ECG Blog #354 — What is an "N" Wave?

The ECG in Figure-1 was obtained from a middle-aged man — who presented with chest discomfort.

QUESTIONS:

• How would YOU interpret this ECG?
• What is an N-Wave?

Figure-1: ECG obtained from a middle-aged man with chest discomfort. (To improve visualization — I've digitized the original ECG using PMcardio).

MY Thoughts on the ECG in Figure-1:

Unfortunately — details are lacking from the clinical history. That said — knowing the patient had “chest discomfort” clearly adds concern.

• There is sinus arrhythmia. All intervals (PR, QRS, QTc) are normal. The frontal plane axis is leftward — but not quite leftward enough to qualify as LAHB (ie, the QRS is not clearly more negative than positive in lead II). There is no chamber enlargement.

Regarding Q-R-S-T Changes:

• Small and narrow Q waves are seen in high-lateral leads I and aVL. These are probably normal septal q waves.
• R wave progression — suggests there is early transition, as the R wave becomes predominantly positive already by lead V2 (usually this does not happen until after lead V2).

The most concerning findings relate to ST-T wave changes:

• In the Inferior Leads — ST-T wave changes look hyperacute. Given the presence of “chest discomfort” and the overall small amplitude of the QRS in these inferior leads (especially in leads II and aVF) — the ST-T waves are more “voluminous” than they should be. Specifically — the ST segment appears coved in leads III and aVF — straightened in lead II — with slight J-point ST elevation and a T wave base in each of the inferior leads that appears wider-than-it-should-be.
• Support that these findings are “real” is forthcoming from the ST-T wave appearance in lead aVL — which manifests reciprocal changes (ie, ST segment flattening, with slight depression and terminal T wave positivity).
• The ST segment in the other high-lateral lead (ie, lead I) — looks uncharacteristically flat. While clearly a nonspecific (and nondiagnostic) change by itself — there should normally be gentle upsloping to the ST segment in this lead.
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• In the Chest Leads — the ST-T wave appearance in leads V2 and V3 is not normal. In addition to the previously noted early transition (ie, the unexpectedly tall [predominant] R wave already by lead V2) — the ST segments in leads V2 and V3 are uncharacteristically flat (as opposed to being gradually upsloping with slight ST elevation, as is normally seen). While admittedly lacking the anterior ST depression that typically characterizes posterior OMI (Occlusion-based Myocardial Infarction) — the abnormal shape of the ST-T waves in leads V2,V3 constitutes a positive "Mirror Test" — that in the context of a patient with new chest discomfort and ECG signs of inferior OMI — strongly suggests acute posterior OMI until proven otherwise (See Figure-2).
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• Finally — the ST-T wave in lead V6 resembles the appearance of the ST-T waves seen in the inferior leads. This is likely to also represent a hyperacute change of lateral involvement.

Figure-2: I've labeled today's tracing to facilitate recognition of acute infero-postero-lateral OMI. RED inserts suggest a positive "Mirror Test" (ie, turning the tracing over and holding it up to the light — which results in a shape that looks like an OMI — as per ECG Blog #80 — Blog #246 — and Blog #285, among others). The RED arrows highlight N-Waves in leads III and aVF (See text).

Putting It All Together:

Admittedly — some of the above noted ECG changes are subtle. That said — in a patient with “chest discomfort” — the sum total of ECG findings described above should suggest infero-postero-lateral OMI until you prove otherwise. 

• To Emphasize: The ECG changes in today's tracing do not qualify as a “STEMI” — because no more than minimal ST elevation is seen in the inferior leads. Nevertheless, in a patient with "chest discomfort" — the “shape” of these ST-T wave abnormalities in the inferior and posterior leads — in association with suggestion of the magical “reciprocal relationship” in lead aVL — strongly suggests acute coronary occlusion (See ECG Blog #193 — for discussion of the concept of OMI rather than "STEMI" as the optimal classification paradigm in 2023). 
• The more leads that are abnormal — the greater the chance that ECG abnormalities are "real" and significant. In Figure-2 — ECG abnormalities are seen in no less than 8/12 leads (ie, leads I,aVL — II,III,aVF — V2,V3 — and lead V6). 
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• Regarding our prediction for the probable “culprit” artery — involvement of the infero-postero-lateral walls suggest the LCx (Left Circumflex) as the most likely candidate.

