ECG Blog #336 — Impossible to Classify. Why?

The CASE: An elderly woman with dizziness for several months.

• The CHALLENGE: As per the title of this post — Is the degree of AV block impossible to classify?

Figure-1: 12-lead ECG and long lead II rhythm strip — obtained from an elderly woman with several months of "dizziness".

MY Approach to the ECG in Figure-1:

As always – I favor beginning interpretation with assessment of the long lead rhythm strip — using the Ps, Qs & 3R Approach to recall the 5 KEY Parameters (See ECG Blog 185). I find it easiest (and most productive) to delay assessing the 12-lead ECG until after I’ve had a chance to look at the rhythm.

• The rhythm is slow and not regular. 
• Although on initial inspection of the long lead II rhythm strip, the QRS does not look overly wide — a quick glance at beats #5 and 6 in simultaneously-recorded leads V1,V2,V3 reveals definite QRS widening. That said — QRS morphology in lead V1 suggests there is RBBB (Right Bundle Branch Block) conduction — which is supported by the wide, terminal S waves in lateral leads I and V6.
• Therefore, despite QRS widening in Figure-1 — this rhythm is unlikely to originate from ventricular myocardium (ie, unlikely to originate from outside of the conduction system). 
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• NOTE: The reason the QRS initially looked "narrow" in the long lead II rhythm strip — is that there is a wide, terminal S wave — and that part of this terminal QRS deflection lies on the baseline.

PEARL #1: As I often emphasize — the simple step of labeling P waves is amazingly helpful for: i) Determining IF there is an underlying regular (or almost regular) atrial rhythm; and, ii) Facilitating assessment as to whether some (or all) of the P waves you identify are (or are not) Related to neighboring QRS complexes.

• In Figure-1 — I began my “Search for P Waves” — by labeling those P waves that I was certain are present. I have done this with RED arrows in Figure-2.
• Setting my calipers to the P-P interval between either of the 2 places in the long lead II rhythm strip where 2 RED arrow P waves occur in a row (ie, to the P-P interval either just before and after beat #3 — or just before and after beat #6) — I was then able to "walk out" a number of partially hidden deflections representing additional P waves (PINK arrows in Figure-2).
• Accounting for slight variation in the P-P interval that is so often seen — it seems logical to anticipate that 2 additional sinus P waves are likely to be hiding within the QRS of beats #1 and 7 (WHITE arrows in Figure-2).

Figure-2: I've labeled definite and presumed underlying sinus P waves in the long lead II rhythm strip with colored arrows (See text).

PEARL #2: When a complex form of AV block is present — my favorite CLUE that a beat is likely to be conducted — is IF you see a beat that occurs earlier-than-expected.

• In addition to its earlier-than-expected occurrence — further support that beat #3 and beat #6 in Figure-2 are both conducted is forthcoming from: i) The fact that both of these beats are preceded by the same normal PR interval (ie, ~0.16 second); ii) A look at simultaneously-recorded lead V1 for beat #6 suggests a very typical RBBB (Right Bundle Branch Block) morphology — in the form of an rSR' (with S wave descending below the baseline in lead V1 — and a terminal "taller right rabbit ear" R' deflection); and, iii) Beat #3 and beat #6 are the only beats in Figure-1 that are preceded by a normal PR interval — whereas none of the other PR intervals preceding the remaining 6 beats on this tracing seem likely to conduct (ie, although the other PR intervals look similar — these PR intervals are quite long, and most are different from one another).
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• NOTE: For more on HOW to tell if a P wave is (or is not) likely to conduct — Check Out the Audio Pearl in the ADDENDUM below.

What We’ve Determined thus Far:

Regarding the rhythm in today’s case — We have so far established the following:

• Although there is slight QRS widening — all 8 beats in today's rhythm appear to be supraventricular — and manifest RBBB conduction (with very typical triphasic RBBB morphology in lead V1 — and upright R wave with wide terminal S waves in lateral leads I,aVL; and V5,V6).
• There is an underlying regular atrial rhythm (RED arrows in Figure-3).
• Beats #3 and #6 are both sinus-conducted — as evidenced by their earlier-than-expected occurrence — and because they are both preceded by a normal and equal PR interval (Figure-4).
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• Review of the carefully measured R-R intervals displayed in Figure-4 — suggests that other than the 2 sinus-conducted beats ( = beats #3 and 6) — the remaining 6 beats on today’s tracing are junctional escape beats. This is because: i) The PR intervals preceding these other beats are long — and for the most part different from one another; ii) The R-R intervals preceding each of these other beats are identical (ie, 1,350 msec.); and, iii) QRS morphology of these junctional escape beats is the same as QRS morphology for the 2 sinus-conducted beats (whereas ventricular escape beats would manifest a different QRS morphology).

