ECG Interpretation Review #66 (2nd Degree AV Block – PACs – Blocked PACs – Mobitz I – AV Wenckebach – Group Beating)

Interpret the two sequential tracings in Figure 1. The patient is hemodynamically stable.

• Note group beating in Tracing A. Is this due to AV block? If so — What type of AV block?
• HINT: Interpretation of Tracing A becomes easier if you first interpret Tracing B obtained from the same patient a lit bit later. This is because the same phenomenon responsible for the abnormal findings is operative in both tracings.

Figure-1: Sequential tracings in a patient who initially manifests group beating (Tracing A) — and later manifests some irregularity with a brief pause following beat #6 (Tracing B). What is the cause of the abnormal findings seen in both tracings? 

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Characteristics of Mobitz I 2nd-Degree AV Block (AV Wenckebach)

    Recognition of the phenomenon of “group beating” should always suggest the possibility of a Wenckebach conduction disorder. That said — this case presents a wonderful example of why group beating does not always mean that a Wenckebach block is present.

• Second-degree AV block of the Mobitz I type (also known as AV Wenckebach) — is by far the most common form of 2nd-degree AV block. Mobitz I is characterized by: i) Progressive lengthening of the PR interval until a beat is dropped; ii) Group beating; iii) A regular (or at least fairly regular) atrial rate; and iv) The pause that contains the dropped beat is less than twice the shortest R-R interval. These features collectively (among a few others) are known as “the Footprints of Wenckebach”. They are all present in Figure 2.

Figure-2: Mobitz I 2nd degree AV block with all of “the footprints” mentioned above. There are two “groups” of beats (beats #1,2,3; and #4,5). The P-P interval is regular (in this case at a rate of 100/minute). Within groups — the PR interval progressively lengthens until a beat is dropped (the P wave following beats #3 and #5 is nonconducted). The pause containing the dropped beat is less than twice the shortest R‑R interval (that is — the pause between beats #3-4 is less than twice the R-R interval between beats #2-3).

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Beyond-the-Core: The reason we specify “AV Wenckebach” rather than just, “Wenckebach” for the conduction disorder shown in Figure 2 — is that there are many types of Wenckebach conduction disorders. Second-degree AV block of the Mobitz I type is just one of these (albeit the most common form seen).

• One may also see Wenckebach-conduction in the form of retrograde block out of the AV node; Ventricular tachycardia with retrograde Wenckebach; SA node Wenckebach; and atrial fibrillation or flutter with Wenckebach conduction, to name just a few.
• Among the above footprints most helpful in identifying some of these other forms of Wenckebach conduction are “group beating” and pause containing the dropped beat being less than twice the shortest R-R interval.

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Why Despite Group Beating – Figure 1 is Not AV Wenckebach

     Before addressing specific interpretation of the rhythm in Figure 1 — We list reasons why sequential Tracings A and B are not the result of Mobitz I 2nd degree AV block. This is easiest to do by adding red and blue arrows to highlight P waves in these tracings (Figure 3). Note the following:

• The PR interval does not progressively increase within the groups of beats (from beat #1-to-2; from beats #3-to-4-to-5; from beats #6-to-7-to-8).
• The atrial rhythm in Tracing A is clearly not regular.

Figure-3: Addition of arrows to Figure 1 to highlight atrial activity (See text).

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Interpretation of the Sequential Tracings in Figure 3

     The underlying rhythm for both tracings in Figure 3 is sinus arrhythmia. In Tracing A — a short pause follows each group of sinus-conducted beats. There is subtle-but-real peaking of the T wave of beats #2, 5 and #8 in Tracing A (blue arrows). That this peaking is real and not due to artifact — is determined by the fact that the T wave of all other beats in Tracing A is smoother and of smaller amplitude. This peaking of T waves must therefore reflect blocked PACs.

• A similar phenomenon is operative in Tracing B — and much easier to see. Note that beat #4 in Tracing B occurs early. The T wave just preceding beat #4 in Tracing B is again peaked (blue arrow) — strongly suggesting that a PAC is hidden within this T wave. The reason the QRS complex of beat #4 looks slightly different — is that beat #4 is a PAC conducted with aberration.
• A short pause follows beat #6 in Tracing B. The commonest cause of a pause is a blocked PAC. Once again — “telltale” notching of the T wave of beat #6 in Tracing B (blue arrow) indicates this is due to another blocked PAC.

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Summary Take-Home Points:

     Awareness and recognition of the phenomenon of “group beating” is an important concept that will often help streamline your interpretation. Some type of Wenckebach conduction will often be the cause. However — there are other reasons for group beating that need to be kept in mind. Remember:

• The commonest cause of a pause is a blocked PAC (not AV block). Although often quite subtle (as they are in Tracing A) — blocked PACs will be found IF you look for them.
• Routine use of a Systematic Approach to rhythm interpretation is essential for accurate interpretation. We use this memory aid: “Watch your Ps & Qs — and the 3Rs”. (See ECG Blog #93 for brief Review.) Using this systematic approach should make it immediately obvious that AV Wenckebach is not the cause of group beating in this example — because the atrial rhythm in Tracing A is definitely not regular!

     In Summary — the 2 tracings in Figure 3 reflect the same phenomenon. There is underlying sinus arrhythmia that is punctuated by multiple PACs. Most of these PACs are blocked.

• The PAC that notches the T wave of beat #3 in Tracing B is conducted with aberration.
• The T wave of beat #6 in Tracing B is unmistakably peaked.
• Recognition that the “theme” for Tracing B is the occurrence of PACs alerts us to carefully focus on T wave morphology at the beginning of each pause in Tracing A. Doing so facilitates identifying the subtle-but-real deformity brought about by hidden PACs within the T waves of beats #2, 5 and 8 in Tracing A.

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- For more information – GO TO:

• See Section 20.0 (from ACLS-2013-ePub) – on the Basics of AV BLOCK – 

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