Saturday, January 6, 2024

Can you spot the problem with the recording of this 12-lead ECG?

Written by Willy Frick

I was reading ECGs in the queue, and the following tracing appeared. I frequently check the chart if I want to know more clinical context (which is crucial for accurate interpretation). But in the reading software, all I am told is that it is a 77 year old man, and that the indication is "NSTEMI."

What do you think?




 






Although not necessarily intentional, the first processing that I do when I see an ECG is to categorize as OMI or not. Here, I do not see OMI (although the ECG is falsely STEMI positive with just over 1 mm STE in V1 and about 2.5 mm STE in V2). 

The Queen of Hearts also sees no OMI with high confidence.

Afterward, I continue to read systematically. My approach (which is not unique) is rate, rhythm, axis, hypertrophy, intervals. (In the days before I learned to look for OMI, back when I was counting ST elevation boxes, I used to save ischemia for last.)

The rate is near 80, normal. As for rhythm, most people are taught that sinus rhythm is recognized by "a P before every QRS and a QRS after every P." This is not what defines sinus rhythm. Sinus rhythm is defined by an electrical impulse which originates in the sinoatrial node, located in the upper right atrium. The electrical wavefront then depolarizes the atria, spreading outward from right to left and top to bottom. What happens afterward (e.g. the presence of a QRS from ventricular depolarization) tells us nothing about where the rhythm originated.

Here are two examples to make the point:

Example 1

(Obviously the V2 cable fell off, ignore that.) The above tracing clearly shows a P before every QRS and a QRS after every P. But it is not sinus rhythm. The P waves are negative in the inferior leads which means the impulse is traveling away from them, i.e. it is headed north! Therefore the impulse must have originated somewhere lower in the atria, perhaps near the coronary sinus.

Example 2

Most people will (correctly) recognize this as complete heart block, but that is not the rhythm per se. Complete heart block is the state of the AV node. Notice the P waves are upright in leads I, II, and aVF (indicating that the impulse is traveling down and to the left) and upside down in aVR (indicating that the impulse is traveling away from the right shoulder, another way of saying down and to the left). Therefore this is sinus (even though there is no relationship between P waves and QRS complexes). The atrial rate is around 120 beats per minute, which indicates high adrenergic state and physiologic distress! This is sinus tachycardia (rhythm) with complete heart block (AV node function) with ventricular escape rate just below 30. 

Never forget that sinus tachycardia is the scariest arrhythmia.

Back to the case:


Looking at the rhythm, we see that the P waves are upside down in leads I and II are upside down, and upright in aVR. This is the opposite of what we would expect for sinus rhythm. One possibility is ectopic atrial rhythm, but this is relatively uncommon. Another possibility is lead reversal. LA-RA reversal is typically the easiest to spot since it usually produces a predominantly negative QRS in I. Other reversals may not be immediately obvious. Is there any other evidence of lead reversal here?

Einthoven's Triangle
Image taken from Life In The Fast Lane

Once I am suspicious of a reversal, I go to Life In The Fast Lane's Limb Lead Reversal page and work my way down the list. LA-RA is unlikely because the QRS in I is still positive. LA-LL is unlikely because that would not explain the P wave inversion. RA-RL, LA-RL, and bilateral arm-leg reversal are all unlikely because none of the leads is zero potential.

This leaves us with the possibility of RA-LL reversal, which should produce the following changes:
  1. Lead II becomes inverted
  2. Leads I and III become inverted and switch places
  3. Leads aVR and aVF switch places
  4. Lead aVL is unchanged
If we "correct" these, we would have the following ECG


Suddenly, the P waves make sense!

Completing our analysis of the ECG, we see marked left axis deviation. Given the small q waves in I and aVL, some might call this left anterior fascicular block. And finally, there is left ventricular hypertrophy with repolarization abnormality and QRS prolongation. 

It turns out, the patient had had a single repeat tracing shown below which confirms that there was lead reversal.