Figure-3: RED arrows highlight a fairly regular atrial rhythm.

Figure-4: Beats #3 and 6 are sinus conducted (each occurring earlier-than-expected — and each is preceded by the identical and normal PR interval = 0.16 second). The remaining 6 beats on this tracing are junctional escape beats (See text).

Putting It All Together:

Today’s rhythm by definition represents some form of 2nd-Degree AV Block because: i) There is an underlying regular (or at least fairly regular) sinus rhythm; and, ii) Some P waves are conducting (ie, beats #3 and 6) — but other P waves that should conduct do not conduct.

• PEARL #3: To determine how “severe” the 2nd-degree AV block in today’s case is — we would need to see how many beats do not conduct despite having adequate opportunity to do so. This is most easily demonstrated by means of the sequential laddergrams that I construct below in Figure-6 thru Figure-11.

PEARL #4: Now that we’ve determined the presence of 2nd-degree AV block — we need to return to the original 12-lead ECG that was shown in Figure-1 — to see IF this reveals a potential cause for this rhythm disturbance. To facilitate this — I've reproduced the 12-lead and rhythm strip for today's tracing in Figure-5.

• We've already noted the presence of RBBB conduction — both for sinus-conducted beats ( = beats #3 and 6) — and for junctional escape beats ( = beats #1,2; 4,5; 7,8).
• There is LVH (Left Ventricular Hypertrophy) — as even in the presence of RBBB (that alters both QRS amplitude and ST-T wave morphology) — the markedly increased R wave amplitude in leads V5 and V6 of Figure-5 clearly satisfies LVH voltage criteria (See ECG Blog #245 for review of LVH criteria).
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• Although it is often more challenging to assess for ischemia on ECG in the presence of bundle branch block — the ST-T wave flattening in multiple leads in Figure-5 (seen in leads I,II,III; aVL,aVF; and V5,V6) — is not a normal response with simple RBBB.
• As emphasized in ECG Blog #204 — it is normal (and expected) with RBBB or LBBB — for the direction of the ST-T wave in the 3 KEY Leads ( = leads I,V1,V6) — to be oppositely directed to the last QRS deflection. Since the last QRS deflection in lead V1 of Figure-5 is the upright R' — the ST-T wave depression that we see in this lead is not abnormal.
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• PEARL #5: With simple RBBB — the relative amount of ST-T wave depression that you see in the anterior chest leads should be maximal in lead V1 — and progressively decrease as you move from lead V1-to-V2-to-V3 — eventually disappearing as you move to more lateral chest leads. This is the opposite of what we see in Figure-5 — since the size of the inverted T wave is maximal in lead V3 (instead of in lead V1) — and T wave inversion continues out to lead V5 (whereas there should normally be no lateral chest lead T wave inversion if the only thing going on was simple RBBB). This strongly suggests that in addition to RBBB — there is also indication of ischemia of uncertain age.
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• CONCLUSION: The history in today's case — is that of an elderly woman with dizziness over a period of several months. Given the presence of RBBB and the above-described ischemic ST-T wave changes — it's possible that the cause of this patient's conduction disturbances are that an "event" (ie, infarction) occurred at some time over these past few months that this patient has had dizziness.

Figure-5: I've reproduced the 12-lead ECG and long lead II rhythm strip from Figure-1 (with RED arrows highlighting regular sinus P waves). We've identified the presence of some form of 2nd-degree AV block. Is there a potential cause for this conduction disturbance?

Final Confirmation of the Rhythm in Today's Case:

The BEST way to demonstrate the etiology of a complex cardiac rhythm — is by sequential construction of a Laddergram — which I illustrate in step-by-step fashion through the next 6 Figures.

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• NOTE: For more on how to read (and/or draw) Laddergrams — Please check out our ECG Blog #188 (which includes teaching aids + LINKS to more than 50 illustrated laddergrams I have published).   

Beginning with Figure-6:

Figure-6: It's easiest to begin construction of a laddergram by indicating atrial activity. Vertical RED lines in the Atrial Tier correspond to P waves (RED arrows) in the long lead II rhythm strip.

Figure-7: Next — I've filled in the Ventricular Tier with RED arrows that correspond to each of the QRS complexes in the long lead II. Note that I indicate all beats on this tracing are conducted with RBBB morphology by a dotted line in the Ventricular Tier that represents the blocked right bundle branch. To Emphasize — It will often be easy to fill in the Atrial and Ventricular Tiers (as I've done in Figures-6 and -7). The "challenge" for solving most complex arrhythmias — generally begins with filling in the AV Nodal Tier (This comes next!).

