Monday, September 27, 2021

ECG Blog #253 (67) — PVCs or Aberrant Conduction?

The 2 tracings that are shown in Figure-1 and Figure-2 were obtained during and shortly after a screening ETT ( = Exercise Treadmill Test).



  • Are the earlier-than-expected wide beats in both tracings PVCs (Premature Ventricular Contractions) or PACs (Premature Atrial Contractions) with aberrant conduction?
  • What are the clinical implications of these early wide beats?


Figure-1: 12-lead ECG and dual-lead rhythm strip obtained from an ETT during exercise (See text).


Figure-2: 12-lead ECG and dual-lead rhythm strip obtained from an ETT after exercise (See text).


  — The Case Continues BELOW today's Audio Pearl ... —




NOTE: Some readers may prefer at this point to listen to the 11:15-minute ECG Audio PEARL before reading My Thoughts regarding the ECGs in Figure-1 and Figure-2. Feel free at any time to refer to My Thoughts on these tracings (that appear below ECG MP-67).


Today's ECG Media PEARL #67 (9:10 minutes Audio) — Applies the wisdom from 2 of my favorite sayings to the "art" of ECG interpretation. These 2 sayings are = "Birds of a Feather" and "Forest from Trees".




Some Brief Concepts on ETT Basics:

Performing and interpreting ETTs is a skill that extends beyond the scope and purpose of my ECG Blog. That said — review of the 2 tracings in Figure-1 and Figure-2 illustrates some important concepts very relevant to distinction between PVCs vs aberrantly conducted PACs.

  • The principle of Exercise Treadmill Testing (ETT) is simple: The patient performs a progressively increasing degree of exercise on a moving treadmill while the clinician monitors a series of parameters, including hemodynamic factors (ie, heart rate, blood pressure) — ST-T wave response to exercise — the cardiac rhythm — and the patient's tolerance for increasingly demanding exercise (ie, Assessing the patient for development of chest pain, shortness of breath, dizziness, fatigue, etc.).
  • Much more than just the ST segment response to exercise is monitored. It is true that the ECG finding of flat or downsloping ST depression in a patient who develops chest pain with no more than low-level exercise during the ETT is a clear indication for prompt cardiac catheterization. But so much more can be learned! For example, the patient who denies all symptoms during the ETT — but who looks to be obviously fatigued at low-level exercise with development of associated arrhythmias and hypotension is at even higher risk of an acute cardiac event — because that patient lacks a "warning system" (ie, It is likely that such a patient will develop and deny fatigue with associated hypotension and arrhythmias during low-level activities of daily living — which poses a real risk of cardiovascular collapse if activity is continued).
  • The treadmill always wins. By progressively increasing the speed and degree of incline of the treadmill — even the most athletically fit individual will eventually fatigue. And THAT is one of the goals of performing an ETT (or "Stress" Test) — namely, that you want to see the ECG response to exercise when your patient is subjected to a level of activity at least comparable (if not exceeding) what that person does every day in an unmonitored situation at home.
  • The final point I'll make relates to the purpose of the ETT in today's case — which was as a screening test to assess this asymptomatic patient for exercise capacity and exercise prescription. How intense of an exercise effort will be appropriate for this patient? Insight into an individual's suitability for a given degree of unsupervised exercise can be gained from their performance on the ETT.
  • NOTE: For those interested — In 1998, I wrote a user-friendly manual regarding the basics of performing and interpreting ETTs in the office (which I did and taught for years). Realizing that this manual is now 20+ years old ... the basic principles are still valid. CLICK HERE for FREE download of my ETT Pocket Brain PDF (Linked Contents in the PDF Bookmarks facilitate a quick overview of the subject).



