What is this Rhythm?

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My Comment by KEN GRAUER, MD (1/26/2023):

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While reading ECGs — Dr. Smith came across the intriguing tracing shown in Figure-1. No history was available. However, the rhythm is indeed challenging — and illustrates a number of important principles in rhythm interpretation of interest to all emergency care providers.

• How would YOU interpret the ECG in Figure-1? 
• Can you come up with a definitive rhythm diagnosis?

Figure-1: 12-lead ECG and simultaneously-recorded 3-lead rhythm for today's case. (To improve visualization — I've digitized the original ECG using PMcardio).

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REVIEW: MY Approach to Rhythm Interpretation

• As always — I favor beginning my interpretation of 12-lead ECGs with a look at the cardiac rhythm. My systematic approach to rhythm interpretation entails assessment of the 5 KEY Parameters, which are easily recalled by use of the memory aid, "Watch your Ps, Qs and 3Rs". The questions to ask when assessing these 5 Parameters are:
• Are there P waves (or other sign of atrial activity)?
• Is the QRS wide?
• What is the Rate? (looking both at the atrial and ventricular rates IF these are different).
• Is the rhythm Regular? (and if not — Is the rhythm “irregularly irregular”, as in AFib — or is there a pattern of “regular" irregularity in the form of group beating?).
• If P waves are present — Are P waves Related to neighboring QRS complexes?
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• NOTE: It does not matter in what sequence you assess the above 5 Parameters — as long as you always assess all 5 of them. I generally look at whichever of the Parameters are easiest to recognize.
• CAUTION: In my experience over decades — Failure to systematically assess each of the above 5 Parameters will result in even the most “experienced” of clinicians missing the diagnosis of a number of non-sinus rhythms.
• To EMPHASIZE: Using the Ps, Qs, 3R system does not slow you down. On the contrary — it organizes my thinking, and allows me within seconds (!) to diagnose almost any arrhythmia.

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MY Approach to the Rhythm in Figure-1:

Using the Ps, Qs & 3R Approach:

• The QRS is narrow in all 12 leads of Figure-1 (So the rhythm is supraventricular!).
• The rhythm is not completely Regular. That said — Parts of the rhythm are regular (ie, Except for the 2 pauses between beats #4-5 and 10-11 — the R-R intervals between all other beats is essentially the same!).
• P waves are present!

PEARL #1: Overall — the best lead to look for P waves in, is lead II. In fact — sinus rhythm is defined by the presence of an upright P wave with fixed PR interval in lead II. 

• Remember that the 2nd-best lead to look for P waves in — is lead V1. 
• KEY Point: There are times when P waves will not be well seen in lead II — and may only be seen in lead V1. Such is the case in today’s tracing. We barely see P waves in the long lead II rhythm strip (and then, only before the 1st QRS complex that occurs at the end of each pause). However — regularly-occurring P waves at a Rate of ~80/minute are clearly seen throughout the entire long lead V1 rhythm strip (ie, RED arrows in Figure-2).

PEARL #2: The KEY for interpretation of complex AV block tracings — will often reside with the 5th Parameter = Whether or not P waves are Related to neighboring QRS complexes. 

• I look to see IF there are any PR intervals that repeat. The best place to look for this — is in front of the QRS complex that terminates a pause in the rhythm. In Figure-2 — Isn’t the PR interval before beats #5 and 11 equal? The fact that the same PR interval repeats before 2 or more beats is unlikely to be due to chance — and suggests that there probably is at least some conduction!

Figure-2: Regularly-occurring P waves are clearly seen in the long lead V1 rhythm strip (RED arrows).

Putting together what we’ve derived thus far:

• The rhythm in Figure-2 is supraventricular (narrow QRS). 
• Regular P waves at a rate of ~80/minute occur throughout the long lead V1 rhythm strip (RED arrows).
• Although the rhythm in Figure-2 is not completely regular — there are groups of beats with a constant R-R interval — that are separated by pauses of almost equal length (ie, The R-R interval between beats #4-5 and between #10-11 is virtually the same!).
• The PR intervals before the 2 beats that terminate the short pauses are equal (ie, the PR intervals before beats #5 and 11 in Figure-3). This tells us that at least 2 beats in this tracing are being conducted to the ventricles, albeit with a markedly prolonged PR interval ( = 0.54 second).

Figure-3: PR interval measurements for the beats just before the pause — and just after the pause.

