ECG Blog #331 — What's The Rhythm?

I was sent the ECG in Figure-1 — but without the benefit of clinical information. There apparently were a number of different opinions by clinicians regarding interpretation of this rhythm.

• How would YOU interpret the ECG in Figure-1?
• Is there complete AV block?

Figure-1: The 12-lead ECG and simultaneously-recorded long lead II rhythm strip in today’s case.

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NOTE: Interpretation of today's rhythm is challenging for a number of reasons. As a result — I go step-by-step in walking through my approach to this tracing.

• IF you prefer to skip the "step-by-step" (and go directly to the answer) — Scroll down to Figure-11 at the bottom of this page (to find the last laddergram with the Final Answer!). 

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MY Thoughts on 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.

PEARL #1: It does not matter in what sequence you choose to assess the 5 KEY Parameters — and I often start with whichever of these parameters is easiest to assess. For example — although I do see a number of P waves in the long lead II rhythm strip in Figure-1 — I found it easier to begin by looking at QRS width and Regularity of the rhythm. I approached this tracing as follows:

• QRS Width: Looking at all 12 leads (to see where the QRS interval is longest) — it should be apparent that the QRS complex is narrow (ie, not more than 0.10 second = not more than half a large box in duration in any lead). Knowing that all QRS complexes in today’s tracing are narrow — tells us that the rhyhthm is supraventricular.
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• Regularity of the Rhythm: It should be obvious that the rhythm in the long lead II rhythm strip is not Regular. Instead — each R-R interval appears to be slightly different than its neighboring R-R interval — so the rhythm is irregular.
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• Rate of the Rhythm: R-R intervals in the long lead II rhythm strip vary from just over 4 large boxes in duration — to a bit over 6 large boxes in duration — which suggests an average heart Rate of ~60/minute.
• Alternatively — You could arrivate at a similar estimated heart rate considering that there are 10 beats in this 10-second rhythm strip.

Continuing with my Ps, Qs, 3R Assessment:

The above process left me still having to assess the 2 remaining Parameters ( = P waves — and — Whether any P waves that are present are Related to neighoring QRS complexes?):

• P Waves: A number of P waves are definitely present!

PEARL #2: As emphasized in previous ECG Blogs — I have found the simple step of labeling P waves to be surprisingly 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.

• I favor beginning my “Search for P Waves” — by labeling those P waves that I am certain are present. I have done this with RED arrows in Figure-2.

Figure-2: I have added RED arrows over those P waves that I am certain are present in the long lead II rhythm strip.

MY Thoughts on Figure-2:

I see at least 9 definite P waves in Figure-2 (RED arrows). For these 9 P waves — the duration of the P-P interval between neighboring P waves looks similar (albeit not quite identical).

PEARL #3: Often when I am able to identify a number of definite P waves in a complex rhythm (as I've done for the RED arrows in Figure-2) — in which the duration of P-P intervals between neighboring P waves is similar (as it is in Figure-2) — additional "on-time" P waves can be found within the QRS complex or ST-T wave of intervening beats.

• Search for such additional “hidden” (or partially hidden) P waves — is facilitated by use of calipers! Simply set your calipers to the P-P interval you measure between 2 of the RED arrows — and see IF there is evidence of notching (or other subtle alteration in one or more T wave deflections) at this caliper-set distance within those R-R intervals that seem to be missing a RED arrow (ie, within the R-R intervals between beats #1-2 — between beats #4-5 — between beats #5-6 — and between beats #9-10).
• I have done this by adding YELLOW arrows to Figure-2 in these places (See Figure-3). It should be apparent that the reason the T waves of beats #1, 4, 5 and 9 all look slightly different from one another in Figure-3 —  is that an “on-time” P wave is partially hidden (and deforming) the T wave for each of these beats.

Figure-3: I've added YELLOW arrows to Figure-2 in places where an "on-time" P wave appears to be hiding (and partially deforming) the T waves of beats #1,4,5 and 9.

An Underlying Sinus Rhythm Is Present!

We have therefore established in Figure-3 — that an underlying sinus rhythm is present in today's rhythm. For clarity — I've highlighted each of the upright sinus P waves in Figure-4 with a RED arrow in the long lead II rhythm strip.

