What exactly is the ECG axis?
The ECG axis describes what is known as the "electrical heart axis" and shows which direction the electrical activity spreads through the heart chambers. Assessing the ECG axis can help to spot abnormalities or heart conditions at an early stage.
A healthy heart has a typical electrical orientation that can vary slightly depending on a person's anatomy and body shape. When the ECG axis is determined, doctors look at how the electrical signal passes from the upper chambers into the lower chambers and whether this signal falls within a normal range.
How is the ECG axis determined?
To determine the ECG axis, an electrocardiogram (ECG) is carried out. Electrical impulses from the heart are picked up using electrodes and displayed as curves on a monitor or paper strip. The key measurements for working out the ECG axis come from the deflections in certain leads, particularly leads I, II, and aVF.
The calculation is based on the largest deflections in these leads. Depending on which direction the main spread of electrical activity points, one of the following axis types is identified:
Normal and abnormal ECG axis types
In medical practice, several different axis types are recognised:
Indifferent type (normal axis): The electrical axis lies between -30° and +90°. This matches the typical electrical heart axis in most healthy adults.
Left axis deviation: The axis shifts to between -30° and -90°. A leftward shift can occur with thickening of the heart muscle (left ventricular hypertrophy) or certain heart conditions.
Right axis deviation: The axis lies between +90° and +120°. A right axis deviation can point to overloading of the right heart chamber, for example with lung conditions or heart valve disease.
Extreme left axis deviation: Values below -90° are rare and often suggest serious conduction disturbances or a left bundle branch block.
Extreme right axis deviation: Values above +120° often occur with chronic lung disease, a right bundle branch block, or congenital heart defects.
When is an abnormal ECG axis a problem?
Not every change in the electrical heart axis automatically means there is a disease. A left axis deviation, for example, is more common in older people or athletes without any condition that needs treatment. Anatomical differences, such as a changed position of the heart within the chest, can also affect the ECG axis.
However, pronounced deviations can be a sign of cardiovascular disease. Especially when shortness of breath, chest pain, or reduced physical performance also occur, further investigation should take place.
Diagnosis and importance in practice
An abnormal ECG axis on its own is not enough to diagnose a heart condition with certainty. The ECG is always considered alongside other tests, such as an echocardiography (heart ultrasound) or a blood test for cardiac markers.
With an unusual ECG axis, it can be helpful to repeat the ECG regularly in order to monitor changes over time. In some cases, it is only a temporary shift that resolves itself without treatment.
What to do if the ECG axis is abnormal?
If an unusual ECG axis is found, it should be checked by a doctor. This is especially important when accompanying symptoms such as dizziness, shortness of breath, or an irregular pulse are present.
In most cases there is no immediate danger, but the ECG axis can give important clues about strain on the heart, reduced blood flow, or congenital heart changes. A careful assessment by a specialist helps to clarify the cause and, if necessary, start treatment.
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