Saturday, May 6, 2023

ECG Blog #378 — Wenckebach with PVCs?

The ECG in Figure-1 — was obtained from an older woman with chest pain.

  • How would YOU interpret this tracing?
  • What kind of AV block is present? — OR — Is there no clear evidence of any AV block?

Figure-1: The initial ECG in today’s case. (To improve visualization — I've digitized the original ECG using PMcardio)

MY Thoughts on the ECG in Figure-1:
This tracing is challenging to interpret because of the changing QRS morphology — and because of the uncertainty regarding atrial activity.
  • The “good news” — is that although the chief complaint from this older woman was “chest pain” — an initial “quick glance” at the 12-lead does not suggest obvious acute changes. In such cases (assuming the patient is hemodynamically stable) — I favor looking closer at the cardiac rhythm before taking a more detailed look at the 12-lead.

PEARL #1: As noted above — the QRS is changing — and atrial activity is uncertain in parts of this tracing. In such cases — I favor starting with the part(s) of the tracing that I am certain about.
  • The underlying rhythm in Figure-1 is sinus — since an upright P wave with constant PR interval is seen before each of the QRS complexes that terminate each of the brief pauses (ie, before beats #3,6,9,12 and 15 — as shown by the RED arrows in Figure-2).
  • The PR interval for each of these sinus beats is slightly prolonged (ie, to slightly more than 1 large box in duration = 0.22 second). Therefore — a 1st-degree AV block is present.
  • The QRS of sinus beats #3,6,9,12 and 15 looks narrow in this long lead II rhythm strip. Returning for a moment to the 12-lead tracing shown in Figure-1 — confirmation that the QRS of sinus-conducted beats is truly narrow, is forthcoming from the fact that the QRS is clearly narrow in each of the simultaneously-recorded leads for beats #3,6,9,12 and 15 in all 12 leads.
  • In contrast — Beats #1, 4, 7, 10 and 13 are wide, different in appearance than the sinus-conducted QRS complexes — and are not preceded by a premature P wave. Therefore — beats #1,4,7,10 and 13 are PVCs (Premature Ventricular Contractions).
  • On the other hand — Beats #2,5,8,11 and 14 are clearly supraventricular — because these beats are narrow and virtually identical in QRS morphology to the sinus-conducted beats ( = beats #3,6,9,12 and 15).

The KEY Remaining Question:
  • Are there additional P waves in Figure-2?

Figure-2: I've labeled (with RED arrows) the definite sinus P waves in the long lead II rhythm strip from the 12-lead tracing shown in Figure-1.

To EMPHASIZE: Interpretation of today’s rhythm depends on answering the above question regarding whether additional P waves are present in Figure-2?
  • Hopefully — You considered the possibility that additional sinus P waves might be “hiding” within the terminal part of the T waves of each of the PVCs (PINK arrows in Figure-3). If this were the case — then the underlying rhythm would be a fairly (but-not-completely) regular rhythm (ie, sinus arrhythmia).
  • Ever-so-slight changes in some of the small upright humps in the T waves under each PINK arrow in Figure-3 — supports the possibility that slightly irregular sinus P waves may be hidden here (ie, the hump in the T waves under the PINK arrows of beats #10 and 13 appears slightly narrower than the hump under the PINK arrows in the T waves of beats #1,4,7).

  • PEARL #2: The reason I knew that P waves were indeed hiding in the T waves under each of the PINK arrows in Figure-3 —  is by use of the concept of simultaneously-recorded leads. The RED arrow in lead V1 of Figure-3 shows what the sinus P wave in this lead looks like. Note that the negative deflection just after the PVC in lead V1 — corresponds precisely with the timing of the small positive hump in the T wave under the PINK arrow following beat #10 in the long lead II rhythm strip (as shown by the vertical PINK time-line in Figure-3).

Remaining QUESTIONS:
  • What then is the rhythm in today’s tracing?
  • WHY is there no “compensatory” pause after each PVC? (ie, after beats #1,4,7,10 and 13)?
  • Since each of the arrows in Figure-3 represent sinus P waves, and, since the PR interval is increasing within each “group” of beats — Why is this not Mobitz I 2nd-degree AV block?

Figure-3: I've added PINK arrows to the long lead II rhythm strip in today's tracing — to show where additional sinus P waves are hiding (See text).

ANSWERS to the Above Questions:
It’s easiest to answer the above questions with a Laddergram (See Figure-4 below).
  • As I’ve often emphasized, although learning to draw laddergrams takes time and practice — learning to read laddergrams that are already drawn for you is not difficult, and can be quickly learned. (For those interested in practice reading and/or learning to draw laddergrams — there are more than 80 practice examples on my ECG Blog #188).

