What is the Rhythm?

Presentation by KEN GRAUER, MD:
The Case: A previously healthy young man presented to the ED for shortness of breath and chest pressure that occurred ~3 hours earlier, when he suddenly felt his heart “skip a beat”, and then begin “racing”. He felt “lightheaded” (presyncopal) during the episode — with the “strong sensation of his heart beating”. He did not feel better until ~45 minutes later. Similar episodes had occurred over the past month — but none lasted as long.  Of note, the patient is an active athlete. There is a family history of a “junctional or other abnormal rhythm”.

Figure-1 shows his initial ECG that was obtained in the ED.
  • Use of calipers is strongly advised for interpreting the rhythm!
Figure-1: The initial ECG in this case (See text). 

  • What is the rhythm in Figure-1?
  • Why is this not, strictly speaking, “isorhythmic” AV dissociation?
  • Might the family history of an abnormal rhythm have anything to do with this case?
  • Clinically — What would you do for this patient?


PEARL #1: The easiest way to instantly enhance your arrhythmia interpretation skills is by using CALIPERS. The cardiologist who does not regularly use calipers for interpretation of complex arrhythmias — is a cardiologist who will miss the diagnosis on more than a few occasions.
  • Obviously, not all arrhythmias are amenable to use of calipers. You’ll often know the etiology of a particular rhythm without the need for calipers. And, in an acute emergency situation — there is often no time to stop and pull out calipers until you’ve taken care of the patient. That said, calipers instantly make you smarter. They excel for assessment of AV blocks and AV dissociation — they allow instant determination if a rhythm is or is not regular — and, calipers are invaluable for deducing whether or not various beats are being conducted.
  • Using calipers will NOT slow you down. On the contrary — with a little bit of practice, you’ll find they dramatically speed up the interpretation of many complex arrhythmias — because you can instantly measure and compare intervals.
PEARL #2: When discussing a complex rhythm — Numbering the beats is of invaluable assistance for clarifying which beat is being referred to (Figure-2).
Figure-2: We have numbered the beats in the long lead II rhythm strip at the bottom of the tracing (See text).
Initial Assessment of the Rhythm: When confronted with a complex arrhythmia — I always like to begin with WHAT I KNOW to be true.
  • I know that the PR interval in front of beat #6 is too short to conduct. The PR interval in front of beat #2 is also too short to conduct. This suggests that there is at least transient AV dissociation — which simply means that there are sinus P waves present that are not related to neighboring QRS complexes.
  • The longest PR interval in Figure-2 appears before beats #8 and 9. The PR interval preceding these 2 beats is equal and normal — and the P wave in this lead II is upright. This suggests that beats #8 and 9 are most probably sinus-conducted.
  • There seems to be indication of LOTS of P waves on this long lead II rhythm strip. PEARL #3: Clarification of a complex rhythm is often much EASIER to attain IF you are able to label underlying atrial activity. We do so in Figure-3.
Figure-3: RED arrows indicate what we believe is the underlying atrial rhythm (See text). 
Assessment of Figure3: Although some of the P waves highlighted by RED arrows in Figure-3 are partially hidden by neighboring QRS complexes — overall P wave morphology appears to be the same throughout this tracing. Therefore, there is an underlying sinus mechanism.
  • BUT — the P-P interval of these RED arrows varies throughout the tracing! This tells us that the underlying rhythm is a fairly marked sinus arrhythmia. It should be emphasized that of itself — sinus arrhythmia is not an abnormal rhythm in an otherwise healthy young adult.
  • All 10 QRS complexes in this tracing are narrow. These QRS complexes all look similar — although there are slight differences in QRS morphology for some of the beats. But the fact that there are some sinus-conducted beats — as well as many other beats with PR intervals that are definitely too short to conduct — tells us that there are junctional escape beats (ie, beats #3, 4, 5 and 6 at least — are all junctional beats).
Putting Together What WKnow Thus Far: The underlying rhythm is sinus bradycardia with marked sinus arrhythmia. As a result of slowing of the sinus pacemaker — the sinus rate occasionally drops below the intrinsic rate of the AV nodal escape pacemaker. This results in transient AV dissociation — until the sinus node speeds up enough to exceed the intrinsic AV nodal escape rate. This phenomenon is best known as, ADissociation by “Default” — ie, “default” of the sinus pacemaker due to marked sinus bradycardia, which transiently allows the AV nodal escape pacemaker to take over. We emphasize the following KEY points:
  • The rhythm in Figure-3 is not “AV dissociation”. AV dissociation is never the etiology of a rhythm. Instead, as we state above — the rhythm is sinus bradycardia with marked sinus arrhythmia. It is because of marked slowing in the rate that AV dissociation arises.
  • There is absolutely no evidence of any form of AV block. This is because we never see any P waves that fail to conduct despite adequate opportunity to conduct! Instead, the reason why certain P waves do not conduct — is simply that the PR interval is too short.
  • This is not “isorhythmic” AV dissociation. The word “iso” — is from the Greek word “isos” — which means “equal”. True “isorhythmic” AV dissociation is an uncommon phenomenon, in which there are independent atrial and ventricular pacemakers that are beating at nearly identical (ie, “equal” ) rates. Most often, there is an underlying sinus (atrial ) rhythm competing with an AV nodal rhythm beating at an almost identical rate. The effect is like a horse race — in which one rhythm temporarily “takes the lead” (ie, takes over the rhythm) — until it either slows slightly, or until the other rhythm accelerates just enough to take over the rhythm — and then this back-and-forth process begins anew. This is not what we see in Figure-3 — as there is an obvious marked difference in P-P and R-R intervals for much of the long lead II rhythm strip.
NOTE — It is important to appreciate that there are 3 potential Causes of ADissociation:
  • AV dissociation due to some form of 2nd or 3rd degree AV Block;
  • AV dissociation by “Usurpation” — in which P waves transiently do not conduct because an accelerated junctional rhythm takes over the pacemaking function (because it is faster than the underlying sinus rhythm); and/or,
  • AV dissociation by Default” — in which a junctional escape rhythm takes over by “default” (ie, because of SA node slowing — as occurs in this case).
But there is MORE to this case … (WARNING: What follows involves advanced arrhythmia concepts! ).

