Wednesday, April 14, 2021

ECG Blog #214 (ECG MP-31) — 12 Leads are Better than One

The rhythm strips in Figure-1 were obtained from 2 different patients. Both patients were hemodynamically stable at the time these rhythm strips were recorded.


WHICH statements are true? (There may be more than 1 correct answer!)

  • A) Tracing A — The rhythm is Mobitz I.
  • B) Tracing A — The rhythm is sinus with PACs (atrial trigeminy).
  • C) Tracing B — This is an SVT (SupraVentricular Tachycardia).
  • D) Tracing B — This might be VT (Ventricular Tachycardia).
  • E) We need more information ... IF so — WHY?


Figure-1: Rhythm strips obtained from 2 different patients. WHAT do they show?



NOTE #1: Some readers may prefer at this point to listen to the 5:30 minute ECG Audio PEARL beforereading My Thoughts regarding the ECG in Figure-1. Feel free at any time to review to My Thoughts on this tracing (that appear below ECG MP-31).


Today’s ECG Media PEARL #31 (5:30 minutes Audio) — WHY do I emphasize the phrase, "12 Leads are Better than One?"



Looking First at Rhythm A:

The underlying rhythm in Tracing A is sinus — as determined by the presence of upright P waves with fixed PR interval preceding beats #1,2; 4,5; 7,8; 10,11; and 13, in this lead II monitoring lead. The QRS complex of sinus beats is narrow.

  • Every-third-beat occurs earlier-than-expected and looks slightly different. That is — the QRS complex of beats #369 and 12 each have a smaller r wave, and a less deep S wave than do sinus beats. The QRS complex for each of these early beats looks to be narrow and preceded by a premature P wave (BLUE arrows in Figure-2 — compared to the RED arrows that highlight sinus P waves).


Figure-2: To Figure-1 — I’ve added RED arrows that show sinus-conducting P waves — and BLUE arrows over what looks to be PACs occurring every-third-beat (See text).

QUESTION: Is there another possible explanation for what we are seeing in Figure-2?

  • HINT #1: Do we really have enough information from Figure-2 to determine IF the QRS complex of each early beat is truly narrow?
  • HINT #2: Look at Figure-3 — in which we have added a simultaneously-recorded lead I rhythm strip. Does the QRS of each early beat still look narrow?

Figure-3: I’ve added a simultaneously-recorded lead I rhythm strip to Figure-2 (See text).

ANSWER: Look at the vertical RED timeline that I’ve added to Figure-3.

  • The QRS complex of every-third-beat in Lead I is actually wide! (ie, beats #3,6,9 and 12).
  • The vertical RED timeline that begins with the onset of the wide QRS complex in lead I shows that the notch in lead II (which looked to be a premature P wave in Figure-2) — is actually the initial part of the QRS complex in simultaneously-recorded lead I. 

12 Leads are BETTER than ONE:

intentionally used Figure-1 as a “trick tracing” to emphasize a number of important points brought about by this case:

  • POINT #1: 12 Leads are Better than One. It is EASY to get fooled when you are not provided with complete information. That said — it is our responsibility as clinicians to reserve making a definitive diagnosis until we have enough information to confidently do so. This often requires more than a single monitoring lead tracing. IF your patient is hemodynamically stable — then by definition, you do have time to obtain additionalmonitoring leads (and ideally, a 12-lead ECG).
  • POINT #2Part of the QRS complex may lie on the baseline. When this happens — the QRS complex may look narrow in one lead — whereas in reality, it is actually quite wide. At other times (as in Figure-3) — what looks like a preceding “P wave”, may actually be the initial part of the QRS complex.
  • POINT #3: Assume that a premature beat is “guilty” (ie, a PVC) until you prove otherwise! Statistically, when the underlying rhythm is sinus — most early-occurring beats that look different and are not clearly preceded by a premature P wave — will be ventricular in etiology. The “onus of proof” always rests with the interpreter to establish that the abnormal-looking beat(s) is(are) aberrantly conducted (and not the other way around). Figure-2 illustrates how you can not “prove” aberrant conduction for beats #3,6,9,12 with the incomplete information provided from a single monitoring lead. 
  • BOTTOM LINE: The rhythm in Tracing A is sinus with ventricular trigeminy (ie, every-third-beat is a PVC). It would be best not to monitor this patient solely in lead II — because doing so would suggest the false impression of atrial trigeminy.