  • For the laddergram in Figure-4 — it is easiest to appreciate the mechanism of today’s rhythm by ignoring beats #1 and 2, and beginning with beat #3.
  • Note that each of the sinus P waves before beats #3,6,9,12 and 15 — are conducted to the ventricles with no more than the very slight delay from the 1st-degree AV block (ie, RED lines passing sequentially through the Atrial, AV Nodal and Ventricular Tiers in the laddergram).
  • Beats #1,4,7,10 and 13 are PVCs — which is why I’ve drawn these impulses starting from the ventricles. The PVCs in the Figure-4 laddergram are "interpolated" — because unlike most PVCs, that are associated with a fully “compensatory” post-ectopic pause (ie, a pause equal to twice the normal R-R interval)“interpolated” PVCs are not followed by this long of a pause (See Figure-5 below for laddergram illustration of a compensatory pause).
  • That said — the retrograde conduction that arises from interpolated PVCs (shown in Figure-4 as dotted RED lines extending backward into the AV Nodal Tier) — may delay conduction of the next on-time sinus impulse, causing an increase in the PR interval of the next sinus-conducted beat (ie, the slight increased inclination of the BLUE lines in the laddergram).

PEARL #3: Although the PR interval before beats #2,5,8,11 and 14 is increased (compared to the PR interval before beats #3,6,9,12 and 15)We can not see why this is happening solely on the basis of the surface ECG tha
t is shown in Figure-3. Instead — We have to postulate that the reason the PR interval lengthens — is that retrograde conduction from the interpolated PVCs must be delaying conduction of the next on-time sinus impulse (as depicted by the dotted RED lines in Figure-4)
  • This phenomenon of trying to explain events that are not evident on the surface ECG — is known as "concealed" conduction. The importance of being aware of this phenomenon — is to recognize that the increase in PR interval for beats #2,5,8,11 and 14 in today's tracing — is the result of concealed conduction (and not due to Mobitz I, 2nd-degree AV block)

Figure-4: Laddergram illustration of today's tracing.


Final POINT: As alluded to above — most PVCs are not interpolated. Instead — most PVCs are followed by a "compensatory" pause, as shown for beat #7 in schematic Figure-5.
  • The significance of interpolated PVCs is the same as for any other PVC. Clinically — the main reason to be aware of the unusual appearance of interpolated PVCs (sandwiched between 2 sinus beats without a full compensatory pause) — is to recognize this phenomenon when it occurs — and to know that the PR interval of the sinus beat following an interpolated PVC may be prolonged as a result of concealed conduction (and that this is not the result of 2nd degree AV block of the Mobitz I type).

Figure-5: Schematic laddergram illustration of premature beats. Beats #1 and 2 are normal sinus-conducted P waves. Beat #4 is a PAC — beat #7 is a PVC — and beat #10 is a PJC. The PVC is said to be "fully compensatory" — because the R-R interval between beats #6-to-8 equals twice the normal R-R interval. This is because retrograde conduction from the PVC prevents the next on-time P wave from being conducted to the ventricles. 


P.S.: Now that we've interpreted the rhythm in today's case — it's important to take another LOOK at the rest of the 12-lead ECG (ie, at Figure-3).
  • The rhythm is sinus with frequent interpolated PVCs. QRS voltage is overall reduced. Intervals and the axis are normal. There is no chamber enlargement.

Q-R-S-T Changes: 

  • There is a QS in leads V1,V2, with slight delay in transition (ie, the R wave does not become taller than the S wave is deep until between leads V4-to-V5). There are no other Q waves.
  • The most remarkable finding — is that T waves in the sinus-conducted beats in chest leads look peaked, and potentially taller-than-they-should-be given R wave amplitude in leads V2-thru-V6.  

  • IMPRESSION: The patient in today's case is an older woman who did present with chest pain. Her ECG does manifest frequent PVCs — and potentially hyperacute T waves. More information and follow-up is needed (ie, serial ECGs, troponins — more history to determine if her symptoms are a result of frequent ectopy or possibly of an acute event). Unfortunately — I have no additional follow-up for this case ...


Acknowledgment: My appreciation to 유영준 (from Seoul, Korea) for the case and this tracing.


Related ECG Blog Posts to Today’s Case:

  • ECG Blog #205 — Reviews my Systematic Approach to 12-lead ECG Interpretation.
  • ECG Blog #185 — Reviews the Ps, Qs, 3R Approach to Arrhythmia Interpretation.

  • ECG Blog #188 — reviews how to read and/or draw Laddergrams (with more than 80 practice examples! ).

  • ECG Blog #68 and ECG Blog #359 — Review of interpolated PVCs.

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