Wednesday, February 17, 2021

Blog #195 (ECG-MP-12): Are P Waves Related to the QRS?

The long lead II rhythm strip shown in Figure-1 was obtained from a previously healthy 27-year-old man who presented with dyspnea.



  • What is going on with the rhythm in Figure-1?
  • Will this patient need a pacemaker?
  • Extra CREDIT: There is a special name for the phenomenon that we see in this rhythm. Do you know what it is?


Figure-1: Lead II rhythm strip, obtained from a 27-year-old man with dyspnea (See text).




MY Approach to the Rhythm in Figure-1:

I find 2 simple steps tremendously facilitate both explaining as well as interpreting complex arrhythmias. These 2 simple steps are: i) Numbering the beats; and, ii) Highlighting P waves with ARROWS (Figure-2):

  • Use calipers! As I continually emphasize — the EASIEST way to instantly become better (and faster) at analyzing complex arrhythmias is to use calipers. P waves are obvious in front of some but not all of the QRS complexes in Figure-2. By using calipers — I instantly remove any doubt that the tiny (partial) deflections I see deforming the onset of QRS complexes #2 and 6 are indeed P waves that are partially hidden by near-simultaneous occurrence of these beats.
  • Isn't it interesting how the PR interval continuously changes from one-beat-to-the-next in Figure-2?


Continuing systematically to assess the rhythm in Figure-2 by the Ps, Qs & 3R Approach (See ECG Blog #185)

  • The QRS is narrow (at least in this single monitoring lead).
  • The ventricular rhythm looks fairly (but perhaps not completely?) regular at a rate between ~50-55/minute. 
  • And, as mentioned — the PR interval continuously changes, so there is no fixed relationship between P waves and neighboring QRS complexes.


Figure-2: I’ve labeled the rhythm in Figure-1 by highlighting P waves (RED arrows) and numbering the beats (See text).


MY Approach to this Rhythm (Continued):

I wondered WHY the PR interval was continually changing in Figure-2?

  • Time to use calipers again! I wondered IF the P-P interval in this rhythm remained constant — and also IF the R-R interval remained constant?
  • For clarity — I show my caliper measurements in Figure-3 for both the P-P interval (RED numbers) and the R-R interval (BLUE numbers). Note that neither the atrial nor ventricular rates remain completely constant in Figure-3. Instead, there is slight variation in both intervals that differ from each other in the way they are changing.


Figure-3: I’ve measured the P-P and R-R intervals in Figure-2 (See text).


Putting It All Together:

As per the Ps, Qs & 3R Approach — the rhythm in today’s case is fairly (but-not-completely) regular, within a rate range for the ventricular rhythm of between ~50-55/minute. The QRS complex is definitely narrow in this single lead II rhythm strip. P waves are present — but they are not related to neighboring QRS complexes (because the PR interval continually changes throughout the tracing).

  • The fact that the PR interval in Figure-2 is constantly changing — tells us that many (if not all) of the P waves are not conducting. Clearly, the PR interval before beats #1, 2, 4; 6, 7 and 8 is too short to conduct. I thought the PR interval before beat #5 was slightly shorter than the PR interval before beat #3. Whether either of these P waves are conducting is debatable — and probably not possible to determine with certainty from this single rhythm strip. Regardless — the “Take-Home” message is that there is AV dissociation for either part or all of the rhythm shown in Figure-2.
  • Because the QRS complex in Figure-2 is narrow and not the result of sinus-conducted beats (with possible exception of beats #3 and 5) — most QRS complexes in this tracing are arising from the AV node (ie, they are junctional beats).


NOTE: Although we have established the presence of AV dissociation for either a part (or all) of the rhythm in Figure-2 — AV Dissociation is never a “diagnosis”. Instead — it is merely a description of the fact that at least temporarily, regular P waves are not related to neighboring QRS complexes.

  • As discussed in detail in ECG Blog #192 — there are Causes of ADissociation to consider. These are: i) 2nd- or 3rd-degree AV Block (in which one or more P waves that should conduct do not conduct)ii) AV dissociation by Usurpation” (in which an accelerated junctional rhythm takes over the pacemaking function)and/or iii) AV dissociation by Default” (in which slowing = “default” of the SA nodal pacemaker allows a junctional escape pacemaker to emerge).
  • There is no evidence of any AV block in today’s rhythm. This is because none of the P waves (RED arrows) that fail to conduct — had a chance to conduct, since the PR interval was too short. This is not to say that there might not be some degree of AV block — but only to say that the rhythm in Figure-2 is too short to answer this question (because we never see P waves occurring at points in the cardiac cycle when they should be expected to conduct, yet fail to do so).
  • The cause of AV dissociation in Figure-2 is also not “usurpation”. Looking at the R-R intervals that I measured (BLUE numbers in Figure-3, that range from 5.7-to-6.1 large boxes in duration) — this corresponds a ventricular rate between ~49-to-53/minute — which falls well within the usual junctional escape rate of 40-60/minute in adults. Thus, there is no “usurpation” — because the junctional escape rhythm is definitely not accelerated!