• IF up until now, you have used calipers to assist in location of the partially hidden P waves — you will have noticed slight variation in duration of the P-P intervals in Figure-4. This is because a component of sinus arrhythmia is present.
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• Did YOU Notice that there are 13 P waves (RED arrows) — but only 10 QRS complexes in Figure-4? This is because not all P waves are conducted. The fact that there are more P waves than QRS complexes in Figure-4 — suggests that some form of 2nd-degree AV block is likely to be present.

QUESTION:

• Do YOU think that any of the P waves in Figure-4 are being conducted to the ventricles? (HINT: Are the PR intervals before any of the 10 beats on this tracing the same?).   

Figure-4: RED arrows highlight a fairly regular sinus rhythm — which manifests slight variation in the P-P interval (sinus arrhythmia). Do YOU think any of the sinus P waves in this tracing are conducting? IF so — Which ones?

Are Any P Waves in Figure-4 Conducting?

For clarity in Figure-5 — I've color-coded the ARROWS highlighting sinus P waves in today's rhythm. There are some identical PR intervals on this tracing. 

• The PR intervals preceding beats #2, 7 and 10 are normal (ie, 0.18 second) — and of equal duration to each other (RED arrows in Figure-5). Note that the R-R intervals preceding each of these 3 beats — are the longest R-R intervals in this tracing.
• PEARL #4: The fact that there are a number of identical PR intervals in today's tracing is unlikely to be due to chance — and tells us that these P waves are being conducted to the ventricles. The presence of conducted beats rules out complete AV block!
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• The PR intervals preceding beats #3 and 8 are of similar (but not identical) duration. I color-coded these 2 P waves with different shades of BLUE to indicate that while this PR interval before beats #3 and 8 is clearly longer than the PR interval highlighted by the RED arrows — there is a slight difference in PR interval duration (ie, the PR interval before beat #8 is slightly longer than the PR interval before beat #3).
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• The PR intervals before beats #4 and 9 are equal and longer still (ie, 0.50 second — highlighted by GREEN arrows in Figure-5)!

PEARL #5: Considering our previous suspicion that some form of 2nd-degree AV block is present (because of the finding that more P waves than QRS complexes are present) — and — the finding of identical PR intervals at the end of each relative pause (ie, before beats #2,7,10 — RED arrows) — and — the finding of progressive PR interval lengthening within groups (ie, from RED-to BLUE-to GREEN arrows) — Doesn't This Suggest that the type of 2nd-degree AV block that is present is most likely to be some form of Mobitz I ( = AV Wenckebach)?

• WHITE arrows near the beginning of each of the longer R-R intervals — appear to represent non-conducted P waves (that are partially hidden within the T waves of beats #1, 6 and 9). It should not be unexpected that these P waves are non-conducted — because they occur so early in the cycle (ie, at a time when the ventricles will probably be refractory).
• This leaves us with only 3 more P waves to account for (ie, the BLACK arrows in Figure-5). Wouldn't It Make Sense for the BLACK arrows after beats #4 and 5 to be conducting with progressively longer PR intervals — until finally, the WHITE arrow P wave after beat #6 is blocked — before the next Wenckebach cycle begins with a much shorter (and normal) PR interval before beat #7 (RED arrow)?

Figure-5: I've color-coded the sinus P waves in Figure-4 — to facilitate recognizing beats with similar PR intervals.

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To EMPHASIZE: Although I have described the above series of rhythm-solving steps in great detail — use of calipers — and awareness of what to look for — allowed me to solve this complex arrhythmia within 2-to-3 minutes! This process can be done quickly!

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PEARL #6: I suspect that virtually all readers of my ECG Blog are familiar with the basic pattern of the Mobitz I form of 2nd-degree AV Block ( = AV Wenckebach). In "typical" Mobitz I — there is a regular atrial rhythm — and, the PR interval progressively lengthens until a beat is dropped. There follows a short pause — and then the next Wenckebach cycle begins again.

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• WHY was today's rhythm so challenging to interpret?

