Thursday, June 20, 2013

ECG Interpretation Review #69 (PVC – PJC – Retrograde – Concealed Conduction – Group Beating)

The lead II rhythm strip shown below in Figure-1 was obtained from an elderly woman with a history of heart failure.
  • What is the cause of the group beating?
  • What is the cause of the brief pauses (between beats #2-3; #5-6; and #8-9)?
NOTE: We acknowledge slight distortion of the ECG grid due to tilting of the ECG paper. This tracing was obtained by smart phone photograph. While less than optimal as a reproduction tool use of smart phone photos tremendously facilitates collaborative interpretation efforts and generally does not prevent arriving at the correct ECG diagnosis.

Figure-1: Lead II rhythm strip from an elderly woman with heart failure. What is the cause of the group beating that is seen? Is this 2nd degree AV block, Mobitz Type I? 

Interpretation of Figure 1:
There clearly is “group beating” in this tracing ­— however, the rhythm is not a manifestation of Wenckebach. We begin interpretation with assessment by the “Ps, Qs, 3R Approach” — We note the following:
  • P Waves — are present and regular (arrows in Figure-2).
  • QRS Complex Width — is narrow. The rhythm is therefore supraventricular (arising from at or above the AV node).
  • Rate & Regularity — As per Figure-2, the atrial (P-P) rhythm is regular (or at least fairly regular) at a rate is just under 60/minute. The ventricular rate varies (there is group beating).
  • Related? — There is sinus conduction (albeit with a long PR interval) for the first 2 beats in each grouping. That is beats #3,4; #6,7; and #9,10 appear to be sinus-conducted with 1st degree AV block (PR interval ~ 0.28 second).

Figure-2: Arrows indicating regularly-occurring P waves have been added to the rhythm strip that was shown in Figure-1 (See text).

The presence of group beating should always suggest the possibility of some type of Wenckebach conduction disturbance. That said — 2nd degree AV block, Mobitz Type I (AV Wenckebach) is not present in this case because: i) the PR interval is not progressively increasing within each group; and, ii) every third beat in this tracing occurs early (Note early occurrence of beats #2, 5 and 8).
  • What is especially unusual — is that no P wave precedes early beats #2, 5 and #8, as would be expected were these PACs (Premature Atrial Contractions). Instead — the next P wave occurs right on time (arrows in Figure-2 that are seen immediately after beats #2, 5, and 8).
  • In addition — No QRS complex follows the P waves after beats #2, 5, and 8. We have to explain why
Events in this interesting rhythm strip are best explained by laddergram. Over the next 7 Figures — We illustrate the steps we use in constructing this laddergram. Much of this construction is intuitive. By the time we are done — it should be readily apparent how useful a laddergram can be in explaining otherwise complex arrhythmia phenomena not immediately recognized from assessment of a rhythm strip alone.

NOTE: Drawing laddergrams is a skill that requires some practice. This may be beyond the level of interest for many providers. We emphasize the following:
  • The vast majority of arrhythmias can be interpreted without drawing a laddergram.
  • While learning to draw laddergrams does require some time and practice learning to read laddergrams already drawn in for you is EASY. You’ll be well on your way toward comfort in this area within the next 5 minutes after completing this ECG Blog.
  • HINT: Use of calipers is essential. There simply is no better way to quickly establish regularity of P waves and QRS complexes, and to determine if a relationship between P waves and QRS complexes exists.

A laddergram simply follows the path of electrical activity through the conduction system. It is broken down into 3 basic tiers: i) the Atrial Tier (the SA node is not shown but implied at the top of this tier); ii) the AV Nodal Tier; and iii) the Ventricular Tier (Figure-3):
  • The element of time occurs horizontally — and corresponds to the passage of time on the ECG tracing.

Figure-3: The framework for our laddergram has been added to Figure-2. Note atrial, AV nodal and ventricular tiers. Vertical lines in the atrial tier have been drawn in to represent all P waves seen on this tracing.

We begin our laddergram by drawing in elements that we KNOW to be correct. Most often — this entails atrial activity:
  • Conduction is fast through the atria. This is illustrated by a vertical red line drawn in the atrial tier corresponding to the occurrence of each P wave (Figure-3).
  • Conduction is slowest through the AV node. This explains why most of the PR interval is spent in the path of the electrical impulse as it travels through the AV node. The KEY to learning to draw laddergrams is to leave the AV Nodal tier for last!
  • We therefore proceed next to drawing in the ventricular tier. We do this with a slightly inclined line — to acknowledge that conduction through the ventricles takes a bit longer than through the atria (Figure-4). Because of the uncertain nature of the early beats in this tracing (beats #2,5,8) — We have decided to save these beats for last. Note in Figure-4 that the inclined lines in the ventricular tier each correspond to a sinus-conducted complex.

