Saturday, March 2, 2024

ECG Blog #419 — The Cause of ECG #1?

I was sent the 2 ECGs shown in Figure-1 — which were recorded from an elderly man whose heart beat "has been irregular for years". No clear history for recent chest pain — but the patient "has not been well" for the previous week. Regarding the 2 ECGs in Figure-1
  • ECG #1 is the initial tracing obtained at the scene by the EMS (Emergency Medical Systems) team — in association with an alert but markedly hypotensive patient.
  • ECG #2 was recorded a short while after ECG #1 by the EMS team — but before any treatment other than ASA was given. As might be imagined — the patient suddenly "felt much better".

  • How would YOU interpret these 2 ECGs?
  •   — How might ECG #2 be related to ECG #1?

Figure-1: The 2 ECGs in today's case. (To improve visualization — I've digitized the original ECG using PMcardio)

MY Thoughts on the 2 ECGs in Figure-1:
Even without a long lead rhythm strip — it is easy to understand why today's patient was markedly hypotensive in association with ECG #1.
  • Atrial activity is present in ECG #1 — in the form of seemingly regular upright P waves in each of the inferior leads — at a regular rate of ~115/minute
  • I suspect that none of the P waves in ECG #1 are conducted.
  • Detection of atrial activity is much more difficult in the chest leads due to a large amount of baseline artifact — although evidence of atrial activity at a similar rate is seen toward the end of lead V1.
  • The above said — intermittent ventricular complexes of varying morphologies are seen (albeit with much distortion from artifact).
  • The longest pause occurs in simultaneously-recorded leads aVR,aVL,aVF — with no ventricular complex seen after the 1st beat in these leads until the lead change to V1,V2,V3 occurs (ie, almost 12 large boxes = 2.4 seconds later). We have no idea how long the pause was before the 1st beat in ECG #1 was recorded.

  • Clinical IMPRESSION: I would interpret ECG #1 as suggestive of near ventricular standstill — with underlying atrial tachycardia, extended pauses, and no reliable ventricular escape focus. This is often a pre-lethal rhythm.

  • PEARL #1: The rhythm in ECG #1 is more than just "complete AV block". This is because in addition to no conduction of any of the sinus P waves — the presence of a fairly regular escape pacemaker that is usually seen with complete AV block is absent. Instead, we see extended pauses that are intermittently interrupted by irregular (and unreliable) ventricular complexes arising from different ventricular sites.

Before any treatmentECG #2 was recorded:
  • Normal sinus rhythm has spontaneously returned! Interestingly — the rate (and morphology) of sinus P waves is virtually unchanged with respect to the unconducted P waves that were seen in ECG #1, the difference being that there is now 1:1 conduction of sinus impulses with a normal PR interval!

NOTE: The QRS complex of conducted beats in ECG #2 is wide. QRS morphology is interesting in that it resembles RBBB (Right Bundle Branch Block) conduction in the chest leads — but LBBB (Left Bundle Branch Block) conduction with marked left axis in the limb leads. 

  • The tall R wave (qR morphology) in lead V1 of ECG #2 — in association with a fairly wide terminal S wave in lead V6 — suggests RBBB conduction.
  • The predominantly upright widened QRS complexes in leads I and aVL of ECG #2 — suggests LBBB conduction — with the predominantly negative QRS complexes in each of the inferior leads indicative of marked LAD (Left Axis Deviation).

PEARL #2: As described in ECG Blog #394 — QRS widening in the presence of sinus rhythm, in which QRS morphology is consistent with RBBB conduction in the chest leads — but LBBB conduction in the limb leads (especially with a leftward axis) — suggests the entity known as MBBB (Masquerading Bundle Branch Block). This is precisely the picture of the QRS morphology that we see in ECG #2.
  • I review my user-friendly approach to the ECG diagnosis of the Bundle Branch Blocks in ECG Blog #282 — and to Hemiblocks and Bifascicular Blocks in ECG Blog #203. In brief — the 3 KEY leads that allow accurate diagnosis of RBBB and LBBB within seconds (!) — are right-sided lead V1 and left-sided leads I and V6.
  • Supraventricular conduction defects not consistent with either RBBB or LBBB in all 3 of these key leads are most easily classified as IVCD (IntraVentricular Conduction Defect) — with this category representing the "end result" of a number of different pathophysiologic processes.

