Sunday, March 21, 2021

ECG Blog #206 — How Many Conduction Defects?

The 12-lead ECG and long lead II rhythm strip shown in Figure-1 was obtained from a 60-something woman, who presented with presyncope. Basic blood work was unremarkable (normal electrolytes).

  • HOW would you interpret this ECG?
  • WHAT treatment is indicated? Why?


Figure-1: 12-lead ECG and long Lead II rhythm strip, obtained from a 60-something woman with presyncope (See text).



MY Approach to the Rhythm in Figure-1:

This is a highly challenging tracing! I made the following initial observations:

  • There is group beating, in the form of alternating short-long cycles with similar (but not identical) R-R interval durations for the shorter cycles (ie, beats #1-2; 3-4; 5-6; 7-8) — and for the longer cycles (ie, beats #2-3; 4-5; 6-7).
  • There are fairly (but not completely) regular P waves which are upright in lead II — so the underlying rhythm appears to be some form of sinus arrhythmia.
  • The QRS is wide (at least 0.12 second in duration). As is best seen in the long lead II rhythm strip — QRS morphology alternates every-other-beat.


PEARL #1: The simple step of labeling P waves in a complex arrhythmia often makes it much easier to appreciate what is going on (RED arrows in Figure-2).

  • Setting my calipers at the P-P interval suggested between any 2 consecutive P waves facilitates “walking out” the atrial rhythm (thereby confirming the underlying sinus mechanism).


Figure-2: I’ve added RED arrows to Figure-1 in order to highlight the underlying regular atrial rhythm (See text).


PEARL #2: It is common to see slight variation in the atrial rate when there is 2nd- or 3rd-degree AV block. This phenomenon is known as ventriculophasic sinus arrhythmia — and is thought to be due to better perfusion, with resultant slight shortening of the P-P interval for those P waves that “sandwich” a QRS complex — and slight lengthening of P-P intervals that do not contain a QRS between them.


PEARL #3: The finding of an underlying sinus mechanism (in this case, ventriculophasic sinus arrhythmia)and group beating should immediately suggest the possibility of AV Wenckebach ( = Mobitz I, 2nd-degree AV block) for the rhythm we see in Figure-2

  • The diagnosis of AV Wenckebach can be confirmed in today’s case by the following additional observations: iThe same PR interval terminates each of the pauses in this rhythm (ie, a PR interval of ~0.16 second is seen before beats #1, 3, 5 and 7)ii) The PR interval then increases by a comparable amount for the 2nd beat in each of the groups (ie, the same PR interval of ~0.24 second is seen before beats #2, 4, 6 and 8); and, iii) The 3rd P wave in each group is non-conducted.
  • Therefore — there is 2nd-Degree AV Block, Mobitz Type I (AV Wenckebach) with 3:2 AV conduction (ie, with 3 P waves in each group, of which only 2 of these P waves are conducted).



The easiest way to illustrate this form of 2nd-degree AV block, is by drawing a laddergram (Figure-3). 

  • NOTE: For review on how to read (and draw) laddergrams See ECG Blog #188).   


Figure-3: Laddergram illustration of the Mobitz I, 2nd-degree AV block with 3:2 AV conduction, as described above in today’s case (See text).


Today’s Case Continued:

There is more to today’s case! We still need to explain WHY morphology of the QRS complex consistently changes from one beat-to-the-next (Figure-4).