Learning points:
  • Always review ECGs systematically to avoid missing things
  • Sinus rhythm is defined by P wave morphology and has nothing to do with QRS complexes
  • Unusual P wave axis and zero potential leads should raise suspicion for reversal
  • As always in ECG, serial tracings can also help identify lead reversal, especially if you see changing polarity of the QRS complexes






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MY Comment, by KEN GRAUER, MD (1/6/2024):
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An often underrated, but extremely important skill in clinical ECG interpretation — is the ability to recognize technical "misadventures". The number of times the failure of even highly experienced providers to recognize these happenings — is much higher than most clinicians appreciate.
  • Today's technical misadventure was astutely recognized by Dr. Willy Frick — as he recounts above in the excellent description of his thought process that immediately clued him in on the correct diagnosis.
  • Today's case is unique in that: i) I believe it to be the 1st time we are publishing an example of this less commonly encountered form of lead reversal in Dr. Smith's ECG Blog; and, ii) Because the patient's underlying baseline ECG abnormalities made recognition of this form of lead reversal that much more difficult to identify.

For clarity in Figure-1 — I've labeled and put together today's initial ECG with Dr. Frick's prediction of what ECG #1a would look like IF limb lead electrodes were correctly placed.


QUESTIONS:
  • What CLUE that I favor for recognizing RA-LL Reversal is highlighted by the colored arrows in Figure-1?
  • What ECG findings in this patient's baseline tracing ( = ECG #1b) make recognition of RA-LL Reversal challenging in today's case?

Figure-1: I've combined today' initial tracing ( = ECG #1a) — with ECG #1b after accounting for the changes caused by RA-LL Reversal (See text).

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NOTE: For readers wanting additional "practice" in recognizing the various forms of lead reversals and sources of artifact — I've assembled a series of cases below in the ADDENDUM. I plan to continue to update this list with future technical "misadventures" we publish that went unrecognized.
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Why Today's Case was so Challenging:
"Common things are common" — and because they are common, we encounter such entities more often, and therefore become much better at readily recognizing them.
  • In the 500+ Comments I have written on Dr. Smith's ECG Blog since becoming an Associate Editor in 2018 — I do not believe we have had a case of RA-LL lead reversal. In contrast — we have had numerous examples of other forms of lead reversal (most commonly LA-RA reversals — but also a fair number of LA-LL reversals).
  • As per Figure-2 below (in the ADDENDUM) — LA-RA reversal is usually easy to recognize by the global negativity it typically produces in lead I (of P wave - QRS - T wave) — in association with a positive QRS in lead aVR.
  • I used to overlook LA-LL reversals — until I "got" that with normal sinus rhythm, the P wave in lead I should not be larger than the P wave in lead II.
  • The KEY to recognizing the much less common RA-LL reversal seen in today's case — is to recognize that we should not normally be seeing negative P waves in all 3 of the standard limb leads (BLUE arrows in Figure-1). Even when a low atrial rhythm is present — the P wave will typically be positive in lead I (as well as in lead aVL). 

Today's Baseline ECG is Very Abnormal:
ECG #1b in Figure-1 (after correcting for the changes produced by RA-LL reversal) — provides us with a picture of the baseline ECG for today's patient.
  • Note that the P wave is now upright in lead II (the RED arrow in ECG #1b) — as it should be with normal sinus rhythm.
  • The QRS complex in ECG #1b is wide (ie, 0.12 second). This most likely is attributable to the combination of marked LVH (R wave ≥12 mm in lead aVL — and, also the presence of very deep anterior S waves!) — and — LAHB (Left Anterior HemiBlock — diagnosed by the markedly leftward axis — clearly exceeding -40 degrees).
  • Lateral lead ST-T wave depression looks consistent with LVH and the conduction defect — and there do not appear to be any acute changes.