  • MBBB is a special type of IVCD that although uncommon, is important to recognize because it identifies a group of patients with: i) Very severe underlying heart disease; ii) A much higher predisposition for developing complete AV block (and needing a pacemaker); andiii) An extremely poor longterm prognosis.

  • NOTE: Variations on this above "theme" of MBBB are common. Thus, the S wave that is typically associated with RBBB patterns in lateral chest leads V5,V6 may or may not be present. In the limb leads, rather than a strict LBBB pattern — more of an extreme LAHB (Left Anterior HemiBlock) pattern may be seen (ie, with wide and predominantly [if not totally] negative QRS complexes in the inferior leads — and with a smaller [blunted] terminal s wave in leads I and aVL).

PEARL #3: Knowing the clinical history may aid in recognition of IVCD patterns that are consistent with MBBB (ie, if the patient has a known history of severe, underlying heart disease)
  • Distinction from simple bifascicular block (ie, with RBBB/LAHB) — may be facilitated by seeing one or more of the following: i) More of a monomorphic upright QRS in lead V1 (which lacks the neatly defined, triphasic rsR' with taller right "rabbit ear" seen with typical RBBB)ii) Lack of a wide terminal S wave in lateral chest lead V6; iii) Seeing an all-positive (or at least predominantly positive) widened QRS in leads I and/or aVL, with no more than a tiny, narrow s wave in these leads; and/oriv) Seeing widened, all-negative (or almost all-negative) QRS complexes in the inferior leads.

What About Today's ECGs?
Although details of the history of today's patient are lacking — the patient is elderly — he has had "an irregular heart beat for years" — and his initial ECG on EMS arrival ( = ECG #1) clearly suggests advanced conduction system disease with a potentially lethal rhythm were it not for spontaneous conversion to the tracing shown in ECG #2.
  • In this clinical context — findings in ECG #2 are clearly consistent with masquerading BBB

  • A permanent pacemaker is needed. 

Acknowledgment: My appreciation to Max-Gordon-Hall (from Batlow, New South Wales, Australia) 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 #282 — reviews a user-friendly approach to the ECG diagnosis of the Bundle Branch Blocks (RBBB, LBBB and IVCD).

  • ECG Blog #203 — reviews ECG diagnosis of Axis, Hemiblocks and Bifascicular Blocks.

  • ECG Blog #394 — reviews another case of Masquerading BBB.


ADDENDUM (3/6/2024): 

In follow-up to the Comment I have just received from H.S.Cho — I would expand on my description of the cardiac rhythm in ECG #1 from today's case.
  • Given the history in today's case (ie, Sudden onset of the rhythm seen in ECG #1 — that within a minute spontaneously resolved) — Rather than "near ventricular standstill" — this rhythm is best described as PD-PAVB (Pause-Dependent Paroxysmal AtrioVentricular Block).

  • For clarity — I have reproduced Figure-1 from today's case.

Figure-1: The 2 ECGs in today's case

Paroxysmal AtrioVentricular Block (PAVB):
As described by Bansal et al (J Arrhythmia 35:870-872, 2019) and Bosah et al (Cureus 14[7]: e27092, 2022) — the entity known as PAVB is a potential cause of syncope that is easily overlooked and which is potentially lethal (probably more often than is commonly realized).
  • PAVB is characterized by the sudden, unexpected onset of complete AV block with delayed ventricular escape — therefore resulting in a prolonged period without any QRS on ECG. Prior to the prolonged pause — the patient manifests 1:1 AV conduction without other evidence of AV block (which is why onset of PAVB is typically so unsuspected!).
  • Because of its totally unexpected onset and propensity to result in sudden death — PAVB is difficult to document and significantly underdiagnosed.
  • Three mechanisms for producing PAVB have been described: i) Vagally mediated (ie, Vagotonic Block — as described in ECG Blog #61with the references listed at the end this Blog post citing instances of transient asystole from excessive vagal tone!); ii) Intrinsic (Phase 4 = pause- or bradycardic-dependent) PAVB; — and, iii) Idiopathic.

i) Vagotonic AV Block:
This form of PAVB is potentially benign when it results from a transient profound surge of parasympathetic tone in an otherwise healthy individual (as might occur with an episode of severe vomiting; a fit of severe coughing; vasovagal reaction from a blood draw).
  • The problem with vagotonic PAVB is localized to within the AV Node.
  • There will often be a "prodome" of diaphoresis, nausea, dizziness — with the patient aware of imminent fainting.

