ECG Interpretation Review #68 (PVC – Interpolated PVC – Concealed Conduction – Compensatory Pause)

     The 12-lead ECG shown below in Figure-1 was obtained from a 50 year old woman with “skipped beats”.

• Is there anything unusual about these ectopic beats and the effect that they have on the normal beats in this tracing? (HINT: What is concealed conduction?).
• What happens at the onset of the last lead change (ie, at the onset where we see leads V4,V5,V6)?

Figure-1: 12-lead ECG obtained from a woman with “skipped beats”. What effect do the ectopic beats have on the normal beats? NOTE — Enlarge by clicking on Figures — Right-Click to open in a separate window (See text).

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Interpretation of Figure 1:

     The underlying rhythm in this tracing is sinus, as determined by the upright P wave with normal PR interval seen for the 3rd complex in lead II.  There are frequent PVCs (Premature Ventricular Contractions) on this tracing — which total four in number. The first three PVCs are said to be “interpolated” — because they occur sandwiched in between two normal sinus beats without the compensatory pause that typically follows a PVC.

• Most PVCs are associated with retrograde conduction of the impulse back toward the AV node. If this retrograde conduction extends far enough to reach the atria, a negative P wave may be seen in lead II after the PVC.
• Even when retrograde conduction from a PVC does not extend all the way back to the atria — retrograde conduction usually lasts long enough to prevent forward conduction of the next sinus P wave. This accounts for the “compensatory pause” that is usually seen following a PVC (as occurs for the last PVC on this tracing).
• IF the timing is just right — a PVC may occur at a moment when enough recovery of the conduction system has occurred to allow forward conduction of this next sinus beat.  This is what happens with interpolated PVCs.

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Taking a Closer Look:

     Events in this tracing are most easily explained by use of a laddergram (Figure-2). Normally conducted sinus beats are beats #3, 6, 7, 10, 12, and 13. The 3 interpolated PVCs are beats #1, 4, and 8. The last PVC in the tracing (beat #11) has a post-ectopic pause.

Figure-2: Laddergram of the 12-lead ECG shown in Figure-1. The first 3 PVCs (beats #1,4,8) are interpolated. The last PVC (beat #11) has a post-ectopic pause (See text).

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     Of interest — we do not see a P wave in front of the 2nd, 5th, and 9th beats on this tracing, even though these are sinus beats!   This phenomenon is known as concealed conduction — in which retrograde conduction from the preceding PVC while not enough to block forward conduction of the next sinus beat, is enough to slightly prolong the ensuing PR interval.

• The term “concealed” is used, because we cannot explain this effect on the ensuing PR interval from what is seen on the actual ECG — but instead must infer there is retrograde conduction from the PVC impeding forward conduction of the next sinus impulse.
• That the sinus P waves preceding the 2nd, 5th, and 9th beats on this tracing are almost certainly “concealed” within the T wave of the PVC that precedes these beats — is deduced from the regularity of atrial impulses (arrows on the laddergram).

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     The final intriguing complexity manifest on this interesting tracing relates to the bizarre occurrence at the onset of the last lead change.  The last PVC on this tracing occurs just before this lead change.

• The discontinuity produced in the T wave of this last PVC (beat #11) results from the fact that the leads change before the T wave is completely recorded.
• Unlike the P waves following the first 3 PVCs — the P wave following beat #11 is not able to penetrate the AV node. As a result — a post-ectopic pause is seen (between beats #11-12) and the next sinus P wave is conducted with a normal PR interval.
• Regarding interpretation of the rest of the ECG — there are small inferolateral q waves of uncertain significance but no acute ST-T wave changes.

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BOTTOM Line:

     Most PVCs are not interpolated — but rather conduct with a post-ectopic pause as seen for the last PVC on this tracing. The clinical significance of interpolated PVCs is the same as for any other PVC. It is good to be aware of the unusual appearance (sandwiched between 2 sinus beats without any pause) — so as to recognize this phenomenon when it occurs.

• It is also good to be aware that the PR interval of the sinus beat following an interpolated PVC may be prolonged — sometimes to a much greater extent then seen for beats #2, 5 and 9 in this tracing. Such PR interval prolongation is the result of concealed conduction (and not due to 2nd degree AV block of the Wenckebach type).

ECG Interpretation Review #67 (PAC – PVC – 12 Leads Better than One)

     The rhythm strip shown in Figure 1 was obtained from a patient with palpitations. The patient was hemodynamically stable.

• Does Figure 1 show atrial trigeminy (every 3rd beat a PAC )?

Figure-1: Lead II rhythm strip obtained from a patient with palpitations. Is this atrial trigeminy? (Figure reproduced from ACLS-2013-Arrhythmias: Expanded Version- pp 229-230). NOTE — Enlarge by clicking on Figures — Right-Click to open in a separate window (See text).

