ECG Blog #68 — Interpolated and Concealed ...

     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.

------------------------------------------

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).

================================

- For more information — GO TO:

• See ECG Blog #69 for Review on How to Draw Laddergrams – 
• For more on Laddergrams — See Section 20.25 (in ACLS-2013-ePub).