Molly Ayala
The mean electrical axis is the primary vector of ventricular depolarization, representing the sum of all waves of depolarization occurring simultaneously. It typically points towards the left ventricle, which is the larger of the two ventricles. Deviations from the mean electrical axis can be caused by right ventricular hypertrophy or a block in the intraventricular conduction system. Determining the electrical axis on an electrocardiogram (ECG) can aid in diagnosing underlying diseases and conditions, such as cardiac chamber enlargement and conduction defects. Left axis deviation (LAD) is a condition where the mean electrical axis of ventricular contraction lies in a frontal plane direction between −30° and −90°. LAD symptoms depend on the underlying cause and can include shortness of breath, chest pain, palpitations, and dizziness. Exercise may trigger or worsen these symptoms in individuals with LAD, particularly if the cause is related to cardiac function or blood flow.
| Characteristics | Values |
|---|---|
| Normal QRS axis in adults | Between -30° and +90° |
| Normal QRS axis in newborns | Between +30° and +190° |
| Normal QRS axis in children (8-16 years) | Between 0° and +120° |
| Left Axis Deviation (LAD) | QRS axis between -30° and -90° |
| Right Axis Deviation | QRS axis greater than +90° |
| Extreme Axis Deviation | QRS axis between -90° and 180° |
| Causes of LAD | Normal variation, thickened left ventricle, conduction defects, inferior wall myocardial infarction, pre-excitation syndrome, ventricular ectopic rhythms, congenital heart disease, high potassium levels, emphysema, mechanical shift, paced rhythm, left bundle branch block, left ventricular hypertrophy |
| LAD Symptoms | Shortness of breath, fatigue, chest pain (especially with exercise), palpitations, dizziness, fainting |
| LAD Treatment | Treat the underlying cause; if the cause is high blood pressure, target treatment at lowering blood pressure and preventing further enlargement of the left ventricle |
| Determining the electrical axis | Inspect limb leads I and aVF, referred to as the quadrant approach or 2-lead method; draw perpendicular lines at +6 for lead I and -7 for lead III, the intersection is the mean electrical axis |
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What You'll Learn
- Left axis deviation (LAD) is a condition where the mean electrical axis is between −30° and −90°
- LAD symptoms depend on the underlying cause, which can include left ventricular hypertrophy, conduction defects, or congenital heart disease
- Determining the electrical axis can help diagnose underlying diseases and narrow down differential diagnoses
- The electrical axis reflects the average direction of ventricular depolarization during ventricular contraction
- Extreme axis deviation occurs when the QRS axis is between -90° and 180°
Left axis deviation (LAD) is a condition where the mean electrical axis is between −30° and −90°
Left axis deviation (LAD) is a condition where the mean electrical axis of ventricular contraction of the heart lies in a frontal plane direction between −30° and −90°. The QRS vector is directed upward and to the left. LAD can be determined by examining the QRS complex in leads I and aVF. If lead I is positive and lead aVF is negative, there is a possibility of LAD. To confirm LAD, examine the QRS complex in lead II. If it is negative, then it is a true LAD; if it is positive, then it is a normal axis.
There are several methods to determine the electrical axis. The quadrant method, which involves examining leads I and aVF, is the easiest. Each of the four quadrants represents 90° and an axis type. For example, 0° to +90° is a normal axis, +90° to 180° is right axis deviation (RAD), 0° to -90° is LAD, and −90° to 180° is extreme axis deviation. Another method is the 3-lead method, which considers leads I, aVF, and II. If the net QRS deflection is positive in lead I but negative in lead II, then it is LAD.
The presence of LAD can be a result of various factors, including normal variation, thickened left ventricle, conduction defects, inferior wall myocardial infarction, pre-excitation syndrome, ventricular ectopic rhythms, congenital heart disease, high potassium levels, emphysema, mechanical shift, and paced rhythm. LAD in athletes is considered a border deviation, and if combined with another borderline feature, further investigation is required due to the increased risk of sudden cardiac death.
While LAD itself does not require treatment, the underlying cause can be addressed. For example, if left ventricular hypertrophy is the cause of LAD, treatment focuses on lowering blood pressure and preventing further enlargement of the left ventricle using medications such as ACE inhibitors, ARBs, calcium channel blockers, diuretics, and beta-blockers. Determining the electrical axis through methods like ECG interpretation can aid in diagnosing underlying disease states and guiding differential diagnoses.
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LAD symptoms depend on the underlying cause, which can include left ventricular hypertrophy, conduction defects, or congenital heart disease
Left axis deviation (LAD) is a condition where the mean electrical axis of ventricular contraction of the heart lies in a frontal plane direction between −30° and −90°. This is reflected by a QRS complex that is positive in lead I and negative in leads aVF and II. The QRS axis moves leftward throughout childhood and continues into adulthood.
Conduction defects such as bundle branch block can cause LAD on an ECG. Conduction disorders refer to issues with how electrical impulses travel through the heart, which can result in abnormal heartbeats or
Congenital heart disease refers to several different types of heart defects, and symptoms can vary widely. General signs can include shortness of breath in babies during feeding, making it hard for them to gain weight, and shortness of breath in older children and adults during exercise. More severe cases can develop shortly after birth, while symptoms in milder cases may not appear until the teenage years or early adulthood. Children and adults with congenital heart disease are also at risk of developing further problems, such as repeated respiratory tract infections and pulmonary hypertension.
