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Teaching Tip: 12 Leads are Better Than One (Or Three)

Years ago, I was tasked with introducing 12-lead ECG interpretation to firefighter/paramedics who had been using ECG for rhythm monitoring for years.  Some were eager to add to their skills, others - not so much.  The feeling was, we have been doing just fine as we are.  When finally convinced that they could interpret STEMI with a 12-lead, many were content to use the 12-lead ECG only for that.  

To illustrate to students the great value of multi-lead assessment, I devised a little "quiz".  I showed the students ten to twelve short rhythm strips, like you see here.  All were cropped from 12-lead ECGs.  I asked my class to interpret the strips as they would if they were taking an ACLS class.  Usually, all did fine, or so they thought.  When shown the 12-lead ECGs the strips were taken from, EVERY student changed his or her mind on EVERY ECG.  The lesson is:  sometimes what we are looking for shows up in some leads and not others.  You can find this illustrated hundreds of times just in the ECG archives on this site.  I will supply some ECGs here on this page over the next few weeks that you could use to show your own students the value of "multi-lead assessment".  

What started as a hard-sell turned out to be a fun exercise.

The ECG shown here is of a patient in V Tach.  There are several strong signs that this is V Tach, including the wide QRS complexes, lack of associated P waves, "backward" axis, also called extreme right axis deviation (Leads II, III, and aVF are all negative and aVR is positive), and V6 is negative.  For more review of the differential diagnosis of wide-complex tachycardias, go to our Ask the Expert answer from Jason Roediger.  This LINK willl take you to Dr. Grauer's informative webpage where he offers a step-by-step guide to differentiating the WCTs.

 The focus of THIS lesson is that, while the patient is in V Tach, and it is in every lead, the tell-tale signs are harder to see in some leads than others.  Remember to show your  Remember to share with your students that the channels of the ECG (in this case three) are run simultaneously, so that the same heartbeat is seen several times - once for each channel.

Dawn's picture

Question: Does an extreme right axis (backward) always indicate a ventricular rhythm?

Today's expert is Jason E. Roediger, CCT, CRAT, who is a highly respected Cardiovascular Technician at the Dept. of Veterans Affairs, Hunter Holmes McGuire VA Medical Center in Richmond, VA. He is known for holding numerous certifications in all levels of ECG interpretation, and also for scoring 100% on the Level IV Advanced ECG Board Certification exam that is usually reserved for cardiologists.

Answer: Always? No. Usually, Yes.
There are exceptions to several "golden rules" in electrocardiography and this one is not exempt. One of the chronic issues contributing towards widespread confusion in understanding electrical axis is a lack of continuity in terminology. There is no general concensus on how to refer to an axis in the right upper quadrant. Depending on which author you are reading, it has traditionally been known by multiple names: Northwest axis. . . upper right quadrant. . . extreme right axis. . . right superior axis. . . "no-man's-land" (i.e., "N-M-L".). . . etc. Because my first exposure to electrical axis was through Dr. Marriott's textbooks and he prefered to use "N-M-L", I have personally latched on to that particular name as well. Even though some persist in calling it an "extreme left axis" or "far left axis deviation", this practice is frowned upon and discouraged. It's important to note that an axis in "N-M-L" is not synonymous with an "indeterminate" axis which occurs when the QRS is essentially isodiphasic or equphasic in all 6 limb leads and therefore the polarity of the QRS cannot be discerned in leads I and aVF.

Definition: An axis in "N-M-L" is recognized when the QRS complex has a predominantly or wholly negative deflection (i.e., down) in leads I and aVF. The axis is −90 to −180 degrees.

Irregardless of which descriptive name you prefer, in the context of a wide QRS complex tachycardia, this particular axis is highly predictive of ventricular tachycardia and is rarely encountered in "conducted" rhythms however some examples of aberrant SVT have been published with an axis in "N-M-L".

In summary: An axis in "N-M-L" implies (but is not proof of) an apical origin to the rhythm and should make one think of and exclude the possibility of ventricular tachycardia. As a general rule, until it is proven otherwise, assume any wide QRS complex tachycardia is ventricular tachycardia. Even though this one clue carries significant weight in supporting the interpretation of ventricular tachycardia, that conclusion can not be made based solely on this single criteria. This axis is just one of a long list of criteria and should be used in conjunction with all of them as they carry alot of strength when used collectively.

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