  • What is atypical about the rhythm in the long lead II of Figure-3, compared to what we most often see when there are junctional escape beats?
  • The answer is revealed in Figure-4, in which careful caliper measurement provides the duration of all P-P intervals (in PURPLE lettering) — and the duration of the R-R interval preceding each of the first 6 beats (in BLUE lettering). What do you see?
Figure-4: We have carefully measured the number of large boxes for the critical intervals illustrated above (See text). 
Assessment of Figure4: Most of the time, the intrinsic rate of the escape junctional pacemaker is constant. We are generally able to use this finding to help determine which beats are junctional vs conducted — because the R-R interval preceding most beats in a junctional escape rhythm is the same. But this is not the case in Figure-4 — because despite our thought that beats #2-thru-6 are all junctional escape beats (since they are all preceded by a PR interval too short to conduct) — the preceding R-R interval varies from 4.8-to-5.1 large boxes.
  • PEARL #4: The QRS complex of junctional escape beats sometimes looks a little bit different than the QRS of sinus-conducted beats. The reason is that the path of the electrical impulse for a sinus beat — may differ slightly from the path of a junctional beat that begins lower down or more to one side or the other of the AV node. We see this in Figure-4. We know beats #8 and 9 are sinus-conducted. We see a small-but-definitely-present initial q wave in these beats that is not seen in junctional beats #3, 4, 5 and 6. In addition, R wave amplitude of junctional beats #4 and 5 is clearly taller than the R wave of sinus-conducted beats #8 and 9! This PEARL is sometimes invaluable for telling us which beats in a tracing with AV dissociation are sinus-conducted vs of junctional origin vs fusion beats.
  • Then WHY does the QRS complex of beat #10 look exactly like the QRS of sinus-conducted beats #8 and 9, despite being preceded by a shorter PR interval? I believe the answer is that this patient manifests a component of Vagotonic AV Block (albeit not yet with clear demonstration of AV block … ).
I discuss in detail the phenomenon of Vagotonic ABlock in THIS CASE. In the interest of brevity here — Suffice it to say that some otherwise completely healthy individuals manifest a marked increase in vagal tone, to the point that significant bradycardia with various forms of AV block may sometimes be seen despite complete absence of underlying structural heart disease. Among the characteristics of this syndrome are marked sinus arrhythmia, variation in the rate of escape pacemakers, variable PR intervals for sinus-conducted beats (that is otherwise hard to explain) — and at times, various forms of transient AV block.
  • Returning to Figure-4 — I believe the fact that the QRS complex of beat #10 is identical to the QRS of known sinus-conducted beats #8 and 9 — tells us that beat #10 is most probably sinus-conducted with a slightly shorter PR interval. The same is likely to be true for beats #1 and 7 — each of which is preceded by a PR interval slightly shorter than the PR interval of known sinus-conducted beats #8 and 9. It’s hard to know about beat #2 (the QRS looks like a sinus-conducted beat — but the PR interval is exceedingly short … ). But overall, considering variation in the R-R intervals preceding known junctional escape beats #3, 4, 5 and 6 — the composite of findings suggest profound vagal influence affecting this patient!
  • The fact that there is a family history of some “abnormal” rhythm — suggests there may be a familial component to this patient’s inherently increased vagal tone.
  • Finally — this patient’s symptoms almost seem out of proportion to the rhythm we see in Figure-4. After all, there are no runs of tachycardia, no profound bradycardia, and as yet no sign of AV block. Close follow-up is essential. Holter monitoring may well reveal other more worrisome arrhythmias over the course of a day. That said, even though vagotonic AV block most often has a benign prognosis — the fact that this patient is so symptomatic (with what sounds like presyncope) merits referral to an EP cardiologist for more thorough evaluation and risk assessment.
BOTTOM LINE — Complete explanation of all findings present in the ECG shown in this case is indeed complex! I would be ecstatic for anyone assessing the rhythm in Figure-1 as showing marked sinus bradycardia and arrhythmia, that sometimes results in sufficient sinus slowing to produce AV dissociation with junctional escape beats. I think it important to appreciate that there is no evidence of AV block in Figure-1. That said — I think it helpful to be aware of the phenomenon of vagotonic AV block, that although uncommon — will be seen from time-to-time by emergency providers (Review of THIS CASE may prove insightful). In this particular case, even though AV block is not seen in Figure-1 — there are a number of ECG findings that in the context of the significant symptoms experienced by this patient, point to an inherent increase in vagal tone that merits referral for more complete assessment.
For More Info:

  • For another example of my step-by-step approach to assessing a case of AV dissociation — CLICK HERE
  • For those wanting a VIDEO Review (58 minutes) of the ECG Diagnosis of AV Blocks & AV Dissociation CLICK HERE. If you click on SHOW MORE (under the video on the YouTube page) — you’ll see a linked Contents of all that is covered in the video. The part regarding AV Dissociation BEGINS HERE.

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