Sunday, May 23, 2021

ECG Blog #227 (43) — Sinus Tach and 1st-Degree AV Block?

The ECG in today's case was obtained from a middle-aged man with a 2-day history of exertional chest pain and shortness of breath (Figure-1). Questions raised by this case include the following:

  • Is the rhythm sinus tachycardia with 1st-degree AV block?
  • Is there evidence of an inferior MI? 
  • Are Sgarbossa criteria applicable to this tracing?


Figure-1: ECG obtained from a middle-aged man with exertional chest pain and shortness of breath. The rhythm was thought to be sinus tachycardia with 1st-degree AV block (RED arrow in lead II highlighting the upright sinus P wave).



NOTE: Unfortunately I do not have follow-up to this case — so I do not know a definitive answer regarding the cardiac rhythm. That said — I believe there are important lessons-to-be-learned with respect to the diagnostic considerations that should immediately come to mind (!) — and THAT is the purpose of today's case.

  • HINT: Today's 1:45 minute ECG Audio PEARL #43 (below) reviews an important arrhythmia concept known as the Bix Rule.


Today’s ECG Media PEARL #43 (1:45 minutes Audio) — Reviews the Bix Rule (which explains why it is uncommon to see sinus tachycarda with 1st-degree AV block).




My initial impression on assessing the ECG rhythm in Figure #1 was sinus tachycardia with 1st-degree AV block. And then I took a 2nd look ...

  • PEARL #1: As the rate of a sinus rhythm increases — the PR interval tends to shorten, probably as a result of reduced basal vagal tone. Because of this, it is uncommon to see sinus tachycardia with a prolonged PR interval. While there clearly are some patients who have baseline PR interval prolongation, who continue to manifest a certain amount of PR interval prolongation with sinus tachycardia — a majority of tachycardias that seem to show a "sinus P wave" in the middle of the R-R interval turn out to have 2:1 AV conduction (from either Atrial Tach or AFlutter). This principle is known as the Bix Rule, named after the Viennese cardiologist who first cautioned against "accepting" the diagnosis of "sinus tach with 1st-degree" before meticulous search for an "extra" P wave hidden within the QRS complex.


Acknowledgment: It is unfortunately impossible to prove that there is 2:1 AV conduction in Figure-1 — without either a prior baseline tracing showing identical QRS morphology with definite sinus rhythm at a slower rate — and/or — without a follow-up tracing showing the same QRS morphology and definite sinus rhythm at a slower rate.

  • That said, I thought: i) The PR interval for the upright deflection in Figure-1 looked "long" given the tachycardia rate; andii) Each of the inferior leads showed a potential "hump" or deflection that might represent a near-simultaneous "extra" P wave in association with the end of the QRS complex — that seemed to "walk out" with calipers (short RED lines in the inferior leads in Figure-2).
  • In lead V1 — we clearly see a negative P wave deflection before each QRS complex — which if "walked out" with calipers, could easily "hide" a second P wave within the QRS complex (short RED lines in lead V1 of Figure-2).
  • PEARL #2: One of the most commonly overlooked arrhythmia findings is subtle 2:1 AV conduction. Over the years — I have seen too-numerous-to-count experienced providers (including many cardiologists) overlook 2:1 AV conduction with either AFlutter or ATach. The BEST way not to overlook subtle 2:1 AV conduction — is to THINK of this possibility with fast supraventricular rhythms, in which you either do not see a clear upright P wave in lead II — or — in which the PR interval looks "longer-than-expected" for a normal sinus P wave ( = the Bix Rule!)NOTE: I fully acknowledge that 2:1 conduction will not be present each time you look for it (!) — but you will not pick up most subtle 2:1 conduction rhythms unless you regularly look for them!

What Happened in Today's Case:

I believe the possibility of 2:1 AV conduction was not considered in today's case — so unfortunately, I do not know "the Answer".

