Wednesday, March 10, 2021

ECG Blog #202 — Do Any P Waves Conduct?

The long lead II rhythm strip in Figure-1 was obtained from an older patient with syncope. The rhythm was diagnosed as probable complete AV Block. 

  • Do YOU agree with the diagnosis of complete (3rd-degree) AV Block?


Figure-1: Lead II rhythm strip, obtained from an older patient with syncope. Is this complete AV Block? (See text).



MY Approach to the Rhythm in Figure-1:

Unfortunately — the tracing is slanted, therefore slightly distorted. That said — we can still appropriately interpret this tracing. By the Ps, Qs & 3R Approach (See ECG Blog #185):

  • The QRS is narrow (at least in this single monitoring lead).
  • Taking into account the slanting, with slight distortion of the ECG paper — the ventricular rhythm overall looks to be regular with exception of beat #4 that occurs earlier-than-expected. 
  • P waves are present. That said — it is difficult to tell from Figure-1 if the atrial rhythm is or is not regular ... At this point I reached for my calipers.
  • NOTE: Before using my calipers — I thought it looked as if several (if not many) of the P waves seen in Figure-1 were not conducted. So, at the least — it seemed probable that some form of 2nd-degree AV Block was present.




NOTE: Some readers may prefer at this point to listen to the 5-minute ECG Audio PEARL before reading My Thoughts regarding the ECG in Figure-1. Feel free at any time to review to My Thoughts on this tracing (that appear below ECG MP-19).



Today’s ECG Media PEARL #19a (6:45 minutes Audio) — Why is this Not Complete AV Block? This recording suggests a few Quick-Things-To-Do that help to rule in or rule out Complete AV Block (P.S. I updated this Audio Pearl on 10/12/2021).



MY Approach to this Rhythm (Continued):

The benefit of using calipers becomes immediately apparent on review of Figure-2:

  • I began by setting my calipers to the P-P interval suggested by any consecutive P waves that I could be certain of (ie, One might use either the P waves seen before and after beat #4 — or — the P waves seen before and after beat #5). Doing so allowed me to find “on-time P waves” at all of the places highlighted by RED arrows.
  • I highlight with WHITE arrows in Figure-2 the 2 places in this rhythm strip where I did not see clear indication of an underlying P wave.

PEARL #1: Most of the time when there is AV block — the atrial rhythm will be regular (or at least fairly regular if sinus arrhythmia or ventriculophasic sinus arrhythmia is present). Therefore, even though we do not see clear indication of sinus P waves under the WHITE arrows — the chances are excellent that on-time P waves are hiding within the QRS of beats #2 and 7.

  • The subtle distortion at the onset of the QRS complex of beat #7 supports my presumption that there are regular P waves throughout this tracing (and the on-time occurrence of this subtle distortion almost certainly marks the beginning of a P wave that is mostly contained within the QRS of beat #7).
  • Unfortunately — all we have in today’s case is this single lead II rhythm strip. But additional monitoring at the bedside and/or looking at simultaneously-recorded leads on a 12-lead tracing are 2 ways to confirm that regular P waves are indeed present!


Figure-2: I’ve labeled atrial activity in Figure-1 with RED arrows highlighting P waves I am certain of (See text).



  • At this point in today’s case — HOW did YOU interpret this rhythm?


MY Thoughts Continued:

Based on the findings described above — we can diagnose the rhythm in Figure-2 as showing some form of 2nd-degree AV Block because: i) There are regular sinus P waves (so we are not dealing with PACs or sinus pauses in a patient with Sick Sinus Syndrome); and, ii) At least some of these P waves are not conducting. In addition — I would emphasize the following:

  • The QRS is narrow everywhere — so the site of the ventricular escape rhythm is within the conduction system (ie, either at the AV node or in the His).
  • PEARL #2: There is AV dissociation — since many P waves appear not to be conducting. But we can not establish the likely severity of AV block until we evaluate how many of these P waves have a reasonable chance to conduct, yet still fail to do so!
  • PEARL #3: The fact that beat #4 occurs much earlier-than-expected in Figure-2 — strongly suggests that this P wave is being conducted! Most of the time with AV block — the ventricular escape rhythm will be regular (or at least, fairly regular). The BEST clue I know when assessing a rhythm to determine if any conduction is present, is to look for beats that occur earlier-than-expected! 
  • NOTE: Unlike any other PR interval in this tracing — the PR interval before beat #4 (which measures ~0.19 second) is of a duration that could be expected to conduct. Do any of the other P waves on this tracing manifest a PR interval that gives them a reasonable chance to conduct?



The easiest way to convey my thinking is by drawing a laddergram (Figure-3). For clarity — I have labeled all P waves in this tracing.

  • I’ve previously reviewed how to read (and draw) laddergrams (See ECG Blog #191). As I noted in Pearl #3 — the much-earlier-than-expected occurrence of beat #4 strongly suggests that P wave “g” is being conducted! As a result — the degree of AV block in this tracing is not complete!
  • All other QRS complexes in this tracing appear to be junctional escape beats — because the R-R interval preceding each of these beats is constant (corresponding to an appropriate junctional escape rate of ~6 large boxes = ~50/minute) — and — none of the other P waves in this tracing have an adequate opportunity to conduct (ie, P waves b, d, f, i, k, m and o all occur too close to the QRS — and P waves a, c, e, h, j, l and all occur quite far away from the next QRS).
  • PEARL #4: Despite the fact that all P waves in this tracing except “g” do not conduct — there is no evidence to suggest “high-grade” AV block. This is because we never see 2 P waves in a row that should conduct, yet fail to do so! 


BOTTOM LINE: I would interpret this tracing as showing 2nd-degree AV Block of uncertain severity. There is evidence of 2:1 AV conduction (ie, every-other P wave that occurs near the middle of the R-R interval should conduct but doesn't) — but the 1 P wave that does conduct (ie, "g"), does so with a normal PR interval. So, it is very possible that the conduction ratio is no worse than 2:1.

  • This most probably is 2nd-degree AV Block of the Mobitz I (AV Wenckebach) Type — because the QRS complex is narrow, and Mobitz I is so much more common than Mobitz II.
  • As discussed in detail in ECG Blog #191 — AV dissociation is never a diagnosis, but instead always arises because of “something else” (ie, because of “usurpation”, “default”, or AV block). In today’ case — the reason for AV dissociation is 2nd-degree AV Block.
  • The patient in today's case may or may not need a permanent pacemaker. We simply can not answer this question based solely on this single short rhythm strip — in which the overall heart rate never drops below 50/minute, and the degree of AV block is not complete. Additional rhythm monitoring (and correlation with the patient's symptoms) — will be needed to answer this question. 


Figure-3: Proposed laddergram for the rhythm in Figure-2 (See text).



  • Acknowledgment: My appreciation to Václav Lejsek (from Prague, Czech Republicfor the case and this tracing.




Additional Relevant ECG Blog Posts to Today’s Case:

  • ECG Blog #185 — Use of a Systematic Approach to Rhythm Interpretation.
  • ECG Blog #191 — Is AV Block Complete? (Assessing AV Dissociation).
  • ECG Blog #188 — How to Read (and Draw) Laddergrams.

Other Posts on Assessing for AV Block:

— NOTE: There are even more cases relevant to assessment for AV Block on this Blog, but those below provide some “practice” for those in search of example cases.

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