Figure-4: Slightly inclined lines have been drawn in to represent sinus-conducted beats in this tracing (beats #1; 3,4; 6,7; and 9,10).

Proceeding with what we KNOW to be true — We now draw in the AV nodal tier for the sinus-conducted beats from Figure-4. We do so by connecting the vertical line at the bottom of the atrial tier for beats #3,4; 6,7; and 9,10 — with the point at the top of the ventricular tier for each of these beats where ventricular conduction begins (Figure-5).

Figure-5: Conduction through atria-AV node-ventricles for each sinus beat has now been completely drawn in.

It is now time to "solve" the laddergram. We accomplish this by filling in the AV nodal tier for those beats we had been uncertain about. It should be emphasized that we are not necessarily certain about the mechanism of the arrhythmia at this point (If we were, we wouldn’t be doing this laddergram). Instead — We are looking for a plausible theory to explain the problematic elements of the rhythm strip. In this case — this entails explaining: i) WHY beats #2,5,8 occur early; ii) HOW the P waves immediately following these beats can occur right on time; and iii) WHY no QRS complex is seen after these non-conducted P waves.

NOTE: If you are drawing your laddergram on paper it is best to initially use pencil rather than pen (just like you would if doing a sudoku puzzle) since you may need to erase and try more than one theory until you arrive at plausible explanation for arrhythmia events.

We presume that early-occurring beats #2,5,8 must be PJCs (Premature Junctional Contractions). These beats are early — they are supraventricular (narrow QRS) — and they are not preceded by premature P waves.
  • PJCs arise from the AV nodal tier. We represent PJCs by an open circle in the AV nodal tier that starts slightly before the QRS of the PJC (Figure-6).
  • Note on the rhythm strip that each early beat looks slightly different compared to the sinus-conducted beats (PJCs are not as tall and somewhat wider). Thus, PJCs are conducted with some degree of aberration. We represent this on the laddergram by adding a dotted line to conduction in the ventricular tier (See beat #5 in the ventricular tier of Figure-6).

Figure-6: We presume beats #2,5,8 are PJCs which conduct with some aberration. Open circles within the AV nodal tier represent the site of origin of these PJCs. We depict aberrant conduction for beat #5 on the laddergram by addition of a dotted slanted line.

In Figure-7 we complete our illustration of aberrant conduction for each of the PJCs (beats #2,5,8) on the laddergram.
  • We KNOW that the on-time P waves occurring after each PJC are not conducted.
  • We illustrate this non-conduction in Figure-7 just for the P wave immediately after beat #5 (addition of a “butt” in the AV nodal tier for the P wave after beat #5).
Figure-7: The P waves after each PJC are non-conducted. We illustrate this with a “butt” ending to atrial conduction for the P wave immediately after beat #5.

We now need to address WHY the P wave after each PJC is non-conducted. We do so by invoking the concept of concealed conduction which means events not visible on the surface ECG must be occurring that physiologically alter the expected patterns of conduction.
  • Figure-8 illustrates our theory. The PJC for beat #5 conducts retrograde (dotted line within the AV nodal tier) as well as forward. In so doing — it renders tissues above it  temporarily refractory. This prevents conduction of next sinus impulse.

Figure-8: Retrograde conduction from PJCs prevent conduction of the next on-time sinus impulse.

Figure-9 completes this exercise — as we draw in retrograde conduction for each PJC that accounts for non-conduction of each subsequent on-time sinus impulse.

Figure-9: Completion of the laddergram.

BOTTOM Line:  Isn’t it now obvious from looking at Figure-9 that we can instantly know what is happening (and why the on-time P waves following beats #2,5,8 are not conducting)?

  • Our THANKS to Yong Chuan Chee (from Penang, Malaysia) for allowing us to use his clinical case and tracing.

  • My ECG Blog #188 — is entirely devoted to Laddergrams (including illustrative ECG Video Pearl on this subject + numerous links to other blog posts with laddergram examples). 

Friday, June 7, 2013

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? (See text).

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.

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).

     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).

     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.

     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).

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