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Determining the electrical axis can help diagnose underlying diseases and narrow down differential diagnoses
The electrical axis of the heart is the sum of all depolarization vectors of the heart. It is a key step in interpreting an electrocardiogram (ECG) and can be determined using various methods. The ventricular (QRS) axis classifications are commonly used in clinical practice, with the normal adult QRS axis being between -30° and +90°, directed down and to the left.
Abnormal axis deviations can indicate underlying pathology. For example, left axis deviation (LAD) is between -30° and -90° and can be caused by inferior myocardial infarction (MI) or left anterior fascicular block, which is common in cats and can accompany feline cardiomyopathy, hyperkalemia, or hyperthyroidism. Right axis deviation (RAD), on the other hand, is a QRS axis greater than +90° and can be caused by right ventricular hypertrophy or right bundle branch block. Extreme axis deviation, where the QRS axis is between -90° and 180°, can be seen in patients with emphysema, pulmonary embolism, or chronic lung disease.
By determining the electrical axis, healthcare professionals can gain insight into underlying disease states and narrow down differential diagnoses. This empowers them to improve patient care and employ vector analysis in various disease states. For example, an emergency department nurse trained in interpreting ECGs can help provide prompt and prudent diagnostic and therapeutic care. Additionally, knowledge of the electrical axis can help identify certain life-threatening arrhythmias, which is crucial for definitive management strategies.
In conclusion, determining the electrical axis is a vital tool for healthcare professionals, helping to diagnose underlying diseases and narrow down differential diagnoses, ultimately improving patient outcomes.
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The electrical axis reflects the average direction of ventricular depolarization during ventricular contraction
The electrical axis is a crucial aspect of electrocardiogram (ECG) interpretation and reflects the average direction of ventricular depolarization during ventricular contraction. This process involves the sequence of vectors associated with different phases of depolarization, which can be visualised through electrode placement on the chest.
Ventricular depolarization is a complex process that begins with the depolarization of the atria, resulting in atrial contraction and blood being pushed to the ventricles. This is represented by the P-wave in the ECG signal. The QRS complex, which is the central component of the signal with a high peak amplitude, corresponds to the depolarization of the ventricular chambers. The QRS complex is of particular interest when determining the electrical axis, with the normal adult QRS axis ranging between -30° and +90°, directed down and slightly to the left.
The mean electrical axis represents the sum of all the mean electrical vectors occurring during ventricular depolarization. This axis typically lies between -30° and +90°. Deviations from this range indicate underlying pathology. A QRS axis less than -30° is classified as left axis deviation (LAD), while a QRS axis greater than +90° is termed right axis deviation. Extreme axis deviation occurs when the QRS axis ranges between -90° and 180°, with the vector directed upward and to the right.
The electrical axis can be determined by evaluating specific leads, such as leads I, II, and aVF. The positive ends of these leads fall within the normal axis region, and the axis is considered normal if all three leads exhibit positive QRS complexes. Another straightforward method is the quadrant approach or 2-lead method, which involves inspecting limb leads I and aVF. By understanding the electrical axis and its deviations, healthcare professionals can improve patient care and make more accurate diagnoses.
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Extreme axis deviation occurs when the QRS axis is between -90° and 180°
The axis of an ECG represents the major direction of the overall electrical activity of the heart. The QRS axis is the most important to determine. The QRS axis in a normal, healthy adult should be within +90° to -30°, directed down and to the left.
Extreme axis deviation is when the QRS axis is between -90° and -180º. This is also referred to as the "Northwest Axis". The situations in which extreme axis deviation may be seen include:
- Misplacement of the limb leads: right arm and left leg leads reversal.
- Ventricular rhythms: ventricular tachycardias, accelerated idioventricular rhythm, and ventricular escape rhythm.
- Hyperkalemia.
- Ventricular pacing.
The most common cause of extreme axis deviation is the reversal of the right arm and left leg leads. When the right arm and left leg leads are reversed, the EKG reveals the following changes:
- Lead I is actually upside-down lead III.
- Lead II becomes upside down.
- Lead III is actually upside-down lead I.
- Leads aVR and aVF are reversed.
- Lead aVL is unchanged.
Extreme axis deviation can also be caused by ventricular pacing. When the stimulation site is near the apex of the right ventricle or the apex of the left ventricle, the QRS complex may have extreme axis deviation.
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Frequently asked questions
A left axis deviation (LAD) is a condition in which the mean electrical axis of ventricular contraction of the heart lies in a frontal plane direction between −30° and −90°. This is reflected by a QRS complex that is positive in lead I and negative in leads II and aVF. LAD can be caused by a variety of factors, including normal variation, thickened left ventricle, conduction defects, inferior wall myocardial infarction, pre-excitation syndrome, ventricular ectopic rhythms, congenital heart disease, high potassium levels, emphysema, mechanical shift, and paced rhythm.
Symptoms of left axis deviation depend on the underlying cause. If left ventricular hypertrophy is the cause, symptoms can include shortness of breath, fatigue, chest pain (especially with exercise), palpitations, dizziness, or fainting. If the cause is a conduction defect such as left bundle branch block, there may be no symptoms unless the defect is caused by heart failure.
There are multiple methods to determine the electrical axis, with the quadrant approach or 2-lead method being the simplest and most adequate. This method involves inspecting limb leads I and aVF. The positive ends of these leads fall within the normal axis region, with lead I at 0°, lead II at +60°, and aVF at +90°. Therefore, if both leads I and aVF have positive QRS complexes, the axis is normal.
