to when it is in a supine position. The T-wave axis, however, remains relatively consistent between the supine and standing positions. The QTc interval also shows notable changes when transitioning from a supine to a standing position.
The mean electrical axis can be determined by identifying the lead with the tallest R wave. It will be within 30 degrees of the positive pole of this lead. For example, if lead II has the tallest R wave, the mean electrical axis is normal. Another method for determining the mean electrical axis is the quadrant graphing method, which uses the hexaxial lead system and involves drawing arrows on leads I and aVF in the direction of the predominant QRS complex deflection.
Shifts in the electrical axis can be caused by various conditions, including right ventricular hypertrophy and right bundle branch block (RBBB). These conditions can be distinguished by the duration of the QRS complex, with right ventricular hypertrophy associated with a normal duration and RBBB resulting in a prolonged duration.
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Frequently asked questions
The mean electrical axis is the sum of all depolarization vectors of the heart. It is directed downward and slightly to the left under normal circumstances.
The mean electrical axis can be determined by identifying the lead with the tallest R wave. It will be within 30 degrees of the positive pole of this lead.
The normal range of the mean electrical axis in humans is between -30° and +90°. In cats, the normal range is between 0° and +160°, and in dogs, it is between +40° and +100°.
Changes in the mean electrical axis during exercise can indicate underlying disease states or conduction abnormalities. For example, a right axis shift may be caused by right ventricular hypertrophy or right bundle branch block.
Body position can affect the mean electrical axis, with significant variations observed between the reclining, sitting, and standing positions. These changes are attributed to alterations in the anatomical orientation of the heart, lung volume, and electrode contact with the skin.