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Interpretation of Figure 1:

     The underlying rhythm in Figure 1 is sinus — as determined by the presence of upright P waves with fixed PR interval preceding beats #1,2; 4,5; 7,8; 10,11; and 13 in this lead II monitoring lead. The QRS complex of sinus beats is narrow.

• Every-third-beat occurs earlier-than-expected and looks slightly different. That is — the QRS complex of beats #3, 6, 9 and 12 each have a smaller r wave and less deep S wave than do sinus beats. The QRS complex for each of these early beats looks to be narrow and preceded by a premature P wave (red arrow in Figure 1). IF this were the case — the rhythm would be atrial trigeminy (every third beat a PAC [Premature Atrial Contraction] ).

QUESTION: Do you agree with the above assessment?

• HINT #1: Do we have enough information from Figure 1 to determine IF the QRS complex of each early beat is truly narrow?
• HINT #2: Look at Figure 2 — in which we have added a simultaneously recorded lead I rhythm strip. Does the QRS of each early beat still look narrow?

Figure-2: A simultaneously recorded lead I rhythm strip has been added to Figure-1. Note from the vertical time line that the notch which looked to be a premature P wave in Figure-1 is actually the initial part of the QRS complex in simultaneously recorded lead I. (Figure reproduced from ACLS-2013-Arrhythmias: Expanded Version- pg 230).

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12 Leads are Better than One

     We fully acknowledge that this is a trick tracing. We nevertheless have chosen it to emphasize a number of important points:

• POINT #1: 12 Leads are Better than One. It is easy to get fooled when you are not provided with complete information. Part of the QRS complex may sometimes lie on the baseline. When this happens — the QRS complex may look narrow in one lead — whereas in reality it is actually quite wide. At other times (as in Figure 1) — what looks like a preceding “P wave” may actually be the initial part of the QRS complex. Use of a simultaneously recorded multi-lead rhythm strip (or 12-lead ECG) may be invaluable in such cases for shedding light on the true nature the rhythm being assessed. With the extra information provided by lead I in Figure 2 — it is now apparent that every third beat (ie, beats #3,6,9,12) occurs early, is wide, and is not preceded by any premature P wave. Every third beat in Figure 2 is therefore a PVC. The rhythm is ventricular trigeminy.
• POINT #2: Assume that a Premature Beat is “Guilty” (ie, a PVC) until Proven Otherwise.  Statistically — most early occurring different-looking beats that are not clearly preceded by a premature P wave will be ventricular in etiology. The “onus of proof” therefore rests with the interpreter to establish that any abnormal-looking beats are aberrantly conducted (ie, supraventricular) — rather than the other way around. Assume a ventricular etiology until proven otherwise. And remember that you can not “prove” aberrant conduction with use of the incomplete information provided from a single monitoring lead.

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- For more information – GO TO:

• Link to ACLS-2013-Arrhythmias(Expanded) — and the epub version of this book.

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ECG Interpretation Review #66 (2nd Degree AV Block – PACs – Blocked PACs – Mobitz I – AV Wenckebach – Group Beating)

     Interpret the two sequential tracings in Figure 1. The patient is hemodynamically stable.

• Note group beating in Tracing A. Is this due to AV block? If so — What type of AV block?
• HINT: Interpretation of Tracing A becomes easier if you first interpret Tracing B obtained from the same patient a lit bit later. This is because the same phenomenon responsible for the abnormal findings is operative in both tracings.

Figure-1: Sequential tracings in a patient who initially manifests group beating (Tracing A) — and later manifests some irregularity with a brief pause following beat #6 (Tracing B). What is the cause of the abnormal findings seen in both tracings? (Figure reproduced from ACLS-2013-Arrhythmias: Expanded Version - pp 264-265). NOTE — Enlarge by clicking on Figures — Right-Click to open in a separate window (See text).

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Characteristics of Mobitz I 2nd-Degree AV Block (AV Wenckebach)

    Recognition of the phenomenon of “group beating” should always suggest the possibility of a Wenckebach conduction disorder. That said — this case presents a wonderful example of why group beating does not always mean that a Wenckebach block is present.

• Second-degree AV block of the Mobitz I type (also known as AV Wenckebach) — is by far the most common form of 2nd-degree AV block. Mobitz I is characterized by: i) Progressive lengthening of the PR interval until a beat is dropped; ii) Group beating; iii) A regular (or at least fairly regular) atrial rate; and iv) The pause that contains the dropped beat is less than twice the shortest R-R interval. These features collectively (among a few others) are known as “the Footprints of Wenckebach”. They are all present in Figure 2.

