Beatrice Lang
The electrical axis of the heart, also known as the heart axis, is a fundamental aspect of ECG diagnostics. Willem Einthoven introduced the idea of the mean electrical axis of the QRS complex in an ECG in 1913. The electrical axis reflects the average direction of ventricular depolarization during ventricular contraction, which is generally to the left and downwards since the left ventricle makes up most of the heart muscle. The mean electrical axis can be determined using the Novosel formula or graphically with the circle of axes. The normal range for the electrical axis is between −30° and +90°, with deviations indicating underlying pathology.
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What You'll Learn
- The normal range for a mean electrical axis is between –30° and 90°
- The QRS axis is the average direction of ventricular depolarisation
- The QRS axis is between -30° and +90° in a normal axis
- Severe hypertrophy of the right ventricle will cause a right axis shift
- The Novosel formula can be used to calculate the mean electrical axis
The normal range for a mean electrical axis is between –30° and 90°
An electrocardiogram (ECG) is a test that records the electrical activity of the heart. The electrical axis of the heart reflects the average direction of ventricular depolarization during contraction. The direction of the depolarization and, thus, the electrical axis is generally alongside the heart's longitudinal axis, to the left and downwards.
The normal axis range is indicative of a QRS vector that is directed downward and slightly to the left. This is due to the left ventricle comprising most of the heart muscle and generating the most electrical force visible on the ECG. The mean QRS vector typically positions at 59° in a normally situated heart.
Abnormal axis deviations can indicate underlying pathology. A left axis deviation (LAD) is indicated by a QRS axis less than -30°, while a right axis deviation (RAD) is shown by a QRS axis greater than 90°. Extreme axis deviation, also known as the "Northwest Axis", occurs when the QRS axis ranges between -90° and 180°.
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The QRS axis is the average direction of ventricular depolarisation
The QRS axis is also known as the electrical axis of the heart, which is normally directed downward and slightly to the left, with values between -30° and +90°. This is because the left ventricle makes up most of the heart muscle under normal circumstances. A QRS axis greater than +90° is referred to as right axis deviation, while a QRS axis less than -30° is referred to as left axis deviation. Extreme axis deviation, indicating underlying pathology, is demonstrated by a QRS axis between -90° and 180°.
The direction of ventricular depolarisation is generally alongside the heart's longitudinal axis, to the left and downwards. This is visualised by a green arrow in figures representing the mean electrical axis. The mean electrical axis is influenced by the ventricular walls, which depolarise from endocardial to epicardial surfaces. This results in a small positive voltage in lead II. The QRS axis must be at ±90° from aVL at either +60° or -120°.
The QRS axis is also used to diagnose left anterior fascicular block, which is indicated by a QRS axis between -45° and 90° with a QR complex in aVL and a QRS duration of 0.12 seconds. Furthermore, the QRS axis is used to determine the presence of ventricular tachycardia, which is characterised by a wide QRS complex and can result in an extreme axis deviation.
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The QRS axis is between -30° and +90° in a normal axis
The QRS axis is considered normal when it falls between -30° and +90°. This range is indicative of a normal axis on an electrocardiogram (ECG) reading. The QRS axis represents the ventricular axis, which is the average direction of ventricular depolarization during ventricular contraction. This axis is generally directed downward and slightly to the left, reflecting the left ventricle's dominance in the heart muscle.
The normal range of the QRS axis can be further understood by examining specific leads on an ECG. Lead aVL, for instance, is isoelectric and located at -30°. The QRS axis must be ±90° from lead aVL, resulting in either a +60° or -120° axis. Leads I, II, and aVF also play a crucial role in determining the normal axis. When these leads are positive, the QRS axis falls within the normal range of -30° to +90°.
The interpretation of the QRS axis is essential for ECG analysis. By evaluating the vectors produced under the electrodes and considering the relationship between them, the direction of ventricular depolarization can be determined. A positive QRS complex in a lead generally indicates that the ventricular axis is directed toward that lead, while a negative QRS complex suggests the axis is in the opposite direction.
