Electrophysiological curiosity. Can you spot it?

        Written by Magnus Nossen (with comments and laddergram by Ken Grauer)

The patient in today’s case is a man in his 60s — who presented with palpitations and lightheadedness. He had no history of syncope. The patient had hypertension, but was otherwise healthy. Due to the reported symptoms, he was admitted for observation and put on telemetry monitoring. Below in Figure-1 is a tracing obtained from the in-house telemetry. 

• The rhythm strip in Figure-1 shows four ECG leads. (aVF, III, V1 and V5). The paper speed is 25mm/s. There is a bigeminal pattern with every other QRS complex being wide. Can you explain what is going on in this tracing? Do you notice anything unusual in this rhythm strip? 

Figure-1: The initial rhythm strip in today’s case.

Below in Figure-2 is another tracing from the same patient. Beat #9 is wide, and manifests a different QRS morphology than the other wide beats. What is the likely explanation for this?

Figure-2: A 2nd rhythm strip from today’s patient.

COMMENT:

Both rhythm strips show sinus rhythm with premature ventricular complexes (PVCs). Following most of these PVCs — there is retrograde conduction that begins from the ventricles, and then travels backward through the AV node and the atria (VA conduction). Because the atria are captured in retrograde fashion — the P waves are negative (with these retrograde P waves being partially hidden within the ST segments of the preceding PVC).

Of note is variation in the RP interval! (ie, the distance from the beginning of the PVC — until the retrograde P wave).

• There are 2 different RP intervals. This can be explained by having dual AV conduction pathways in the retrograde direction. One of these pathways conducts significantly slower than the other.
• When the retrograde impulse travels through the slower pathway — conduction time and RP interval is longer. 
• The opposite occurs when retrograde conduction is through the faster pathway (ie, faster retrograde conduction results in a shorter RP interval).

Because of added complexity in the 2nd tracing — I have labeled this 2nd rhythm strip in Figure-3:

• Green arrows point to sinus P waves in Figure-3 — with normal sinus conduction of beats #1,3,5,7 and 10.
• Beats #2,4,6 and 8 are PVCs with a similar (albeit not identical) QRS morphology. These PVCs most probably originate from the same ectopic site in the ventricles.
• The Orange arrow points to a retrograde P wave with a longer RP interval (295 msec.) — that is conducted from PVC beat #2.
• The Blue arrow points to a retrograde P wave with a shorter RP interval (170 msec.) — that is conducted from PVC beat #4.
• There is no retrograde conduction following the PVC beat #6.
• The Red arrow points to a retrograde P wave with a longer RP interval — that is conducted from the PVC beat #8. Note that this retrograde RP interval from beat #8 is slightly longer than the RP interval from beat #2 (330 msec. vs 295 msec.).

Figure-3: I’ve labeled Rhythm #2.

What about Beat #9?

• Note that beat #9 is wide — but with a different QRS morphology when compared to wide beats #2,4,6,8 (ie, Beat #9 manifests a predominantly negative QRS in all 4 monitoring leads).
• Beat #9 follows after the longest RP interval in this rhythm strip. The reason beat #9 looks different — is that beat #9 is a ventricular “echo” or reciprocal beat. The mechanism operative for this reciprocal beat in Figure-3 — is illustrated in Panel C of Figure-4.

Figure-4: Illustration of the mechanism for reciprocal (echo) beats. (Adapted from Strasberg et al — Am J Cardiol 48(4): 639-646, 1982 ).

KEY Clinical Point (Grauer):

The importance of Figure-4 — is that it shows the mechanism for initiation of many reentry SVT rhythms! (ie, See My Comment at the bottom of the page in the March 6, 2020 post in Dr. Smith's ECG Blog — in which I illustrate near the very end of my March 6, 2020 Comment the initiation of "fast-slow" form of AVNRT by what we see here in Panel B).

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Returning to the mechanism in Figure-3 of today's case:

Grauer: For clarity in Figure-5 — I've kept the same color coding, and illustrate Dr Nossen's above explanation of reciprocal beating in a Laddergram that concentrates on lead III from Figure-3.