What is an "N-Wave"?

In 2011 — Niu et al described the presence of an "N-Wave" — or delayed activation wave of the left ventricular basal region. Because this area of the heart is typically supplied by the LCx — this is likely to be the "culprit" artery when N waves are seen as part of the pattern of acute ischemia (Int J Cardiol 162(2): 107-111, 2013).

• As highlighted in the March 26, 2022 post in Dr. Smith's ECG Blog — N waves are recognized by the transient appearance of notching (usually ≥2 mm in size, with respect to the PR segment) — with resultant slight widening of the QRS complex. Because N-waves typically last <24 hours after acute occlusion — they may be smaller than 2 mm, depending on when during the process they are seen.
• As opposed to J-wave notching (that is more likely seen in the chest leads) — N-waves are usually seen in one or more of the inferior leads, and/or in leads I, aVL.
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• NOTE: When I first read about N-waves — I was hesitant to use this ECG finding in my assessment, for fear of "overcalling" occasional QRS fragmentation as being something else. But I thought today's case provided superb illustration of probable N-waves (RED arrows in Figure-2) in this patient with "chest discomfort" and ECG signs that are strongly suggestive of acute infero-postero-lateral OMI from a LCx culprit. I wish we had cath confirmation on today's case.

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Acknowledgment: My appreciation to Arron Pearce (from Manchester, UK) for the case and this tracing.

• My appreciation also to Kianseng Ng (from Malaysia) — for enhancing my attention to the phenomenon of N-Waves.

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Related ECG Blog Posts to Today’s Case:

• ECG Blog #205 — Reviews my Systematic Approach to 12-lead ECG Interpretation.
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• ECG Blog #193 — illustrates use of the Mirror Test to facilitate recognition of acute Posterior MI. This blog post reviews the basics for predicting the "Culprit" Artery (as well as reviewing why the term "STEMI" — should replaced by "OMI" = Occlusion-based MI).
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• ECG Blog #285 — and ECG Blog #246 — and ECG Blog #80 — for examples of acute posterior MI (with use of the Mirror Test to facilitate diagnosis).
• ECG Blog #317 — reviews use (or not) of Posterior Leads.
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• ECG Blog #184 — and ECG Blog #167 — review the "magical" mirror-image opposite relatinship between lead III and lead aVL that helps to confirm acute OMI.
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• The February 16, 2019 post in Dr. Smith's ECG Blog — My Comment (at the bottom of the page) emphasizes utility of the Mirror Test for diagnosis of acute Posterior MI. 
• Diagnosis of an OMI from the initial ECG — Serial tracings with spontaneous reperfusion — then reocclusion! — See My Comment at the bottom of the page in the October 14, 2020 post on Dr. Smith's ECG Blog.
• Acute OMI that wasn’t accepted by the Attending — See My Comment at the bottom of the page in the November 21, 2020 post on Dr. Smith’s ECG Blog.
• Another overlooked OMI (Cardiologist limited by STEMI Definition — OMI evident by Mirror Test) — See My Comment at the bottom of the page in the September 21, 2020 post on Dr. Smith’s ECG Blog.
• Recognizing hyperacute T waves — patterns of leads — an OMI (though not a STEMI) — See My Comment at the bottom of the page in the November 8, 2020 post on Dr. Smith's ECG Blog.
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• ECG Blog #258 — How to "Date" an Infarction based on the initial ECG.
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• ECG Blog #294 — Reviews how to tell IF the "culprit" artery has reperfused.
• ECG Blog #230 — Reviews how to compare Serial ECGs.
• ECG Blog #115 — Shows how dramatic ST-T changes can occur in as short as an 8-minute period.
• ECG Blog #268 — Shows an example of reperfusion T waves.
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• ECG Blog #337 — A "NSTEMI" that was really an ongoing OMI of uncertain duration (presenting with inferior lead reperfusion T waves).