ECG Blog #484 — What is Not Blocked?

The ECG in Figure-1 was obtained from a man in his 80s with known coronary disease — who presented for routine follow-up. 

• The patient was found to have an irregular pulse — but he was asymptomatic with a normal blood pressure.
• He was not on any rate-slowing medication ...

QUESTIONS:

• How would you interpret the rhythm in Figure-1?
•   — What treatment is indicated?
• 
  
• Beyond-the-Core: Today's fascinating rhythm turned out to be more fascinating (and more complicated) that I initially realized. 
• HINT: Are there any PR intervals that repeat? If not — Can you explain why not?

Figure-1: The initial ECG in today's case.

=================================

How I Approached Today's Tracing:

I have found the simple steps of numbering beats and labeling P waves amazingly helpful in: i) Facilitating communication with other health care professionals about what we think is going on (because we can refer to the beats by number!); and, ii) Labeling P waves makes it so much easier to appreciate the relationship (if any) between P waves and neighboring QRS complexes.

• There is group beating in the long lead II rhythm strip in Figure-1 (ie, alternating shorter-then longer R-R intervals). In all — there are 4 groups of 2 beats ( = beats #1,2; 3,4; 5,6; and 7,8).
• P waves are present (RED arrows in Figure-2). These P waves appear to be fairly regular (easily appreciated by "following" the nearly equal distancing between RED arrows in Figure-2).
• There are more P waves than QRS complexes (ie, The RED arrow P waves that are seen to fall near the middle of each of the longer R-R intervals are not followed by any QRS complex). Thus, despite the fairly regular atrial rhythm — there are on-time sinus P waves that are not conducted. This defines the presence of some form of 2nd- or 3rd-degree AV block.
• PEARL #1: We can instantly know that the rhythm in Figure-2 is not 3rd-degree AV block — because the ventricular rhythm is not regular (as it usually is when AV block is complete — because most of the time, the "escape" rhythm with complete AV block tends to be surprisingly regular).
• PEARL #2: We can instantly suspect that some form of 2nd-degree AV block of the Wenckebach type is present — because of the presence of group beating (which is so commonly seen in Wenckebach rhythms).
• PEARL #3: It's important to appreciate that the overall ventricular rate in Figure-2 is slow (The long lead II rhythm strip is 10 seconds long = 10/60 seconds, or 1/6  of a minute. There are 8 beats in this 10-second rhythm strip — and 6 X 8 = 48 beats/minute). This degree of bradycardia, by itself — may be problematic in a patient in his 80s.

QUESTIONS:

• What about the QRS complex in Figure-2?
• Why is the QRS wide?
• Why does QRS morphology change every-other-beat?
• Are any beats in Figure-2 conducted to the ventricles?

Figure-2: I've numbered the beats — and have labeled P waves with RED arrows.

================================

ANSWERS:

QRS morphology in Figure-2 for each of the beats in the chest leads — is consistent with RBBB (Right Bundle Branch Block) because: i) There is a predominantly positive (qR) pattern in right-sided lead V1; and, ii) In left-sided lead V6 — there is a predominantly positive R wave, that is followed by a wide terminal S wave. 

• PEARL #4: QRS morphology in the limb leads alternates between LPHB (Left Posterior HemiBlock) and LAHB (Left Anterior HemiBlock) conduction.
• Note that each of the odd-numbered QRS complexes ( = beats #1,3,5,7) manifest LPHB morphology (rS in lead I; qR in leads II,III).
• Each of the even-numbered QRS complexes ( = beats #2,4,6,8) manifest LAHB morphology (Rs in lead I; rS in leads II,III).
• 
  
• PEARL #5: In cases such as this one, in which the cardiac rhythm is so challenging — there is a tendency to overlook assessment of the 12-lead ECG for potential acute changes! Note in the limb leads — that there is deep inferior lead T wave inversion for odd-numbered beats #1 and 3 — and overly tall positive (hyperacute?) T waves in leads I and aVL. In the chest leads — odd-numbered beats #5 and 7 show persistent ST-T wave depression as far laterally as lead V5 (which should not be seen with simple RBBB conduction). Although nonspecific — these abnormal ST-T wave changes could reflect recent ischemia and/or infarction as the potential cause of this patient's conduction defects!

Putting It All Together:

Today's patient is an asymptomatic man in his 80s, with known coronary disease — who was being seen for "routine" follow-up.

• I initially interpreted today's rhythm as some form of 2nd-degree AV Block, presumably Mobitz Type I (which is also known as AV Wenckebach).
• I initially thought that since the PR interval preceding beats #2,4,6 and 8 looked to be the same (albeit prolonged at >0.20 second) — that these beats were being conducted to the ventricles.
• The PR interval preceding beats #1,3,5 (and probably also for beat #7) — looks to be too short to conduct. This suggests that Wenckebach cycles are being terminated by junctional "escape" beats #1,3,5,7.
• There is significant bradycardia (The overall ventricular rate for this rhythm = 48/minute).
• QRS complexes manifest an alternating form of bifascicular block (RBBB/LPHB alternating with RBBB/LAHB).
• Considering all of the above findings together (despite the patient's lack of symptoms) — this older man has significant bradycardia with indication of severe conduction system disease (ie, 1st-degree AV block + RBBB + alternating LAHB and LPHB). As a result — I suspected that a permanent pacemaker would probably soon be needed.
• KEY Point: Repeat ECGs and serial Troponins are needed prior to pacemaker placement to rule out a recent event (especially given the abnormal ST-T wave abnormalities described above in PEARL #5).