Figure-8: It's time to start filling in the AV Nodal Tier. It's easiest to begin with those beats that you know are conducting ( = beats #3 and 6, as discussed earlier — as these 2 beats both occur earlier-than-expected — and  are preceded by the same normal PR interval).

Figure-9: I next focused on the remaining 6 beats — each of which appears to be a junctional escape beat given identical QRS morphology, equal preceding R-R intervals — and lack of a preceding P wave with reasonable chance for conduction. BLUE circles within the AV Nodal Tier suggest junctional origin for each of these beats.

Figure-10: I've added slanted BLUE lines that fail to make it through the AV Nodal Tier — to represent those P waves that clearly do not have a "chance" to conduct (because they either occur simultaneous with the QRS complex — or very soon thereafter, presumably during the absolute refractory period). Dotted BLUE lines emanating from the site of AV Nodal escape schematically represent retrograde conduction that should be expected to impair forward conduction of the blocked atrial impulses. 

Figure-11: I then completed my proposed laddergram by assessing the remaining P waves that have not yet been accounted for. BLUE lines ending in butt-ends stalled within the AV Nodal Tier suggest that despite seemingly having "adquate opportunity" to conduct — none of the BLUE arrow P waves are conducted to the ventricles.

CONCLUSION: It should be apparent from the final Figure-11 in my proposed laddergram — that the mechanism I favor for today's rhythm is 2nd-degree AV Block.

• It is impossible to tell from the short 8-beat rhythm strip available — whether today's rhythm represents the Mobitz I or Mobitz II form of 2nd-degree AV block. This is because we never see 2 conducted beats in a row. Therefore, we have NO idea IF the PR interval would increase IF it was given a chance to do so.
• In favor of Mobitz I — is that this form of 2nd-degree AV block is far more common than the Mobitz II form (In my experience — over 90-95% of all 2nd-degree AV blocks are Mobitz I). This is "good news" — since longterm prognosis of Mobitz I is generally much better than for Mobitz II, which almost always requires permanent pacing.
• That said — the presence of QRS widening (here in the form of underlying RBBB) — does increase the possibility of Mobitz II.
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• PEARL #6: Note in Figure-11 — that we never see 2 BLUE arrow P waves in a row! That is, we never see 2 P waves in a row that should conduct, yet fail to do so. As a result — we have NO idea if the 2nd-degree block in today's case represents a "high-grade" form of AV block (or simple Mobitz I without an increasing PR interval because of interruption from junctional escape beats).
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• Unfortunately — I do not have follow-up on this case. Assuming nothing "fixable" is found (ie, No recent infarction; normal thyroid and electrolyte studies) — I suspect this patient will end up with a permanent pacemaker because: i) She is elderly; ii) Her overall ventricular rate is in the 40s; and, iii) This patient has been symptomatic for months. 

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Acknowledgment: My appreciation to 陳俊宏 = Chun-Hung Chen (from Taichung City, Taiwan) for the case and this tracing.

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Related ECG Blog Posts to Today’s Case:

• ECG Blog #205 — Reviews my Systematic Approach to 12-lead ECG Interpretation. (This block post also reviews the concept of "Trifascicular Block").
• ECG Blog #185 — Use of a Systematic Approach to Rhythm Interpretation. 
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• ECG Blog #188 — Reviews how to read (and/or draw) Laddergrams (plus LINKS to more than 50 clinical examples of laddergrams I have drawn).
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• ECG Blog #203 — Reviews the ECG diagnosis of Axis and Hemiblocks.
• ECG Blog #204 — Reviews a user-friendly approach for diagnosis of the Bundle Branch Blocks.
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• ECG Blog #186 — and ECG Blog #236 — for review on the basics of 2nd-degree AV Block.
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• ECG Blog #192 — Reviews the 3 Causes of AV Dissociation — and emphasizes why AV Dissociation is not the same thing as Complete AV Block. 
• ECG Blog #191 — Emphasizes the difference between AV Dissociation vs Complete AV Block.
• ECG Blog #202 — and ECG Blog #257 — Review cases regarding HOW to tell if there is (or is not) Complete AV Block.
• ECG Blog #247 — Reviews a complex case with AV Dissociation.

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ADDENDUM (9/24/2022): I've reviewed some KEY material related to today's case:

• Audio Pearl on HOW to tell if a given P wave in an AV block tracing is likely to be conducting.

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ECG Media PEARL #61 (5:45 minutes Audio) — Reviews HOW to Tell IF a P Wave is Conducting? Being able to answer this question is KEY for determining the etiology of complicated AV Block/AV Dissociation tracings.