  • Characteristic ECG findings of vagotonic PAVB include progressive sinus rate slowing — often associated with an increasing PR interval and a narrow-QRS escape focus — followed by recovery with progressive return to a normal sinus rate and normal PR interval.

ii) Intrinsic PAVB:
Several names have been attached to this mechanism of PAVB — including most commonly "Phase 4 AV block" and/or PD-PAVB (Pause-Dependent Paroxysmal AtrioVentricular Block).
  • PD-PAVB is the most likely mechanism for the cardiac rhythm in ECG #1 from today's case. The underlying pathology is severe His-Purkinje System disease (strongly suggested by the presence of MBBB in ECG #2 of today's case). This form of PD-PAVB is likely to be fatal unless the patient receives a permanent pacemaker.
  • The interesting pathophysiology of PD-PAVB results from chance occurrence of an "appropriately-timed" PAC or PVC that partially depolarizes the diseased HPS (His-Purkinje System) at a specific point in the cycle that renders the poorly-functioning HPS unable to complete depolarization. The resultant prolonged pause in ventricular depolarization may only resolve if another "appropriately-timed" PAC or PVC occurs at the precise point needed to "reset" the HPS depolarization cycle (which presumably explains why the patient in today's case spontaneously recovered).
  • Of note — although severe underlying HPS disease is evident from the MBBB seen in ECG #1 of today's case — up to 1/3 of patients with PD-PAVB do not show evidence of conduction defects on ECG, thereby complicating documentation of this diagnosis.

  • PEARL #4: In addition to seeing MBBB in ECG #2 — the fact that the atrial rate remains the same in both ECG #1 and ECG #2 is yet one more reason why the rhythm in ECG #1 does not represent simple vagotonic PAVB.

iii) Idiopathic PAVB: 
This is the most recently described form of PAVB — in which findings are not consistent with either of the other 2 forms.
  • The baseline ECG before idiopathic PAVB tends to be normal.
  • No "trigger" for PAVB is evident (ie, no source of excessive vagal tone — and no precipitating PACs/PVCs are seen).


NOTE: The conduction disturbance shown in ECG #1 from today's case differs from that shown in ECG Blog #342 — in which the initial rhythm was AFib (ie, no P waves present for the first 6 beats in this tracing) — followed by a prolonged flat line pause (nearly 5 seconds in duration) — until finally a QRS complex preceded by a P wave (that may or may not have been conducting) was seen.
  • Whether the prolonged flat line pause in this Blog #342 example represents another variation of PD-PAVB due to severe underlying disease of the His-Purkinje System — or reflects severe SA Node disease (ie, Sick Sinus Syndrome) — or most likely represents some combination of the two, is uncertain from the single ECG I was provided with.
  • That said — the BOTTOM Line remains the same, namely that assuming nothing "fixable" is found — permanent pacing will be needed.



  1. This patient may have been suffering from paroxysmal AV block(such as phase 4 AV block) that also requiring permanent pacemaker implantation..

    1. Excellent thought! Nice write-up of PD-PAVB ( = Phase 4 or Pause-Dependent Paroxysmal AtrioVentricular Block) by Bosah et al may be found here — — As per discussion by Bosah et al — sudden block of repetitive atrial impulses may be seen in patients with severe underlying Purkinje system disease if an "appropriately-timed" PAC or PVC occurs. The condition may similarly remit if another "appropriately-timed" PAC or PVC occurs — but longterm, pacing definitely needed! THANKS again for your comment!

    2. To H.S.Cho — Just wanted you to know that your comment motivated my further research of the concept you raised ( = PD-PAVB) — and as a result, I have acknowledged you in the ADDENDUM that I just wrote to this ECG Blog #419. THANKS again for your comment! — :) Ken