ANSWER:

There are numerous variations on the theme of Wenckebach blocks (See ECG Blog #251 for discussion — with an Audio Pearl on the "Footprints" of Wenckebach). PEARL #6 (above) — is to appreciate how today's rhythm represents a more subtle variation on the "usual" Wenckebach theme. Reasons why this rhythm may not have been recognized by the clinicians who initially saw the patient include:

• The pauses containing the dropped beats are not much longer than many of the other R-R intervals in this tracing. At first glance — today's rhythm looks to be almost regular.
• The 2 "groups" of beats in today's tracings (ie, beats #2-thru-6; and #7-9) do not "look" similar (whereas many Mobitz I tracings feature similar 2-beat or 3-beat groupings).
• With typical Mobitz I tracings — the greatest "increment" in PR interval (ie, the amount that the PR interval increases from one beat-to-the-next) — occurs between the 1st and 2nd beats within a group. This is not what we see in today's tracing — as the greatest increment in PR interval is seen when moving from BLUE arrow to GREEN arrow P waves.
• Finally — The BLACK arrow P waves before beats #5 and 6 are conducted with unusually long PR intervals (ie, 0.64 and 0.68 second, respectively). As a result — it may not seem likely that these BLACK arrow P waves can be conducting.

Final Thoughts on Today's ECG:

The rhythm in today's case represents an unusual example of 2nd-degree AV Block of the Mobitz I Type ( = AV Wenckebach). Whether the unexpected increase in PR interval between the 2nd and 3rd beat in each grouping is the result of dual AV Nodal pathways — or simply "variation-on-the-theme" is uncertain — and probably not important for this patient's clinical course. The "good news" — is that the overall ventricular response is not slow — so it is possible that this patient may not be symptomatic. Unfortunately — We were not provided with any clinical history.

• After interpreting the rhythm in today's case — we need to assess the remainder of the 12-lead ECG. Although we do not know the age of this patient — QRS amplitude appears to be markedly increased (ie, very deep S wave >25 mm in lead V2 — and tall R waves in the lateral chest leads) — so LVH is suspected.
• The negative T wave in lead III is not necessarily abnormal given predominant negativity of the QRS in this lead. There do not appear to be any acute ST-T wave changes.
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• Clinical Correlation will be essential for optimal management in today's case! In ECG Blog #307 — we explored the most important clinical conditions associated with 2nd-degree or 3rd-degree AV block (See Figure-6 in ECG Blog #307). Although acute inferior infarction is probably the most common cause of AV Wenckebach — there is no indication of this on today's ECG. There is however probable LVH — so full evaluation of this patient for an underlying cardiac cause of this AV conduction disturbance is in order.

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 we 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 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 the arrhythmia — usually 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 #2, 7 and 10 — each of which are preceded by the same normal PR interval).

Figure-9: I next focused on the 1st group of beats (ie, beats #2-thru-6). It seemed logical that the BLUE arrow P wave would be conducting beat #3 (with a slight increase in PR interval compared to the PR interval of beat #2) — to be followed by the GREEN arrow P wave conducting beat #4. Looking "ahead" — this leaves us with the remaining 3 RED arrow P waves in this 1st group — but with only 2 QRS complexes ( = beats #5,6).

Figure-10: I've connected the RED arrow P waves that occur within the T waves of beats #4 and 5 — to the only logical QRS complexes that might be conducted by them. This means that the RED arrow P wave that occurs just after the QRS of beat #6 must be non-conducted!

Figure-11: I then completed my proposed laddergram by connecting the remaining RED arrow P waves to the only logical QRS complexes that might be conducted by them. As deduced earlier — it is logical that the WHITE arrow P waves are not conducted — because they occur so early in the cycle (very soon after the previous QRS complex).

CONCLUSION: It should be apparent from this laddergram that there are 2 Wenckebach groupings — in which PR intervals progressively increase from beats #2-thru-6 (until the WHITE arrow P wave occurring just after beat #6 is blocked) — and a shorter Wenckebach grouping (beats #7-9), with increasing PR interval until the WHITE arrow P wave after beat #9 is blocked. Thus, the rhythm in today's case = 2nd-degree AV Block, Mobitz Type I (AV Wenckebach) with 6:5 and 4:3 AV conduction.

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Acknowledgment: My appreciation to Arkan KaDhim (from Baghdad, Iraq) 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. 
• ECG Blog #185 — Reviews my 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 #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.