It is important to note that the QRS axis moves leftward throughout childhood and continues into adulthood. At birth, the normal QRS axis ranges from +30° to +190°, gradually shifting leftward between 8 and 16 years to a range of 0° to +120°. This leftward shift reflects the maturation of the heart's electrical axis.
Any deviation from the normal QRS axis range of -30° to +90° may indicate underlying pathology. A QRS axis less than -30° is classified as left axis deviation, while a QRS axis greater than +90° is considered right axis deviation. Extreme axis deviation occurs when the QRS axis falls between -90° and 180°. These deviations can provide valuable insights into the electrical activity of the heart and help identify potential cardiac abnormalities.
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Severe hypertrophy of the right ventricle will cause a right axis shift
The mean electrical axis normally points towards the left ventricle, as this is the largest mass of the myocardium. This means that the surface ECG predominantly records its activity. However, severe hypertrophy of the right ventricle will cause the electrical activity of the right heart to dominate over that of the left on the ECG. This results in a right axis shift of the mean electrical axis.
Right ventricular hypertrophy (RVH) is an abnormal enlargement or pathologic increase in the right ventricular muscle mass. It is often a maladaptive response to chronic pressure overload, commonly arising from chronic, severe lung disease. The right ventricle is considerably smaller than the left ventricle in a normal heart, and its electrical forces are largely obscured by the larger left ventricle.
In the case of RVH, the right ventricle becomes enlarged, leading to a right axis deviation (RAD) and a reversal of the normal R wave progression in the precordial leads. This shift in the mean electrical axis can be observed on an electrocardiogram (ECG or EKG), which measures how well the heart conducts electrical impulses that trigger heartbeats. If the right side of the heart is larger, it will have a harder time conducting these impulses, and the ECG will reflect this by showing a right axis deviation.
The normal axis of the QRS complex on an ECG is between -30° and +90°. A right axis deviation is indicated by a QRS axis greater than +90°. This deviation can be a sign of RVH, with the QRS axis shifting between +90° to +180°. Associated right atrial overload and ST-segment and T-wave abnormalities may also be present, reflecting subendocardial ischemia or repolarization issues.
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The Novosel formula can be used to calculate the mean electrical axis
The Novosel formula, derived by D. Novosel et al. in 1999, can be used to calculate the mean electrical axis of the heart (EA) from standard electrocardiogram (ECG) recordings. The EA is a mandatory topic in almost any undergraduate course on cardiac electrophysiology and is used clinically to detect hypertrophy, cardiac conduction disturbances, and the origin of arrhythmias.
The EA is the direction of the net current observed during cardiac contraction, which is represented by the QRS complex on the electrocardiogram due to the depolarization of cardiomyocytes. The direction of depolarization is generally alongside the heart's longitudinal axis, to the left and downwards, and the mean EA normally points towards the left ventricle because it makes up most of the heart muscle.
The Novosel formula can be used to calculate the EA from leads I and aVF:
EA= +/- Arctan ((2*aVF)/(sqrt(3)*I))
The correction factor 2/sqrt(3) is required because unipolar and bipolar leads have different strengths. The Novosel formula provides a much easier and quicker means for determining the mean EA than the "circle of axes" method and may be useful in a clinical setting.
Dahl and Berg (2020) discussed the principles of the mean EA, its background, underlying physiological processes, and its trigonometry. They also referenced the Novosel formula and highlighted the differences between the theoretical assumptions of the EA and its practical application.
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Frequently asked questions
A normal mean electrical axis on an EKG, or electrocardiogram, is between -30° and +90°. This measurement reflects the average direction of ventricular depolarization during ventricular contraction.
The mean electrical axis of the QRS complex is the sum of all depolarization vectors of the heart. It was first introduced by Willem Einthoven in 1913 and remains a clinically important aspect of ECG diagnostics.
The mean electrical axis can be calculated using vector analysis, specifically by measuring the net QRS voltage in two bipolar limb leads, usually leads I and II.