Looking Next at Rhythm B:

Take another look at Tracing B (Figure-4) — and at the Questions we posed earlier at the beginning of this case. Keeping in mind the insights gained in our discussion of Tracing A — HOW would you now respond?


WHICH statements are true about Tracing B? (There may be more than 1 correct answer!)

  • C) Tracing B — This is an SVT (SupraVentricular Tachycardia).
  • D) Tracing B — This might be VT (Ventricular Tachycardia).
  • E) We need more information ... IF so — WHY?


Figure-4: Tracing B was obtained from a patient who was hemodynamically stable. HOW would you interpret this rhythm?




  • A single V1 monitoring lead is shown in Figure-4. At first glance — this tracing suggests there is a regularSVT (ie, narrow-complex) rhythm, at a rate just over 150/minute, but without clear sign of sinus P waves.
  • That said — it is impossible to be certain where the QRS complex ends from the single monitoring lead shown in Figure-4 (and it looks as if there may be an extra “shoulder” attached to the end of the QRS complex in lead V1).
  • More monitoring leads are needed. Since this patient is hemodynamically stable — a 12-lead ECG is indicated (Figure-5).


Figure-5: The 12-lead ECG from which the lead V1 rhythm strip in Figure-4 was taken.



Interpretation of Figure-5:

It should be obvious that the rhythm in Figure-5 is VT. The QRS complex is wide — and several ECG findings that are virtually diagnostic of VT are present. These findings include: i) QRS morphology in Figure-5 does not resemble any known form of conduction defect; ii) The QRS complex in a number of leads shows delay in the initial deflection of the QRS (ie, a slow downsloping S wave in leads I, aVL, and V2-thru-V6); iii) There is global negativity of the QRS in all 6 of the limb leads; andiv) There is extreme axis deviation (ie, the QRS in lead I is all negative).

  • In Figure-5 — the QRS complex looks narrow in 2 of the 12 leads (ie, in leads aVR and V1). The QRS looks wide in each of the remaining 10 leads!
  • BOTTOM LINE: Part of the QRS may sometimes lie on the baseline (as it does in leads aVR and V1). When this happens — it could be easy to be misled IF the single lead you are monitoring happens to be the lead in which part of the QRS lies on the baseline (as we saw in Figure-4). Therefore, whenever your patient is hemodynamically stable — 12 Leads will be BETTER than ONE!


  1. Dr Ken, I fully appreciate the invaluable purpose and theme
    "12 Leads are better than one" in April 14th 2021 blog.This
    blog, indeed, has portrayed this theme very dramatically.
    May I give my comments:
    In rhythm strip A, if we surmise the notches preceding the
    early QRS # 3,6,9 & 12 as PACs, then their PR interval can
    not be so short unless we also surmise WPW intermittently.
    Also the compensatory pause is expected to be typically incomplete - here it is quite complete.
    Now, coming to rhythm strip B- it is wide complex (120ms)
    tachycardia @ 166/mt.There is a negative bump on every 4th
    T wave. I think it is likely to be due to retrograde 4:1
    VA conduction ( AV dissociation ) thus confirming VT.
    With regards, Dr.R.Balasubramanian. PONDICHERRY-INDIA.

    1. THANKS as always so much for your insightful comments. As to A — Depending on WHERE in the atria a PAC arises — the PR interval COULD be as short as the one seen here. And while true that a PAC more likely manifests a less-than-complete compensatory pause — there is OVERLAP in both ways (ie, both for PACs and PVCs) regarding duration of the post-ectopic pause, depending on whether a PVC conducts retrograde or not (and if so, for how far) — and for PACs depending on how much SA node suppression is seen — so in my opinion — it’s not possible to rule out PACs solely from the appearance of Tracing A.

      As to B — On the 12-lead ECG — I’m not convinced that the negative “bump” in V1 is a retrograde P wave — as I don’t see any convincing suggestion of retrograde P waves elsewhere ( = my opinion). — 1:1 retrograde VA conduction could be seen with either reentry SVT rhythms or VT. Retrograde Wenckebach conduction points to VT — but to me, even though there is variation in morphology of this retrograde bump in V1 — I see slight variation in the QRST in mulitple leads, such that I’m not convinced this confirms retrograde atrial activity. And on the 12-lead for B — I don’t see a negative bump in every 4th T wave … Again — you could be correct. I just see it differently. THANKS again for your comments! — :)

  2. Great...sir n thanks for whole explanation