  • IF indeed, the rhythm in Figure-2 was sinus tachycardia — then there would be no need to "treat the rhythm". Instead — the treatment of choice for sinus tachycardia is to treat the underlying cause (in this case, whatever was causing this patient's exertional chest pain and shortness of breath) — in which case, the rate of sinus tachycardia should slow as the patient's clinical condition improves, and clear definition of the rhythm will then most probably be forthcoming!
  • Alternatively — another lead system (such as use of a Lewis Lead) might be tried in an attempt to better visualize atrial activity (See Figure-3 below in the Addendum).

Figure-2: I've added short RED lines to Figure-1 at places where I suspected there might be underlying P waves with 2:1 AV conduction (See text).


Other ECG Findings in Figure-2:

  • Although the QRS complex in Figure-2 looks to be a little bit wide (ie, slightly more than half a large box in duration) — it does not look wide enough by my calculation to qualify as a complete LBBB (ie, not as wide as ≥0.12 second in duration).
  • In addition to not being overly wide — QRS morphology in Figure-2 "looks" supraventricular. Note how QRS amplitude is dramatically increased! For clarity (because of the marked overlap in complexes in Figure-2) — I have outlined in PURPLE the limits of the QRS in lead V3 — and in PINK, the limits of the QRS in lead V5. Both of these QRS complexes are cut off at their peak, but already measure more than 25 mm!
  • PEARL #3: The precise cause of slight QRS widening in this case is difficult to determine. I suspect there is a combination of marked LVH (a thicker left ventricle takes up to 0.01-to-0.02 seconds more to traverse) and possible incomplete LBBB.To my knowledge — there is no literature validation on use of Sgarbossa criteria for incomplete LBBB (unlike complete LBBB — incomplete LBBB is too uncommon to objectively study). That said — common sense would suggest that the rule of proportionality conveyed by the Smith modification of Sgarbossa criteria should still apply. I see no abnormal-looking ST elevation in Figure-2. And although the amount of J-point ST depression in lateral chest leads V4-thru-V6 is marked (especially in lead V4) — given huge R wave amplitude in these leads, my "sense" is that J-point ST depression is probably not "disproportionate" given how huge R wave amplitude is.
  • As to the question of possible inferior MI of uncertain age — an isolated inferior Q wave in lead III is not necessarily abnormal, especially in association of possible incomplete LBBB.


Putting It All Together:

The ECG in today's case shows a supraventricular rhythm at a rate just over 100/minute. The QRS complex appears to be slightly wide, albeit with a supraventricular-like morphology most consistent with marked LVHand possible incomplete LBBB.

  • The cardiac rhythm is uncertain. The rhythm could be sinus tachycardia with 1st-degree AV block — or — this could be atrial tachycardia with 2:1 AV conduction.
  • The "good news" — is that there do not appear to be acute ST-T wave changes on this ECG. One would "have to be there" in order to know how best to proceed. IF the patient was (and remained) hemodynamically stable — then treating associated disorders (ie, the cause of this patients exertional chest pain and shortness of breath) should result in a slowing of the rate if the rhythm was sinus tachycardia.
  • Finding a prior tracing — considering additional lead systems (such as use of a Lewis Lead) — and/or — repeat ECGs as the patient is treated would almost certainly clarify what the true cardiac rhythm is.



Acknowledgment: My appreciation to Abdullah Al Mamum (from Dhaka, Bangladesh) for allowing me to use this case and this tracing.


ADDENDUM (May 23, 2021): I've excerpted from my ACLS-2013-ePub directions on how to obtain a Lewis Lead, to help visualize atrial activity (as discussed in ECG Blog #223).


Figure-3: How to record a Lewis Lead.


Related ECG Blog Posts to Today’s Case: 

  • ECB Blog #221 — Reviews my "user-friendly" approach for diagnosing the BBBs (Bundle Branch Blocksin less than 5 seconds — including Audio Pearl-38 on HOW to diagnose acute MI when there is BBB (with review of modified Smith-Sgarbossa criteria). 
  • ECG Blog #223 — Assessing P Waves and Atrial Activity (including use of a Lewis Lead).

No comments:

Post a Comment