=====================================

The CASE Continues:

A prior ECG from 6 months earlier was found (See Figure-3).

QUESTION:

• What do we learn from this prior ECG?

Figure-3: Previous ECG from 6 months earlier on today's patient.

=================================

ANSWER:

The previous ECG on today's patient (that is shown in Figure-3) — shows sinus rhythm with 1st-degree AV block and RBBB/LAHB.

• There is no sign of LPHB conduction — and no indication of acute ST-T wave findings in this prior tracing.

MY Thoughts: The fact that this patient previously had 1st-degree AV block + RBBB/LAHB — suggests this as his "baseline" conduction defect.

• The fact that this patient’s current ECG now shows LPHB conduction instead of LAHB conduction for the odd-numbered beats in Figure-1 — may either be due to: i) Phase 4 or "bradycardia-dependent" BBB — since LPHB conduction is only seen after a longer preceding R-R interval (See My Comment in the August 17, 2020 post in Dr. Smith's ECG Blog); — or — ii) A true manifestation of alternating hemiblock conduction due to true trifascicular disease.
• In any case — prudence suggests serial tracings and Troponins be checked to rule out a recent event (given the abnormal ST-T waves in odd-numbered beats in Figure-1).

=================================

Beyond-the-Core: And then I looked closer ...

Take another LOOK at today's initial tracing ...

• To facilitate assessment of the rhythm — I show only the long lead II rhythm strip in Figure-4. I've left the RED arrow sinus P waves from Figure-2 in place.

QUESTIONS:

• Is the PR interval the same before beats #2,4,6 and 8? (as I initially thought in Figure-2?).
• Is the R-R interval before beats #3,5 and 7 the same? 
•    — What might the answers to these 2 questions mean?
• 
  
• Extra Credit: Do you think that any of the P waves in Figure-4 are being conducted to the ventricles?

Figure-4: Focus on the long lead II rhythm strip from ECG #1 — in which RED arrows highlight a fairly regular underlying sinus rhythm.

ANSWERS:

• The easiest of the above questions to answer relates to the length of the R-R interval before beats #3,5 and 7. 
• YES — The R-R interval before beats #3,5 and 7 is the same. This tells us that beats #3,5 and 7 are likely to be junctional “escape” beats (ie, As shown in Figure-5 — the R-R interval preceding beats #3,5,7 measures 7.5 large boxes = 1500 msec. — which corresponds to a ventricular rate of 300 ÷ 7.5 = 40/minute  = a rate that corresponds to the 40-60/minute range expected for a junctional escape rhythm).
• In further support that beats #1,3,5,7 are each junctional "escape" beats — is that the PR intervals preceding these beats are all different, as well as being too short to conduct. In other words — before any of the PINK arrow P waves have a chance to conduct to the ventricles, 1500 msec. have elapsed — which is the amount of time that triggers an AV Nodal escape focus (that is "set" at an escape rate of ~40/minute) to put out a junctional escape beat.

Figure-5: The R-R interval preceding beats #3,5,7 = 7.5 large boxes — which corresponds to a junctional "escape" rate = 300 ÷ 7.5 = 40/minute.

Advanced PEARL #6: Instead of beats #1,3,5,7 representing "escape" from a junctional site — these beats could represent an "escape" focus arising from the left anterior hemifascicle (which typically manifests RBBB/LAHB morphology at an escape rate of ≤40/minute).

=====================================

QUESTION:

• Are the PR intervals before beats #2,4,6 and 8 really the same? (as I initially thought when I first saw at this tracing?).

ANSWER:

• NO. As shown in Figure-6 — the RED numbers indicate precise measurement of the PR intervals preceding beats #2,4,6 and 8 as equal to 270, 310, 340 and 300 msec.
• Therefore — None of the PR intervals in today's arrhythmia repeat. Almost always — AV Wenckebach rhythms are identified by recognizing "Footprints of Wenckebach" — with 2 of the most helpful "Footprints" being: i) Group beating (that is seen in today's case); and, ii) Repetition of at least some PR intervals (that tells us there is conduction of at least some P waves). 
• Today's case is unusual — because none of the PR intervals are the same!

PEARL #7: For the interpretation of complex rhythm disorders — the use of calipers may prove invaluable! Calipers instantly make you “smarter” while also saving time — because they make it so easy to determine the precise duration of intervals.

• To Emphasize: Calipers are usually not needed for basic assessment of arrhythmias sufficient to enable appropriate initial management (ie, My initial assessment of bradycardia in today's case, in association with some form of 2nd-degree Wenckebach conduction in which there are junctional escape beats — is more than adequate for initial management of today’s patient). But precise interpretation of the mechanism of today’s rhythm is all but impossible without the use of calipers to precisely measure and compare PR and R-R intervals — as shown below in Figure-6.