Figure-2: Mobitz I 2nd degree AV block with all of “the footprints” mentioned above. There are two “groups” of beats (beats #1,2,3; and #4,5). The P-P interval is regular (in this case at a rate of 100/minute). Within groups — the PR interval progressively lengthens until a beat is dropped (the P wave following beats #3 and #5 is nonconducted). The pause containing the dropped beat is less than twice the shortest R‑R interval (that is — the pause between beats #3-4 is less than twice the R-R interval between beats #2-3). (Figure reproduced from ACLS-2013-Arrhythmias: Expanded Version- pg 237).

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Beyond-the-Core: The reason we specify “AV Wenckebach” rather than just, “Wenckebach” for the conduction disorder shown in Figure 2 — is that there are many types of Wenckebach conduction disorders. Second-degree AV block of the Mobitz I type is just one of these (albeit the most common form seen).

• One may also see Wenckebach-conduction in the form of retrograde block out of the AV node; Ventricular tachycardia with retrograde Wenckebach; SA node Wenckebach; and atrial fibrillation or flutter with Wenckebach conduction, to name just a few.
• Among the above footprints most helpful in identifying some of these other forms of Wenckebach conduction are “group beating” and pause containing the dropped beat being less than twice the shortest R-R interval.

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Why Despite Group Beating – Figure 1 is Not AV Wenckebach

     Before addressing specific interpretation of the rhythm in Figure 1 — We list reasons why sequential Tracings A and B are not the result of Mobitz I 2nd degree AV block. This is easiest to do by adding red and blue arrows to highlight P waves in these tracings (Figure 3). Note the following:

• The PR interval does not progressively increase within the groups of beats (from beat #1-to-2; from beats #3-to-4-to-5; from beats #6-to-7-to-8).
• The atrial rhythm in Tracing A is clearly not regular.

Figure-3: Addition of arrows to Figure 1 to highlight atrial activity (See text). (Figure reproduced from ACLS-2013-Arrhythmias: Expanded Version - pp 264-265).

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Interpretation of the Sequential Tracings in Figure 3

     The underlying rhythm for both tracings in Figure 3 is sinus arrhythmia. In Tracing A — a short pause follows each group of sinus-conducted beats. There is subtle-but-real peaking of the T wave of beats #2, 5 and #8 in Tracing A (blue arrows). That this peaking is real and not due to artifact — is determined by the fact that the T wave of all other beats in Tracing A is smoother and of smaller amplitude. This peaking of T waves must therefore reflect blocked PACs.

• A similar phenomenon is operative in Tracing B — and much easier to see. Note that beat #4 in Tracing B occurs early. The T wave just preceding beat #4 in Tracing B is again peaked (blue arrow) — strongly suggesting that a PAC is hidden within this T wave. The reason the QRS complex of beat #4 looks slightly different — is that beat #4 is a PAC conducted with aberration.
• A short pause follows beat #6 in Tracing B. The commonest cause of a pause is a blocked PAC. Once again — “telltale” notching of the T wave of beat #6 in Tracing B (blue arrow) indicates this is due to another blocked PAC.

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Summary Take-Home Points:

     Awareness and recognition of the phenomenon of “group beating” is an important concept that will often help streamline your interpretation. Some type of Wenckebach conduction will often be the cause. However — there are other reasons for group beating that need to be kept in mind. Remember:

• The commonest cause of a pause is a blocked PAC (not AV block). Although often quite subtle (as they are in Tracing A) — blocked PACs will be found IF you look for them.
• Routine use of a systematic approach to rhythm interpretation is essential for accurate interpretation. We use this memory aid: “Watch your Ps & Qs — and the 3Rs”. (Click HERE for brief Review.) Using this systematic approach should make it immediately obvious that AV Wenckebach is not the cause of group beating in this example — because the atrial rhythm in Tracing A is definitely not regular!

     In Summary — the 2 tracings in Figure 3 reflect the same phenomenon. There is underlying sinus arrhythmia that is punctuated by multiple PACs. Most of these PACs are blocked.

• The PAC that notches the T wave of beat #3 in Tracing B is conducted with aberration.
• The T wave of beat #6 in Tracing B is unmistakably peaked.
• Recognition that the “theme” for Tracing B is the occurrence of PACs alerts us to carefully focus on T wave morphology at the beginning of each pause in Tracing A. Doing so facilitates identifying the subtle-but-real deformity brought about by hidden PACs within the T waves of beats #2, 5 and 8 in Tracing A.

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- For more information – GO TO:

• Brief review of the Ps,Qs & 3R Approach to Rhythm Interpretation.
• The Basics of AVBlock (including links to examples of AV block mimics).
• Link to ACLS-2013-Arrhythmias(Expanded) — and the E-Pub version of this book.

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