My Sequential THOUGHTS on Today's Case:

Realizing that we lack historical details about the patient in today's case — as well as not knowing for how long he/she exercised on the treadmill, nor their physiologic response — we still can interpret these 2 tracings with regard to the etiology of the early ectopic beats:


ECG #1 — Obtained during Exercise:

  • The underlying rhythm appears to be sinus tachycardia at a rate close to ~150/minute. Although we do not know the age of the patient, nor the stage of exercise during the ETT that Figure-1 was obtained — the chances are good that at the least — a reasonable degree of exercise has been achieved by the test (ie, probably at least 85% of maximal predicted heart rate for age).
  • A number of wider beats are seen in Figure-1. The 2 most obvious wide beats are seen in simultaneously-obtained leads V1, V2 and V3 (ie, beats #15 and 19).
  • Looking at the long lead rhythm strips (of lead II and lead V5) — at least a few other wider (different-looking) beats are seen (ie, beats #7, 11, 23). 
  • Each of the different-looking beats occur a little bit early! That said — I could not tell from Figure-1 if these wider (different-looking) beats were PVCs or PACs with aberrant conduction.
  • Clinically — it would not matter if these slightly early, different-looking beats were PVCs or aberrantly conducted PACs because: i) The ST segment response in Figure-1 shows no more than a minimal amount of rapid-upsloping ST depression, which is a normal (non-ischemic) response to exercise; andii) If the patient was appropriately tolerating the amount of exercise — PVCs at the frequency seen here, that do not occur in runs are not necessarily an indication to stop the test.
  • ETT NOTE: Fusion beats are common during exercise testing. This is because as heart rate increases — the R-R interval correspondingly decreases, which means there is a much greater chance for the occurrence of any PVCs that are seen to coincide with sinus-conducted beats. This explains why beats such as #7, 11 and 23 that do not look extremely wide may nevertheless represent PVCs (ie, if these beats are fusion beats).
  • ETT NOTE: In general, the amount of artifact increases at increasing degrees of exercise — because the patient is having to walk up an ever increasing incline at faster and faster speeds. As a result — rhythm interpretation and assessment of the ST segment response to exercise may be challenging. This is why the most helpful tracing for assessing ST-T wave changes during ETT is usually the immediate post-Exercise tracing — because the heart rate will still be fast, with great reduction in the amount of artifact compared to tracings obtained during exercise.


ECG #2 — Obtained after Exercise:

  • ETT NOTE: ECG monitoring is usually continued for between 6-10 minutes after the completion of maximal exercise (unless the patient is still having symptoms or arrhythmias after this period of time). ST segment depression that only occurs at peak exercise, and is not accompanied by chest pain — and, which promptly resolves in recovery (ie, within 2-4 minutes after stopping the test) is less likely to indicate significant underlying coronary disease.
  • Although we are not told when during the post-exercise period ECG #2 was obtained — the fact that the sinus rate is barely over 100/minute in ECG #2 (compared to ~150/minute in ECG #1) suggests that at least several minutes have already passed in the recovery period after stopping the test. That said — there are still frequent wider (different-looking) beats, as is best seen in the long lead II and V5 rhythm strips in Figure-3.
  • Sinus P waves are highlighted by RED arrows in Figure-3. Note that different-looking QRS complexes occur every 4th beat (ie, Beats #2, 6, 10 and 14 in Figure-3 are each wider and very different-looking compared to the other sinus-conducted beats in this tracing).
  • Note that there are different QRS morphologies for the wider beats in Figure-3, that I have labeled X1 and X2. The KEY for determining the etiology of these wider beats is highlighted by the YELLOW arrows in the long lead II rhythm strip. There is an unmistakeable notch under each of these YELLOW arrows that is not present either in or just after the T wave in the other sinus-conducted beats (ie, No notch is seen over the vertical BLUE lines in the long lead II rhythm strip). This proves that these YELLOW arrows each highlight a premature P wave (PAC), that precedes each of the wider beats in the long lead II rhythm strip in Figure-3.
  • PEARL #1: The best clue that a wider, different-looking early beat is an aberrantly conducted PAC — is when this beat is preceded by a PAC (which it is in Figure-3, as proved by the notch under each of the YELLOW arrows).
  • PEARL #2: The next best clue in favor of aberrant conduction is the finding of a QRS morphology for the wider, different-looking beats that resembles some form of known conduction defect. Note that QRS morphology for each of the wider beats labeled X1 in Figure-3 is consistent with LBBB conduction (ie, upright [albeit notched] QRS in lateral leads I, V5 and V6 — and predominantly negative QRS with extremely steep S wave downslope for beat #10 in lead V1).
  • Bottom Line: The underlying rhythm in Figure-3 is sinus with atrial quadrigeminy — in which every 4th P wave is a PAC (YELLOW arrows) that conducts with LBBB aberration (X1 beats).



