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What Is an ECG Axis?

By Jo Dunaway
Updated Jan 30, 2024
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An electrocardiograph (ECG) axis is the direction in which all electric activity of the heart is focused. Electrode receptors placed in pairs known as leads on specific vector locations on the body, can map the heart’s electrical activity through ECG machine. This cardiac test is known as an electrocardiogram. The electrocardiogram will reveal measurements of overall heart rhythms and any weaknesses in heart muscles. Once the direction is located, it is possible to determine how much of a focus it might be.

There are usually 12 leads when recording an ECG, six placed at junctions of limbs and six placed in chest areas. By imagining a line drawn between the lead pairs one can see vectors of leads in a circumference ray diagram of zero degrees, 60 degrees, 120 degrees, and so forth, around an elliptical circle. Each of these lead pairs views the heart from a different angle and reads for electrical activity. Electrolyte imbalances causing abnormal rhythms, known as myocardial infarction (MI), can be detected and areas of heart muscle that are affected can be located. The pumping ability of the heart, however, cannot be measured in an ECG; thus it is possible to have a good ECG even while in cardiac arrest.

In the scenario of a healthy human heart, the ECG axis would point toward the largest muscle in the heart, which is normally the left ventricle, also known as left chamber. This mass of muscle in the left chamber is normally assisted somewhat by the right chamber. The left chamber is responsible for blood flows running in a line from the left leg to the right shoulder. If the left ventricle enlarges, it is said to be in a left ECG axis deviation if this deviation is more than 30 degrees off a normal ECG axis. A deviation of 30 to 90 degrees off a normal axis may signal MI or emphysema, or it could be the normal reading for a pregnant woman.

The left and right ECG axis deviations point to differing health conditions. The left deviations, as stated above, can be abnormal heart rhythms in the left chamber. Some causes of an ECG axis deviation to the right are chronic lung disease, a pulmonary clot, or right chamber enlargement when the right ventricle muscle has become thickened. Another cause of such a right deviation is called atrial septal defect, when the upper walls of the heart do not close completely, in a congenital heart defect.

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