Saturday, November 4, 2023

ECG Blog #402 — Will Adenosine Convert This?

You are told that the patient next door is in the regular SVT (SupraVentricular Tachycardia) rhythm shown in Figure-1.


  • Is the rhythm AVNRT or AVRT?  
  • Is Adenosine likely to convert this rhythm?

Figure-1: How would YOU interpret this ECG?

MY Thoughts on the ECG in Figure-1:
When faced with a challenging cardiac arrhythmia — It is a "luxury" to have access to a long lead rhythm strip containing 3 simultaneously-recorded leads. This provides the optimal chance that QRS size and atrial activity (if present) will be readily detectable.

PEARL #1: The 3 leads I favor for rhythm determination are precisely the leads that are shown in Figure-1. These are: 
  • Lead II — which is clearly the BEST lead in most cases for visualing P waves and/or other atrial activity. Sinus rhythm is defined by the presence of an upright P wave with constant PR interval in lead II.
  • Lead V1 — which is the 2nd-best lead for visualizing P waves (although the P wave will not necessarily be upright in lead V1 with sinus rhythm). Lead V1 offers the best perspective of right-sided QRS morphology.
  • Lead V5 — which offers an excellent perspective of left-sided QRS morphology.

And then I took a closer LOOK — at the rhythm in Figure-1.


  • Is the rhythm in Figure-1 truly regular?
  •    — HINT: Before answering — LOOK at Figure-2.

Figure-2: Is the rhythm truly regular? (See text).

Is Today's Rhythm Truly Regular?
Although today's rhythm looks regular — it is not completely regular, as I highlight in Figure-2:
  • Even without calipers — Doesn't it appear that the R-R intervals within the RED dotted ovals are a little bit longer than the R-R intervals within the BLUE ovals? This establishes that the rhythm is irregularly irregular. Otherwise:

  • The rate of today's rhythm is fast (close to ~150/minute).
  • The QRS complex is narrow in all 12 leads.
  • Although difficult to assess atrial activity given all of the baseline artifact — it appears that no P waves are seen.

  • Therefore: The finding of an irregularly irregular rhythm with a narrow QRS complex, and the complete absence of P waves — establishes the diagnosis as AFib (Atrial Fibrillation), here with a rapid ventricular response.
  • Otherwise — the axis is normal — R wave progression is appropriate — there is no chamber enlargement — and some nonspecific ST depression is seen in the lateral chest leads that is probably rate-related.

 When the rate of AFib is rapid — this irregular tachycardia may look regular when it is not
  • That the rhythm is AFib — is easier to appreciate in Figure-3.

Figure-3: Today's rhythm is not regular (See text).

Figure-3 shows Today's Rhythm is Not Regular
The double RED arrows that I've drawn near the middle of the long lead V5 rhythm strip are all the same length.
  • Although extremely subtle — the 5 double arrows in the top 2 rows of the long lead V5 rhythm strip — are each a little bit shorter than the 2 double arrows in the lower row.

The CASE Continues:
Today's patient was treated with IV, and then PO Metoprolol. The repeat ECG after this treatment is shown in Figure-4.
  • Following treatment with a ß-blocker — the rate of today's rhythm has slowed considerably. Although baseline artifact remains — it should be evident that no P waves are present.
  • The irregular irregularity of today's rhythm in Figure-4 is now obvious. Clearly, the rhythm is AFib — here with a controlled ventricular response.
  • Note that the ST depression that was previously seen in the lateral chest leads of Figure-3 — has now resolved, confirming that this ST depression was indeed rate-related.

Figure-4: Repeat ECG after IV and PO Metoprolol.

Would Adenosine have Worked for Today's Rhythm?
Adenosine was not used in today's case. Instead — rate control of this patient's AFib, as well as confirmation of this rhythm diagnosis was obtained by treatment with a ß-blocker.
  • To EMPHASIZE: It would be extremely easy not to realize that the rapid SVT (SupraVentricular Tachycardia) rhythm that today's patient presented with was irregular — because (as per PEARL #2)When AFib is rapid, it may look quite regular. Clinically, this is usually not problematic — since initial emergency treatment of many SVT rhythms is similar.

The above said — When possible, optimal management of SVT rhythms is much easier to achieve when you know what the specific rhythm diagnosis is.
  • ECG Blog #240 — reviews my approach to the ECG assessment of regular SVT rhythms.

  • Adenosine is a wonderful drug for emergency treatment of reentry SVTs. Even in cases in which Adenosine does not convert the rhythm — it will often facilitate rhythm diagnosis by its transient rate-slowing effect. Although the drug is usually safe (because of its ultra-short half-life) — side effects can occur, and these are not uniformly short-lived. Therefore — Adenosine is probably best avoided for treatment of rhythms for which the drug has little to no chance of being effective (ie, If you know that the SVT you are treating is AFib — then its better to use some other rate-slowing or antiarrhythmic agent — rather than risking side effects from a drug that is unlikely to work).

  • See Figure-5 and -6 for more on Adenosine.   

Figure-5: Pages 1 and 2 on Pros & Cons of using Adenosine (excerpted from my ACLS-2013-ePub).


Figure-6: Pages 3 and 4 on Pros & Cons of using Adenosine (excerpted from my ACLS-2013-ePub).


Acknowledgment: My appreciation to Mubarak Al-Hatemi (from Qatar) for the case and this tracing. 


Related ECG Blog Posts to Today’s Case:

  • ECG Blog #205 — Reviews my Systematic Approach to 12-lead ECG Interpretation.
  • ECG Blog #185 — Reviews the Ps, Qs, 3R Approach to Rhythm Interpretation.

  • ECG Blog #240 — Reviews the approach to a regular SVT rhythm.


  1. V2 and V3 in the initial ECG appear to have possible flutter waves (or deflections where a wave might be) before and after the T wave. Perhaps it is Atrial Flutter with a 2:1 block? I feel like A Flutter and A Fib are sometimes on a continuum where a patient might slip in and out of one to the other and back where organized, regular flutter waves become irregular fibrillation waves. Could the irregularity be from ectopic beats or the equivalent of ‘sinus’ arrhythmia? The quality of the tracing makes it hard to tell.

    Regardless, adenosine would not have converted that, either. And the repeat ECG is clearly A Fib

  2. THANK YOU for your comment! As you highlight — suboptimal quality of this tracing makes it difficult to draw conclusions. That said — I ALWAYS look for AFlutter (as I cannot tell you HOW MANY times I have seen this rhythm overlooked, even by cardiologists). I also always have my calipers nearby (to take out when/if the patient is stable). The ventricular rate is clearly faster than the usual 140-160/min range that is most commonly seen with untreated AFlutter — and although I looked in ALL 12 leads — I could not get 2:1 atrial activity to "march out" in any lead. You can see irregularly irregular AFlutter — but in my opinion, I did not see flutter activity.

    That said — You are completely correct that AFlutter and AFib may evolve into the other, sometimes back-and-forth over a period of time. But I didn't think there was AFlutter in this tracing. THANKS again for your comment! — :)