MicroEKG Manual

QRS Axis

MicroEKG Details

Axis Deviation:

The QRS axis is the “average” direction of electrical activity during ventricular depolarization. The QRS axis may shift due to physical change in the position of the heart, chamber hypertrophy, or conduction block.

Normal QRS axis is from around -30 to +90 degrees. More negative than -30 is called left axis deviation. More positive than +90 is called right axis deviation.

The average direction of electrical activity is the "QRS axis."
See the diagram: if the axis is in the direction labeled "II" the axis is +60 degrees.

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Right axis deviation is seen on the ECG when more electrical forces are moving to the right than normal. This is usually due to hypertrophy of the right ventricle (RVH). Causes of right axis deviation include COPD, pulmonary emboli, valvular disease, septal defects, and pulmonary hypertension.

An axis of +90 is common in persons with emphysema. This so-called “vertical heart” reflects both the rotation of the heart downward as the diaphragm position drops due to air trapping, and some degree of hypertrophy of the right ventricle.

Left axis deviation occurs when additional electrical forces move to the left (hypertrophy), or when the time required for the electrical activity to move over the ventricle is prolonged (LBBB, left ventricular dilation).

Causes of left axis deviation include hypertension, aortic stenosis or regurgitation, subaortic stenosis, mitral regurgitation, and left ventricular conduction defects.

The QRS axis may shift during the respiratory cycle if elevation of the diaphragm changes the physical position of the heart. Beat-to-beat variation in QRS axis (an every-other-beat change in QRS shape) is called “electrical alternans.” This is thought to be caused by the heart physically swinging back and forth in a pericardial effusion.  [Chapter Menu]

Determining QRS axis by inspection:

The “inspection” method of determining axis requires that you check QRS orientation in specific leads. It is usually much faster than the vector method, and provides the same clinical information. QRS Axis by Inspection Method
lead I positive, lead III positive = normal axis
lead I negative (+/- R positive) = RIGHT axis
lead III negative, lead II negative = LEFT axis

First check lead I. If the QRS is “positive” overall (comparing negative deflections to positive deflections), right axis deviation is ruled out.

If the QRS in lead I is negative, right axis deviation is mild. Now check lead R. If the QRS is overall positive, right axis deviation is definite.

Now check for left axis deviation by inspecting lead III. If the QRS is overall positive, left axis is ruled out.

If the QRS is negative in III, check lead II. If lead II also shows an overall negative QRS, left axis deviation is diagnosed.

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In this ECG, lead I is positive. Next we look at lead III and note it’s negative. We check lead II. Because lead III and lead II are both negative, we diagnose left axis deviation by the “inspection method.”  [Chapter Menu]

Vector method for QRS axis:

Determining QRS axis by vector method is most easily done using lead F and lead I. These leads are convenient because they are at right angles to each other.

First determine lead I’s QRS size and orientation by subtracting the S wave height from the R wave height. Then determine lead F’s size in the same way.

vectaxis.gif (2741 bytes) Plot out the overall QRS size on the line representing lead I. Positive is to your right, negative to your left. Plot out the QRS size on the line representing lead F. If F’s QRS is positive, draw downward. If negative, draw up.

Draw lines perpendicular to the end points of your lines, to form a rectangle. Draw a line from the centerpoint to the corner of your box. This is the electrical vector. Its orientation represents the electrical axis

Remember that lead I is pointing to zero degrees, directly to your right (the patient’s left). Clockwise from lead I is positive, counterclockwise is negative. In the diagram on the previous page, we would estimate the axis at minus 40 degrees.  [Chapter Menu]

Equiphasic lead method for QRS axis:

Another alternative for estimating QRS axis is the “equiphasic lead” method. Locate a lead that has the smallest total QRS complex and/or is equiphasic. The QRS axis should be at 90 degrees to this lead.
Now look at the lead that (on the vector diagram) is 90 degrees from the equiphasic lead. If this lead’s QRS complex is positive, the QRS axis is in the direction of that lead. If negative, the QRS axis is 180 degrees opposite.

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In this ECG, lead III is most nearly equiphasic. The QRS axis will therefore be at 90 degrees to it. We would estimate the axis at around plus 30 degrees.  [Chapter Menu]

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Go to Chapter 4, Ventricular Block

All material referenced through this menu is excerpted from copyrighted works by Bruce Argyle, MD. You are welcome to use selected portions, as long as appropriate credit is given. The credit for the text referenced through this menu is:

Argyle, B., MicroEKG Computer Program Manual.
Mad Scientist Software, Alpine, Utah

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