Figure-3: I have labeled ECG #2 to highlight key features of the different-looking beats (See text).



Taking Another Look at ECG #1 — Obtained during Exercise:

  • In Figure-4 — I have labeled the slightly early, different-looking beats from Figure-1 with either an X1 or X2 notation — corresponding to the 2 different QRS morphologies that we identified in Figure-3.
  • As previously stated — the rapid rate and amount of artifact induced by exercise made it extremely difficult to confidently identify premature atrial activity in ECG #1. That said — we still can assess QRS morphology for the 2 clearly widened beats in simultaneously-obtained leads V1, V2, V3 of Figure-4.
  • The beat in lead V1 that is labeled X1 (in Figure-4manifests a predominantly negative QRS with very steep S wave downslope that looks identical to QRS morphology of beat #10 in lead V1 in Figure-3. 
  • QRS morphology in the long lead V5 rhythm strip for the 3 complexes labeled X1 in Figure-3 manifests a notched-R morphology. This is similar to the notched-R morphology in the long lead V5 rhythm strip for beat #15 in Figure-4. So, even though we are not able to identify a premature P wave before beat #15 in the long lead V5 of Figure-4 (because of all the artifact produced during exercise) — it is likely that the etiology of this X1 beat in Figure-4 is the same as the etiology that we proved for the 3 X1 complexes in Figure-3 = aberrantly conducted PACs).
  • Taking another look at beat #19 in Figure-4 (that we have labeled with an X-2 notation) — there is an rsR' morphology for this beat #19 in lead V1, in which the S wave descends below the baseline — and the R' is slender and manifests a taller right "rabbit ear". This QRS morphology for beat #19 in lead V1 is extremely typical for RBBB aberration — which is supported by the finding of a wide terminal S wave for this same beat #19 in the long lead V5 rhythm strip.
  • PEARL #3: By the "Birds of a Feather" concept (See the above Audio Pearl #67) — Since the "theme" of the slightly early, different-looking beats in these 2 tracings is that these are aberrantly conducted PACs (as proven by the YELLOW arrows that highlight premature P waves!) — the chances are that other slightly early, different-looking beats are also aberrantly conducted supraventricular impulses (and not PVCs). 


CONCLUSION: These 2 tracings, taken from a screening ETT — provide an insightful example of how careful observation allows identification of subtle-but-evident premature P waves (PACs) that confirm a supraventricular etiology for the sighty early, different-looking beats. 

  • PACs are conducted with varying degrees of RBBB and LBBB aberration. 
  • The modest amount of rapid-upsloping ST depression during exercise, that resolves in recovery suggests a negative screening test for ischemia (although clearly, additional details about the patient and specifics on exercise parameters during each stage of the test would be needed for full analysis).


Figure-4: I have labeled ECG #2 to highlight key features of the different-looking beats (See text).



Acknowledgment: My appreciation to Massimo Bolognesi (from Cesena, Italy) for the case and this tracing.




Related ECG Blog Posts to Today’s Case: 

  • ECG Blog #128 — Reviews the concept of Fusion Beats.

  • ECG Blog #211 — Reviews why some beats (and rhythms) are conducted with aberration (and why the most common form of aberrant conduction manifests RBBB morphology)
  • ECG Blog #33 — Reviews a case showing blocked and aberrantly-conducted PACs. 
  • ECG Blog #66 — Reviews a case showing blocked and aberrantly-conducted PACs. 
  • ECG Blog #140 — Reviews a case showing PACs with varying aberrant morphology.

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