ECG Blog #55 — Group Beating with Acute MI

The Lead II rhythm strip shown in Figure 1 was obtained from a 50-year-old woman with new-onset chest pain.

• Can you explain the 3 pauses seen (between beats #5-6; 8-9; and 11-12)?
• What might a 12-lead ECG show?

Figure 1: Lead II rhythm strip obtained from a woman with chest pain. What is the rhythm? 

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INTERPRETATION: The best way to approach the rhythm strip in Figure 1 is to step back a short distance from the tracing. Doing so should allow appreciation of the phenomenon of group beating. That is – there is a pattern to the irregularity seen in this lead II rhythm strip:

• Each “group” of beats (ie, beats #1-thru-5; 6,7,8; 9,10,11; and 12,13,14) – is separated by a short “pause”.
• Each of the short pauses (ie, the distance between beats #5-6; 8-9; and 11-12) – is approximately the same duration.
• Each of the pauses terminates with a QRS preceded by a fixed (albeit slightly prolonged) PR interval.
• The pause that contains the dropped beat is less than twice the shortest R‑R interval.
• Beyond-the-Core: The R‑R interval following each pause is longer than other R‑R intervals in each group (ie, the R‑R interval between beats #6-7, 9-10, and 12-13 – is longer than the R-R interval between beats #7-8; 10-11, and 13-14).

The above 5 bullets above are characteristic “footprints” of Wenckebach Conduction. Specifically – the above conduction disturbance manifest in Figure 1 is 2nd degree AV Block, Mobitz Type I (= AV Wenckebach). Additional clues in support of this diagnosis are the following:

• There appears to be ST segment elevation in this lead II rhythm strip.
• The patient has new-onset chest pain.
• The conducting beats (ie, beats #6,9, and 12) manifest 1st degree AV block.

Armed with the knowledge that AV Wenckebach is likely – we pursue definitive diagnosis by determining IF a regular atrial rhythm is present. Use of calipers greatly facilitates the process. Starting with one of the conducting P waves (say, beat #6) – One can set calipers to the P‑P interval between beat #6 and the obviously-hiding P wave that provides an extra peak in the ST segment of beat #5. Doing so allows us to walk out regularly-occurring P waves throughout the tracing (short vertical lines in the Atrial Tier of Figure 2).

• The remaining 2 tiers in Figure 2 (AV node and Ventricles) – clearly demonstrate documentation of AV Wenckebach with progressive lengthening of the PR interval until a beat is dropped within each group. The sequence of progressive PR lengthening then begins again as new “groups” begin with beats #6, 9, and 12.
• For a Review on the Basics of Laddergrams – See ECG Blog #188.

Figure 2: Laddergram of the Lead II rhythm strip shown in Figure 1. 

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COMMENT on MOBITZ I – 2nd DEGREE AV BLOCK:

     Awareness of the “footprints” of Wenckebach greatly facilitates recognition of this conduction disturbance. This is especially true when the clinical setting predisposes to development of 2nd degree AV block, Mobitz Type I (= AV Wenckebach). Thus, we specifically look for Mobitz I in the setting of acute inferior infarction, especially when we see groups of beats followed by pauses in the rhythm. Many of the “footprints” are present in Figure 2:

• Group beating.
• Progressive lengthening of the PR interval within each group until a beat is dropped.
• Pause containing the dropped beat is less than twice the shortest R-R interval (unlike what may occur when there are sinus pauses or blocked PACs).
• Progressive decrease in the R-R interval within groups (because the greatest increment in PR interval is usually between the first and second beats in any one group).
• Conducted beats manifest 1st degree AV block (since with inferior infarction both 1st degree block and Mobitz I are often the result of AV nodal ischemia).

Not all of the above “footprints” will always be present – and they are not all needed to make the diagnosis of Mobitz I. But awareness of the relationships illustrated in Figure 2 should make recognition easier in cases like this one when the atrial rate is relatively rapid and P waves are partially hidden within ST segments of preceding beats.

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ACCOMPANYING 12-LEAD ECG:

     The 12-lead ECG that accompanies the lead II rhythm strip shown in Figure 1 is shown below (in Figure 3).

• How do you interpret this 12-lead ECG?
• Does this 12-lead tracing provide a reason for the 2nd degree AV block, Mobitz Type I conduction disturbance seen in Figure 1?
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Figure 3: 12-lead ECG from the 50-year-old woman with chest pain whose lead II rhythm strip was shown in Figure 1.

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INTERPRETATION OF ACCOMPANYING 12-LEAD ECG:

     As already noted – the rhythm in Figure 3 is 2nd degree AV Block, Mobitz Type I (AV Wenckebach). The tracing shows evidence of acute inferior STEMI with probable posterior involvement. Important ECG findings include the following:

• ST elevation in each of the 3 inferior leads (leads II,III,aVF).
• Reciprocal ST depression in leads I and aVL.
• Positive “mirror test” ST depression in leads V1,V2. This suggests either: i) reciprocal ST depression; ii) posterior wall involvement; iii) concomitant ischemia in the LAD (left anterior descending) coronary artery distribution; or iv) some combination of i, ii, iii.
• ST elevation in lead III > lead II – and – marked reciprocal ST depression in lead aVL (more than in lead I) – both of which suggest a proximal RCA (Right Coronary Artery) occlusion.
• Given the usual vascular supply of the RCA (that most often supplies the AV node and the posterior wall of the left ventricle) – acute RCA occlusion may be commonly associated with both AV Wenckebach and posterior as well as inferior wall involvement.
• FINAL Clinical ECG Point: Note how ST segment and T wave morphology seems to vary from beat-to-beat. Much of this is simply the result of the regular atrial rhythm that “marches through” the Mobitz I AV conduction disturbance. Appreciation of the distorting effect of regular atrial activity explains the difficulty in definitive assessment of ST-T wave morphology.

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• ACKNOWLEDGMENT: My appreciation goes to Ankur Jain, MD for allowing me to use this ECG and this clinical case.

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• See also ECG Blogs #19, 20, 21 – and Section 20.0 in our ACLS-2013-ePub to download PDF Review on the AV Blocks. Review of Laddergrams begins in Section 20.25 in this pdf.
• Link to Section 10.0 for pdf download on the ECG Diagnosis of Acute MI (from our ECG-2014-ePub).