PEARL #3: Our above description thus far of the 5 Parameters — should immediately suggest that of some form of AV Wenckebach (ie, 2nd-degree AV block of the Mobitz I type) is present in Figure-3. This is because:

• The QRS is narrow (The QRS is usually narrow with Mobitz I. The QRS is almost always wide with Mobitz II).
• There are regular P waves, as highlighted by the RED arrows in Figure-3 (The finding of a regular or at least almost regular atrial rhythm makes other causes of group beating such as PACs and sinus pauses far less likely).
• There is group beating — in which the R-R intervals of the 2 short pauses are approximately equal in duration (I always immediately suspect Wenckebach if the QRS is narrow and P waves are regular and there is group beating!).
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• The PR intervals before beats #5 and 11 are equal — which tells us that these beats are conducting to the ventricles. In contrast — the P waves that occur just after the QRS of beats #4 and 10 are not conducted — which in the setting of a regular atrial rhythm defines this tracing as some form of 2nd-Degree AV Block.
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• PEARL #4: Statistically — Mobitz I 2nd-degree AV block is much more common than Mobitz II (ie, In my experience — at least 90-95% of all 2nd-degree AV blocks I have seen over the years are Mobitz I. Mobitz II is distinctly uncommon).
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• PEARL #5: The finding of 1st-degree AV block (especially given the markedly prolonged PR interval in today's case) — further increases statistical likelihood of Mobitz I (There commonly occurs a progression from 1st-degree AV block — to Mobitz I 2nd-degree block).

PEARL #6: We are used to recognizing Mobitz I, 2nd-degree AV block by obvious progressive lengthening of the PR interval until a beat is dropped. The reason Mobitz I is difficult to recognize in today's tracing — is that we are presented with a long Wenckebach cycle, in which there is only minimal increase in the PR interval increment from one beat-to-the-next.

• In such cases — the EASY way to immediately recognize that the PR interval is in fact increasing — is to measure the PR interval just before the pause (ie, the PR intervals before beats #4 and 10 in Figure-3) — and to compare this to the PR interval before the first beat that ends each pause (ie, the PR intervals before beats #5 and 11 in Figure-3). Doing so confirms that the PR interval is ever-so-slowly increasing until a P wave is non-conducted.

Laddergram Illustration:

The laddergram in Figure-4 — illustrates the mechanism of today's rhythm. I've carefully measured each PR interval in milliseconds.

• NOTE: Many patients "don't read the textbook". There simply is not always a measurable beat-to-beat increase in the PR interval with all cases of Mobitz I, especially when there is a long Wenckebach cycle (as is seen from beat #5-to-#10 in Figure-4).
• That said — there should be no doubt that the PR interval before the first beat in this grouping ( = 540 msec. for the PR interval before beat #5) — is less than the last PR interval in this group ( = 640 msec. before beat #10).
• The next on-time P wave (ie, the YELLOW arrow that occurs just after beat #10) — is non-conducted. There follows a short pause (between beats #10-to-11) — after which the PR interval again shortens (to 540 msec. before beat #11) — as the next long Wenckebach cycle begins. This sequence of events confirms the diagnosis of a long Wenckebach cycle.

PEARL #7: We often think of 1st-degree AV block as a benign rhythm. This is because most patients with an isolated 1st-degree AV block remain asymptomatic without need for intervention. This is especially true when the severity of 1st-degree AV block is not great (ie, a PR interval less than 0.30 second).

• Once the PR interval extends much beyond 0.30 second — the delay in ventricular contraction that occurs may result in the atria contracting against closed AV valves, with reduction in cardiac output. This may lead to a series of symptoms similar to “pacemaker syndrome” (ie, dizziness, fatigue, light-headedness, presyncope/syncope, dyspnea and/or chest pain).
• On occasion — implantation of a permanent pacemaker may be needed in a patient with a marked 1st-degree AV block (ie, PR interval significantly greater than 0.30 second) — IF the patient is symptomatic as a direct result of PR interval prolongation (For an example of this — See My Comment at the bottom of the page in the May 24, 2020 post in Dr. Smith's ECG Blog). 
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• Mobitz I, 2nd-degree AV block is also often a relatively benign rhythm — in that many patients with Mobitz I will not need a pacemaker. That said — the fact that the shortest PR interval in today's case for sinus-conducted beats is 0.54 msec. — suggests there is significant conduction system disease with high risk of symptoms similar to "pacemaker syndrome" (as described above for patients with a markedly prolonged 1st-degree block).

Final POINT in Today's CASE: To emphasize — that although the PR interval increment from one beat-to-the-next in today's rhythm is small (and not-at-all obvious) — Attention to the ECG findings highlighted above in Pearls #3-thru-6 allowed me to know that the rhythm was Mobitz I in less than 10 seconds.

Figure-4: Laddergram illustration of today's rhythm. The YELLOW arrows are on-time non-conducted P waves in each of the long Wenckebach cycles (See text).