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.
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:
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.
In this ECG, lead I is positive. Next we look at lead III and note its 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 Is QRS size and orientation by subtracting the S wave height from the R wave height. Then determine lead Fs size in the same way.
Remember that lead I is pointing to zero degrees, directly to your right (the patients 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.
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