• GREEN arrows and lines in the laddergram — show normal sinus-conducted beats.
• Beats #2,4,6 and 8 are PVCs (The PINK circles schematically show origin of these PVCs from the ventricle).
• The ORANGE dotted line — shows PVC beat #2 conducting backward to produce a retrograde P wave with an RP interval = 295 msec.
• The BLUE dotted line — shows PVC #4 conducting backward over the slower AV nodal pathway, to produce a retrograde P wave with an RP interval = 170 msec.
• PVC #6 — fails to make it all the way back to the atria.
• But — PVC #8 not only conducts backward, here over the slower AV nodal pathway to produce a retrograde P wave (the RED dotted line in the laddergram, leading to the RED arrow) — but because of its longer RP interval (330 msec.) — the faster AV nodal pathway has enough time to recover forward conduction properties — and therefore produces beat #9 (in PURPLE) — which is an "echo" beat that is conducted with rate-dependent aberrancy because beat #9 occurs so soon after beat #8.

Figure-5: My proposed Laddergram illustrating the mechanism of the "echo" beat in Figure-3.

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The CASE Continues:

The patient had many episodes of NSVT (Non-Sustained Ventricular Tachycardia) — like the one shown below in Figure-6. He experienced palpitations — both when having PVCs, as well as during his episodes of NSVT and VT. 

• Can YOU identify atrial activity in Figure-6?
•    Why does beat #3 prove that the wide beats are ventricular?

 

Figure-6: Subsequent 5-lead rhythm strip with an episode of NSVT.

Atrial Activity in Figure-6:

The rhythm in Figure-6 begins with a series of regular sinus P waves (GREEN arrows in Figure-7).

• It could be easy to overlook the 2nd and 3rd GREEN arrows — because they are partially hidden within the ST-T waves of beats #2 and 4. 
• The KEY is to recognize the 4th, 5th and 6th GREEN arrows — and to note that the P-P interval is fairly constant for these 3 P waves.
• Setting your calipers to approximately this P-P interval — should facilitate finding the 2nd and 3rd GREEN arrow P waves!
• Remember — Since the 1st beat in Figure-6 is sinus-conducted, It is logical for the underlying atrial rhythm to be regular, at least for the beginning of this tracing. So, we are looking for signs of an underlying regular atrial rhythm (with perhaps slight sinus arrhythmia) — and it is much easier to find this when you know what you are looking for! (and using calipers makes the partially-hidden P waves so easy to find! ).
• 
  
• We know beats #2 and #4 are PVCs — because these beats are wide, very different-looking from sinus-conducted beats, and these beats are not preceded by a premature P wave.
• 
  
• Isn't beat #3 intermediate in its QRS morphology? — compared to the morphology of sinus-conducted beat #1 and PVCs #2 and #4? The reason for this intermediate QRS morphology — is that beat #3 is a Fusion beat! ("F") — and this proves that all of the beats that look like #2 and #4 are also of ventricular etiology! (See "Lesson #3" in My Comment at the bottom of the page of the April 2, 2022 post for more on the diagnostic value of fusion beats).
• 
  
• Finally, knowing that today's patient shows a tendency for ventricular beats to conduct retrograde — I suspect that the YELLOW arrows in Figure-7 represent retrograde P waves following ventricular beats in the run of NSVT.

Figure-7: I've labeled Figure-6.

CASE Conclusion:

Today's patient had a structurally normal heart and a normal cardiac catheterization. A permanent pacemaker was placed and the patient was atrial paced at 60bpm. After pacemaker placement — a ß-blocker was initiated. The  hope was that the increased heart rate (from atrial pacing) combined with ß-blocker treatment would suppress most ventricular ectopic activity and the runs of NSVT. When these measures did not work — the patient underwent PVC ablation, which did achieve a good clinical result.

Learning points:

• Dual AV physiology in the retrograde direction does not necessarily mean there is dual AV conduction in the antegrade direction. 
• Dual AV physiology is necessary for AVNRT and reciprocal or "echo" beats. 
• 
  
• As shown above in Figure-4 — Echo beats provide a common mechanism for initiation of reentry SVT rhythms! (and — understanding the mechanism to produce "echo" beats provides insight for understanding how a run of NSVT may even produce a subsequent run of reentry SVT à la Panel C in Figure-4).