Mastering Axis Determination: Part 6
Updated: Aug 14, 2021
By now you can predict the QRS axis in the frontal plane within 15 degrees as long as you have an equiphasic (or isoelectric) lead in the frontal plane. So what constitutes a normal QRS axis? What is a left axis deviation? A right axis deviation?
If you don’t have a copy of the hexaxial reference system, go back to Part IV and print yourself out a copy.
As a review, lead I cuts the hexaxial reference system in half horizontally, and lead aVF cuts the hexaxial reference system in half vertically. You can think of this as an x and y-axis that divides the hexaxial reference system into quadrants. Hence, you can use leads I and aVF to place the heart’s electrical axis into one of the four quadrants. This is sometimes called the Quadrant Method for axis determination.
Remember that the normal QRS axis goes from a right shoulder-to-left leg direction in most patients. In other words, it tends to point down and to the left, or toward the left inferior quadrant of the hexaxial reference system, which ranges from 0 to +90 degrees. When the QRS axis in the frontal plane is in the normal quadrant, you will have positive QRS complexes in lead I and positive QRS complexes in lead aVF.
When the QRS axis is 0 to -90 degrees, we call it a left axis deviation. This is the left superior quadrant of the hexaxial reference system. When the QRS axis is in the left superior quadrant, you will have positive QRS complexes in lead I and negative QRS complexes in lead aVF.
In reality, the QRS axis can be slightly into the left superior quadrant and still be considered normal.
When the axis is between 0 and -30 degrees, it is sometimes referred to as a physiological (as opposed to pathological) left axis deviation. With a physiological left axis deviation, lead II is usually equiphasic (remember that lead II is perpendicular to lead aVL and lead aVL points to -30 degrees on the hexaxial reference system). For a good example of this, see the ECG from Part V. Is this ECG normal? Absolutely not! But the axis is technically normal, even though it extends into the left superior quadrant at -26 degrees.
The most common causes of pathological left axis deviation are left anterior fascicular block or Q-waves from inferior wall myocardial infarction. Some sources say that left ventricular hypertrophy pulls the axis to the left, and while this seems logical, in most cases patients with left ventricular hypertrophy have a normal QRS axis. Electrolyte derangements and ventricular rhythms may also present with a left axis deviation. Paced rhythms in particular should have a left axis deviation if the pacing lead is in the apex of the right ventricle.
If the QRS axis in the frontal plane is +90 to 180 degrees, it is a right axis deviation. This is the right inferior quadrant of the hexaxial reference system. With a right axis deviation, you will have negative QRS complexes in lead I and positive QRS complexes in lead aVF.
A right axis deviation is usually abnormal. It might indicate pulmonary disease, right ventricular hypertrophy, Q-waves from lateral wall myocardial infarction, left posterior fascicular block, electrolyte derangement, tricyclic antidepressant overdose, or a ventricular rhythm.
If the QRS axis is -90 to 180 degrees, something is very wrong (possibly your lead placement). This is the right superior quadrant of the hexaxial reference system, but in various publications, it can be called an extreme right axis deviation, an indeterminate axis, or a right shoulder axis. It’s bad because it means the heart is depolarizing in the wrong direction. With an extreme right axis deviation, you will have negative QRS complexes in lead I and negative QRS complexes in lead aVF.
Finally, here’s a cheat sheet you can fall back on if all else fails. This one relies only on leads I, II, and III (although you can substitute lead aVF for lead III). This method works pretty well because, as we saw earlier, by looking for an equiphasic QRS complex in lead II we can distinguish between physiological and pathological left axis deviation.
Remember, QRS complexes in lead III are allowed to be negative. However, negative QRS complexes in lead I or lead II are abnormal.
Do I expect you to remember all this right now? No, I do not. Experience is the best teacher, and there’s nothing like holding an ECG in your hand and associating it with a particular patient. My goal is simple. I want you to start seeing it.
When you capture a 12-lead ECG with good data quality, in most cases you’ll get a computerized interpretive statement at the top. You’ll also get the computer measurements of the heart rate, PR interval, QRS duration, QT/QTc interval, and P-QRS-T axis. When you see that the QRS axis is -66 degrees and the interpretive statement says “Left axis deviation” I want you to take a good look at the ECG. Do you notice that the QRS complexes are positive in lead I and negative in leads II, III, and aVF?
A deeper understanding of axis determination helps you really see the 12-lead ECG, not just lead II, and it ultimately helps you consider various possibilities that you hadn’t considered before. It’s also the key to understanding bifascicular blocks, and can be helpful in the differential diagnosis of wide complex tachycardias.