Sunday, October 21, 2007

An EKG Axis Pearl

"Pearls" are simple, easy-to-understand gems that medical students and residents love to acquire in training because they can quickly simplify a tough concept in medicine, and likewise can be easily shared with colleagues, making them look really intelligent to their colleagues who haven't heard the "pearl" yet. Here's an example:

Recently, a conversation with an internal medical resident went like this on rounds as we reviewed the patient's EKG:

"What the main QRS axis of this patient's EKG?" I asked.

"Well, if the QRS is positive in lead I and negative in lead aVF, it must be left axis."

"Not necessarily. Sounds like you learned EKG axis from Dubin's text, eh."

"Yeah. We have always been told to look at lead I and aVF."

"Well, you should really look at leads I and II."
When I was in medical school, I learned EKG reading from the classic book by Dale Dubin, MD, called "Rapid Interpretation of EKG’s". Now in its 6th edition, the book serves as a cornerstone for the interpretation of EKG’s for most medical students. Even though Dr. Dubin is frowned upon as a textbook for EKG's by some because of his past transgressions, his classic textbook has withstood the test of time and has undoubtedly taught more physicians how to read an EKG than any other textbook.

But it contains some errors, too, and anyone wanting to become an avid EKG reader should be aware of the text's shortcomings. EKG axis determination is one of the most glaring examples.

Perhaps the most challenging concept for most medical students to grasp is that of EKG Axis, that is, the general direction of the heart's depolarization wavefront (or mean electrical vector) in the frontal plane in which the the ventricles electrically activate.

Dubin simplifies axis determination by dividing the frontal plane into four equally-divided major quadrants: normal quadrant (0-+90 degrees), left axis quadrant(-1 to –90 degrees), right axis quadrant (+91 to +180 degrees)and extreme right axis quadrant(-91 to –180 degrees). To identify an EKG with a “normal” axis, then, Dubin suggests students use leads I and aVF to determine the main QRS axis. According to his algorithm, if the main QRS deflection in I and aVL are positive, then the axis should be considered “normal.”

But Dubin’s method fails to acknowledge that QRS axes between 0 and -29 degrees are also considered normal (physiologic). Only at –30 degrees and less, is left axis deviation considered. So Dubin's algorithm is flawed.

So is there a simple way to quickly determine if an EKG has a frontal plane QRS normal axis? Sure! Just look at leads I and II instead of leads I and aVF.

If leads I and II both have a QRS deflection that is mostly positive, then the QRS axis of the EKG is normal. If the QRS in lead I is positive and II is negative, then the QRS axis is defined as left axis deviation. If lead I is negative and II is positive, then the QRS axis is defined as right axis deviation, and if both leads I and II QRS deflections are mainly negative, then the EKG is axis is considered to be extreme right axis deviation. Wikipedia has a nice graphic (but would have been better is they excluded the bottom row, Lead III, and just focused on Leads I and II):

So just by looking at the main QRS deflection in leads I and II on the EKG, rather than leads I and avF, you can always quickly determine whether an EKG has a normal axis, left axis, right axis, or extreme right axis deviation.

Here ends the EKG axis pearl-of-the-day.



Toni Brayer, MD said...

After 20 plus years of reading EKGs, this simple tip is really neat. Thanks!

Pragmatist said...

Heh, thanks for the link, but I should note that while I detailed Dr. Dubin's "transgressions," I ultimately endorsed the book as it is still a good introduction to EKGs.

However, you do make a good point about determining the axis from EKGs that Dubin fails to describe properly, so thanks for the tip!


C said...

Using lead I and II descriminates well between normal axis and left and right axis deviation.
Yet, at +150 lead II is iso-electric and therefore it does not discriminate correctly between right axis deviation and extreme axis, because the transition between these is of course at 180.

Tom B said...

As the creator of said Wikipedia graphic, I appreciate your constructive criticism! If you would like a copy of the graphic with lead III excluded just let me know.