ECG Blog #186 (ECG MP-4) — AV Blocks - Mobitz I

One of the most problematic areas in arrhythmia interpretation is assessment of the AV Blocks. This doesn't have to be so difficult ... I review the basics in today's ECG Blog #186. You'll get to TEST your skill with a challenging arrhythmia case in the very next ECG Blog (ECG Blog #187 — coming a few days after this one). 

• My ECG Media Pearl (#4) — that appears below, offers brief overview of the subject (in ~ 5 minutes).
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• In ECG Blog #236 — my 15-minute ECG Video Pearl #52 reviews how to recognize the 2nd-Degree AV Blocks (including "high-grade" AV block).
• Section 2F (6 pages = the "short" Answer) from my ECG-2014 Pocket Brain book provides quick written review of the AV Blocks (This is a free download).
• Section 20 (54 pages = the "long" Answer) from my ACLS-2013-Arrhythmias Expanded Version provides detailed discussion of WHAT the AV Blocks are — and what they are not! (This is a free download).

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ECG Media PEARL #4 (4:30 minutes Audio): — takes a brief look at the AV Blocks — and focuses on WHEN to suspect Mobitz I.

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NOTE: The tracing below highlights many of the KEY concepts to appreciate in honing your skill for interpreting the AV Blocks. What is going on here?

Figure-1: This ECG was sent to me without the benefit of clinical information. No 12-lead ECG was available from the patient. That said — Consider the following: 

• The 1st thing to note is Group Beating! — and this should immediately prompt you to consider the possibility of Mobitz I (ie, AV Wenckebach). 
• I next looked for definite P waves — which are visible prior to the 1st beat in each of the groupings. The fact that the PR interval preceding beats #1, 3, 6, 8, 10 and 13 are all the same tells you there is conduction — and should prompt you to look to see if an underlying regular sinus rhythm might be present. 
• The BEST (and fastest) way to determine IF a regular P wave rhythm is present is with CALIPERS, setting your calipers to an interval defined by definite P waves and subsequent notches in the early part of the ST segment that you suspect might be P waves. Doing so should allow you to walk out (and therefore confirm!) a regular P wave rhythm. 
• It should then be EASY to establish progressive PR interval lengthening until a beat is dropped.
• NOTE: Although the above clinical approach to this rhythm might seem complex — the fact that you instantly recognized the GROUP beating was the KEY clue that facilitated each of the following step I describe. (Full details of this case, including explanatory laddergram are discussed in ECG Blog #164).