  • Look at lead V1 in Figure-4. Although QRS morphology is different for the 2 beats seen in this lead (corresponding to beats #5 and 6 in Figure-4) — both of these QRS complexes seen in lead V1 are consistent with RBBB (ie, an rsR’ pattern with QRS widening). That RBBB is present in Figure-4 is confirmed by the presence of wide, terminal S waves in lateral leads I and aVL.
  • KEY Point: The fact that we have a simultaneously-obtained long lead II rhythm strip at the bottom of the 12-lead ECG in Figure-4 is invaluable in today’s case for clarifying which beats in each of the 12 leads correspond to which of the 2 QRS morphologies.
  • QRS morphology in the KEY leads for the Bifascicular Blocks was reviewed ECG Blog #205. For clarity, I’ve added a visual reminder at the top of Figure-4 showing how to identify the 2 forms of bifascicular block ( = RBBB/LAHB and RBBB/LPHB).
  • LOOK in Figure-4 at QRS morphology for beat #1 in simultaneously-recorded leads I, II and III. The qRS pattern in lead I, with wide terminal S wave in this lead is consistent with RBBB. In association with RBBB — the predominantly negative QRS in leads II and III for beat #1 is consistent with LAHB (Left Anterior HemiBlock). Therefore, beat #1 conducts with a RBBB/LAHB pattern of bifascicular block.
  • Now LOOK at beat #2 in simultaneously-recorded leads I, II and III. There is still a wide terminal S wave in lead I, consistent with RBBB. But note how steep the initial (ie, straight) portion of S wave descent is in lead I. Coincident with this — note the change in QRS morphology in leads II and III for beat #2, which now manifest a qR pattern consistent with LPHB (Left Posterior HemiBlock). Beat #2 therefore conducts with a RBBB/LPHB pattern of bifascicular block.
  • Applying these observations to the changing QRS morphology that we see occurring every-other-beat in the long lead II rhythm strip — we can deduce that there is a pattern of alternating hemiblock, in which odd-numbered beats (ie, beats #1,3,5,7) all manifest RBBB/LAHB conduction — and even-numbered beats (ie, beats #2,4,6,8) manifest RBBB/LPHB conduction.


Figure-4: The rhythm in today’s case manifests RBBB with alternating hemiblock conduction (See text).

LADDERGRAM with Events in the 3 Major Conduction Fascicles:

To better illustrate the complex alternating conduction pattern in today’s case — I schematically represent in Figure-5 the events that occur in the Ventricular Tier for the 3 major conduction fascicles, each of which is color-coded.

  • RBBB is present for all beats on this tracing. This block in conduction through the right bundle branch is represented by the PURPLE lines with butt ends in the Ventricular Tier for all 8 beats in Figure 5.
  • As previously noted — odd-numbered beats (ie, beats #1,3,5,7) are conducted with a RBBB/LAHB pattern of bifascicular block. This means that after passing through the AV Node and the Bundle of His — conduction through the ventricles occurs over the intact Left Posterior Hemifascicle (RED arrows within the Ventricular Tier for beats #1,3,5,7).
  • In contrast — even-numbered beats (ie, beats #2,4,6,8) are conducted with a RBBB/LPHB pattern of bifascicular block. This means that after passing through the AV Node and the Bundle of His — conduction through the ventricles occurs over the intact Left Anterior Hemifascicle (light BLUE arrows within the Ventricular Tier for beats #2,4,6,8).


Figure-5: Schematic representation of events in the 3 major conduction fascicles (See text).

BOTTOM LINE: It should be evident that this 60-something woman who presented with presyncope has severe conduction system:

  • The underlying rhythm is a ventriculophasic sinus arrhythmia. Because of frequent non-conducted beats — the overall ventricular rate is slow.
  • There is abnormal ST-T wave inversion in leads I, aVL and V2, with some ST-T wave flattening elsewhere. Whether or not this represents ischemia of recent origin is difficult to determine given the bradycardia and conduction defects.
  • There is 2nd-Degree AV Block, Mobitz Type I (with 3:2 AV conduction).
  • There is complete RBBB.
  • There is alternating Hemiblock (with the pattern of bifascicular block shifting from one-beat-to-the-next from RBBB/LAHB to RBBB/LPHB).


Follow-Up to this Case: A dual chamber pacemaker was implanted in this patient given her symptoms occurring in association with severe conduction system disease.



Additional Relevant ECG Blog Posts to Today’s Case:

  • ECG Blog #186 – When to suspect Mobitz I 
  • ECG Blog #62 — Mobitz I vs Mobitz II 2nd-degree AV block.
  • ECG Blog #164 — Review of another case with step-by-step analysis of Mobitz I. 
  • ECG Blog #188 — How to Read (and Draw) Laddergrams.
  • ECG Blog #205 — Review of the Bifascicular Blocks.
  • ECG Blog #130Alternating Bundle Branch Block (RBBB/LBBB).
  • ECG Blog #232 — For more on Bigeminal Rhythms (when the 1st beat in each pair is sinus-conducted).



  • Acknowledgment: My appreciation to Ser Barsa (from Yerevan, Armenia) and to J.B. (from the UKfor the case and this tracing.



  1. Many thanks, Ken, masterly interpretation as always, one never stops learning from your pearls! I want to thank my UK colleague - cardiology registrar J.B. who keeps sending me these real-life ECG puzzles ))

    1. My Pleasure Sergey! I just now added J.B. to my Acknowledgment for this case! — :)