PEARL #8: As noted above, and as discussed in detail in the ADDENDUM to ECG Blog #458 (as well as in many other Blog posts) — AV Wenckebach is most easily recognized by seeing one or more of the "Footprints of Wenckebach". Among the least appreciated of these "Footprints" — is RP/PR Reciprocity (ie, The longer the RP interval — the more time the AV node has to recover, and the shorter the PR interval of the next beat will be).

• Recognition of today's case as some form of Mobitz Type I, 2nd-degree AV Block — is an example in which the possibility of AV Wenckebach is suggested by recognition of group beating — and then verified by the concept of PR/RP Reciprocity — despite the lack of any repetitive PR intervals.
• We see RP/RP Reciprocity in Figure-6. Thus, the shortest PR interval (ie, 270 msec. before beat #2) occurs in association with the longest RP interval (ie, 860 msec. just after beat #1).
• In contrast — the longest PR interval ( = 340 msec., before beat #6) occurs in association with the shortest RP interval (ie, 680 msec. just after beat #5).
• And, as the RP interval progressively increases (ie, from 680 — to 720 — to 770 msec.) — the PR interval progressively shortens (from 340 — to 310 — to 300 msec.) — until we see the shortest PR interval ( = 270 msec.) occur in association with the longest RP interval.
• KEY Point: The reason conditions that set up PR/RP Reciprocity occur in today's rhythm — is a result of ventriculophasic sinus arrhythmia (See ECG Blog #344 for more on this special form of sinus arrhythmia that so commonly occurs in association with 2nd- and 3rd-degree AV block).
• To Emphasize: It is rare that I need to invoke the principle of PR/RP Reciprocity in order to prove Wenckebach conduction. Today's case is one of those rare times.

Figure-6: Demonstration of PR/RP Reciprocity in this subtle case of AV Wenckebach with junctional escape.

==================================

Laddergram Illustration:

The BEST way to illustrate the mechanism of today's rhythm — is by laddergram, as shown below in Figure-7.

• Small RED circles within the AV Nodal Tier highlight the 4 junctional "escape" beats (beats #1,3,5,7).
• Retrograde conduction from these junctional escape beats influences the timing of subsequent sinus P waves — and it is this effect that enables conduction of these P waves with constantly changing PR intervals.

Figure-7: My proposed laddergram for today's rhythm.

Advanced PEARL #9: The degree of AV block in today's rhythm may appear to be higher-grade than it actually is. We generally define 2nd-degree AV block as being a "high-grade" block — IF we see 2 or more on-time sinus P waves fail to conduct despite occurring at a point in the rhythm in which there is adequate opportunity to conduct. But we have no idea from the single tracing we are given IF beats #1,3,5,7 would have conducted IF the rhythm had not been interrupted by the occurrence of appropriately-timed junctional escape beats.

• I suspect beats #1,3,5,7 would have been able to be conducted — IF the P waves before these beats would have had a chance (ie, a little more time) to conduct before being interrupted by the junctional escape beats.
• KEY Point: I bet that an additional 30-to-60 seconds of telemetry monitoring would have answered the question as to whether there is true "high-grade" block — because the changing timing that we see (in association with the underlying ventriculophasic sinus arrhythmia) would almost certainly have afforded an opportunity for these P waves to conduct without interruption by junctional escape beats.

FINAL Thought: I do not know the final outcome of today's case. But as alluded to earlier — the combination of an elderly patient with significant bradycardia and ECG evidence of trifascicular involvement — most probably will eventually lead to pacemaker placement.

==========================================

Acknowledgment: My appreciation to William Santos (from Nadal, Brazil) for the case and this tracing.

==========================================

 

============================== 

Related ECG Blog Posts to Today’s Case:

• ECG Blog #185 — My Ps, Qs, 3R System for Rhythm.
• ECG Blog #188 — Reviews how to read and draw Laddergrams (with LINKS to more than 120 cases — many with step-by-step sequential illustration of how to construct the laddergram).

• ECG Blog #205 — Reviews my Systematic Approach to 12-lead ECG interpretation.

================================

• ECG Blog #192 — 3 Causes of AV Dissociation.
• ECG Blog #191 — AV Dissociation vs complete AV Block.
• 
  
• ECG Blog #389 — ECG Blog #373 — for review of some cases that illustrate "AV block problem-solving".
• ECG Blog #236 — for an ECG Video Pearl on the 3 Types of 2nd-degree AV block.
• ECG Blog #344 — thoroughly reviews the Types of 2nd-degree AV Block (Mobitz I vs Mobitz II vs 2:1 AV Block).
• 
  
• ECG Blog #267 — Reviews with step-by-step laddergrams, the derivation of a case of Mobitz I with more than a single possible explanation.
• ECG Blog #164 — Step-by-Step laddergram of Mobitz I.
•  
• ECG Blog #195 — Isorhythmic AV Dissociation.