This leaves us with “default slowing of the sinus pacemaker) as the cause of the AV dissociation that we see in today’s rhythm. It is because the SA node slows — that a junctional escape rhythm is able to emerge.

  • That said — there is a difference between today’s rhythm, and the much more common variation of AV dissociation by “default” that we saw in ECG Blog #192. Most of the time, escape rhythms are surprisingly regular — as was the junctional escape rhythm in Blog #192.
  • Instead, in today’s case — the P-P and R-R intervals that I measured in Figure-3 (RED and BLUE numbers in this Figurecontinually vary. Yet despite this — a 1-P wave to 1-QRS complex ratio persists, albeit with P waves moving in-and-out of the QRS.


This phenomenon of nearly equal sinus and junctional escape rates that continue to vary slightly, yet somehow maintain a close (yet slightly variable) proximity of P wave to QRS over time is known as isorhythmic Adissociation

  • As I discuss in today’s ECG Media PEARL #12 (6:40 minutes Audio) — this unusual form of AV dissociation is defined by its name. Both pacemakers are “isorhythmic” (ie, nearly equal rates for sinus P waves and the junctional rhythm) — and — there is AV “dissociation” (ie, the atria and AV node beat independently from each other — and there is no conduction of sinus impulses as long as the rhythm lasts).
  • NOTE: The colorful term, “accrochage” (from the French word for “hanging on” or “hooking” onto something— has often been used to refer to the uncanny pursuit of P waves to neighboring QRS complexes, despite slight variation in the rates of these P wave and QRS rhythms. The less colorful term, “synchronization” is then preferred when the phenomenon of isorhythmic AV dissociation is longer lasting.

CLINICALLY — The significance of AV dissociation (be it isorhythmic or otherwise) depends on the clinical setting in which it occurs. 

  • As emphasized earlier — there is no evidence of any AV block in today’s tracing. Assuming this lack of evidence for AV block continued with additional monitoring — the clinical course of this 27-year old man’s isorhythmic AV dissociation is most likely to be benign.
  • NOTE: AV dissociation by “default” is not uncommon among otherwise healthy, athletic individuals — because predisposing sinus bradycardia is common with increased resting vagal tone. Having such individuals exercise briefly is usually enough to sufficiently increase the sinus rate — which when the rhythm is benign, will result in sinus P waves regaining control of the rhythm (and overriding the resting junctional escape rate).
  • I suspect that the patient’s dyspnea in today’s case is not related to his cardiac rhythm. There is no evidence to suggest that a pacemaker will be needed for this patient.

Today’s ECG Media PEARL #12 (6:40 minutes Audio) — reviews the entity known as isorhythmic AV Dissociation.



Proposed Laddergrams for Today’s Case:

I conclude this case by proposing 2 potential laddergrams for today’s rhythm.

  • I suspect the laddergram I’ve drawn in Figure-4 is more likely — because most of the time with true isorhythmic AV dissociation, for as long as this phenomenon lasts — sinus and junctional pacemakers continue to discharge within the Absolute Refractory Period of the other, resulting in non-conduction of all sinus P waves.
  • That said, because we have not seen a prior tracing from this patient in which he was normal sinus rhythm — we have no idea of what his “normal PR interval” is when sinus beats are conducted. It is therefore impossible to know if the PR interval of 0.14 second that is seen before beat #3 is (or is not) sinus-conducted (Figure-5).
  • In Theory — true isorhythmic AV dissociation in which there is no conduction of any sinus impulses, will continue until a significant change in either the sinus or junctional rate occurs.
  • Practically Speaking — it does not matter if beat #3 is or is not a sinus-conducted beat. Regardless — the underlying mechanism of today’s rhythm is sinus bradycardia that results in a period of isorhythmic AV dissociation with a junctional escape rhythm — and which in an otherwise healthy young adult, is almost certain to be benign.


Figure-4: Proposed Laddergram #1 — which I think is the more likely mechanism, since “capture” beats are generally not seen with true isorhythmic AV dissociation until a significant change in rate of either the atrial or ventricular rhythm occurs (See text).


Figure-5: Proposed Laddergram #2 — in which the QRS complex preceded by the longest PR interval (ie, beat #3) might be a sinus-conducted beat. It’s impossible to rule this in or out, however — without knowing what this patient’s “normal PR interval” is during a normal sinus rhythm (See text).


Acknowledgment: My appreciation to Feroz Haroon (from Srinagar, Kashmir, India) for the case and this tracing.



  1. Why there is variation in PP intervals also RR intervals ?

    1. @ Dola — Excellent question — and one that I do not find a truly good answer for in the literature. Numerous sources cite the near identical sinus and AV nodal heart rates, and the “back-and-forth” relationship between P waves and neighboring QRS complexes — but I’ve yet to see a convincing mechanism. My understanding is that both sinus and AV nodal rates are very close but INDEPENDENT of each other — and somehow “compete” for control of the rhythm — with both subect to slightly differing influences. Autonomic reflexes and vagal tone seem to be involved — and my hunch is that slight variation of these influences contributes to the slight variation we see in rate. Wish I saw more clear-cut evidence of what is truly going on …