  • NOTE #1: One of the usual clues to RA-LL reversal — is that the QRS complex in lead aVR will be positive rather than negative. But because of this patient's marked left axis deviation on their baseline tracing (producing predominant inferior lead negativity) — the QRS when there is RA-LL reversal, will also be predominantly negative in lead aVR (as can be seen in lead aVR of ECG #1a).
  • NOTE #2: Because of the increased QRS amplitude from marked LVH — the tall qR pattern seen in leads I and aVL of ECG #1a looks "as expected" for a patient with marked LVH.
  • PEARL: As per Dr. Frick — it is the qR pattern in lead II that simply does not make sense! Given near total negativity of the QRS complexes in leads III and aVF of ECG #1a — it would seem bizarre for the QRS complex in the 3rd inferior lead ( = lead II) to be predominantly positive.


Some Final "Editorializing" Thoughts on ECG Interpretation:
There are many "correct" ways to interpret an ECG. As with so many skills we learn as medical providers — the approach one develops is most often based on: i) How (and by whom) we are taught; ii) What we learn from our reading and clinical experience; and, iii) Our personal preferences and "what works" for us.
  • While the importance of prompt ECG recognition of acute OMI in need of immediate cath lab activation cannot be overstated — I still favor systematic interpretation of each ECG encountered before assessing for acute ischemia and OMI.
  • My reason for this preference — is based on my decades of seeing even experienced providers jump to "more spectacular findings" (such as ST elevation) — while overlooking that the rhythm is not sinus (ie, 2:1 AFlutter will often produce a "pseudo-infarct" pattern by distortion of ST segments produced by partially hidden flutter waves).
  • "Systematic" Interpretation entails looking 1st for P waves. If the P wave in lead II is not upright — then by definition, the rhythm is not sinus. The only 2 exceptions to this rule (ie, when the rhythm will still be sinus despite a negative P wave in lead II) — are IF you have lead misplacement or dextrocardia.
  • Systematic interpretation also entails determining IF the QRS complex is wide before you go further in your assessment. This is because IF the QRS is wide (ie, due to BBB, VT, WPW, Hyperkalemia, etc.) — then all of your criteria for assessing the rest of the ECG (including assessment for the likelihood of acute OMI) will be different than if the QRS is narrow.

  • My Bias: With regular practice — Use of a Systematic Approach will not slow you down. On the contrary — it speeds you up, because you won't have to go back over the same parts of the tracing (ie, If you didn't realize the rhythm was not sinus — or did not account for the potential effect of QRS widening on ST-T wave appearance). You'll also overlook fewer potentially important ECG findings by being systematic. I interpret tracings systematically in "real time" (including my assessment for acute ischemia).


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ADDENDUM:
My favorite on-line “Quick GO-TO” reference for the most common types of lead misplacement comes from LITFL ( = Life-In-The-Fast-Lane). I have used the superb web page they post in their web site on this subject for years. It’s EASY to find — Simply put in, “LITFL Lead Reversal” in the Search bar — and the link comes up instantly. 
  • This LITFL web page describes the 7 most common lead reversals. There are other possibilities (ie, in which there may be misplacement of multiple leads) — but these are less common and more difficult to predict.
  • When I suspect a type of lead reversal that I am less familiar with — I simply review those 7 most common types of lead reversal on the LITFL page — and see if any of the ECG examples they provide seems to apply. 

  • PEARL: — I’ve summarized in Figure-2 those tips that have helped me most over the years to rapidly recognize tracings in which lead reversal is likely.

Figure-2: Tips for recognizing lead reversal (See text).


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OTHER Examples of Lead Reversal on Dr. Smith's Blog:
Technical errors featuring a variety of lead reversal placements remain a surprisingly common “mishap” of everyday practice. As a result — we'll continue to periodically publish clinical examples of lead misplacement. For review — GO TO:

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Links to Examples of ARTIFACT 
More technical "misadventures" are referenced here — some from Dr. Smith's ECG Blog — some from other sources (NOTE: As I did not previously keep track of these — there are additional examples of artifact sprinkled through Dr. Smith's ECG Blog that I have not yet included here ... ).
 
 

 


Posted by Willy Frick at 8:01 AM
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