ECG Blog #129 — Late-Cycle: How Many Children?

How would you interpret the lead V1 rhythm strip shown in Figure-1?

• How certain are you of your diagnosis?
• How would you describe this rhythm in words?
• Why does beat #8 look so different from all other beats in this tracing?
• What clinical situation is commonly associated with arrhythmias such as the one shown here?

Figure-1: Long lead V1 rhythm strip showing a changing rhythm. Can you explain what is happening? 

PEARL: 3 of the most Helpful Steps for facilitating interpretation of the mechanism of complex arrhythmias are: i) To look first for an underlying rhythm; then, ii) To mark the presence of sinus P waves that you can clearly see; and, iii) To use calipers (as by far the fastest, easiest, and most accurate way to seek out atrial activity). We have done this in Figure-2:

Figure-2: We have marked (with RED arrows) the presence of sinus P waves that we clearly saw in Figure-1 (See text).

Interpretation: It should now be apparent that the underlying mechanism of the rhythm in Figure-2 is sinus. This is true even though the number of sinus-conducted beats in this tracing is limited. Nevertheless, similar-shaped sinus P waves with a constant and normal PR interval precede beats #1, 3, 6, 7, 9, 11 and 13.

• To determine the rate of the underlying sinus rhythm — We look for 2 consecutive sinus-conducted beats. This occurs for beats #6 and 7 — which tells us that the underlying sinus rate is just under 100/minute (because the R-R interval between beats #6-7 is just over 3 large boxes in duration).
• Beats #2, 4, 5, 10 and 12 are wide. These beats are either not preceded by any P wave — or preceded by an on-time P wave that notches the very beginning of the QRS complex with a PR interval that is too short to conduct. Therefore these beats must be ventricular in etiology. We call these beats “PVCs”  ( = Premature Ventricular Contractions) — even though they occur relatively late in the cycle (usually just before the next on-time sinus P wave is able to conduct). So these ventricular beats do occur “early” (ie, all of them except perhaps beat #5 occur before the next on-time sinus-conducted QRS complex would be seen) — but barely so. As a result, these late-cycle PVCs are also known as end-diastolic PVCs.
• Clinically — this late-cycle feature of the PVCs seen in Figure-2 is similar to the phenomenon of AIVR (Accelerated IdioVentricular Rhythm), in which a ventricular rhythm at a slightly accelerated rate (usually between 60-110/minute) is seen in patients with recent acute infarction who have just reperfused the infarct-related artery. Note that this is the picture we see for ventricular beats #4 and 5, which if they were followed by additional ventricular beats at similar R-R interval spacing, would constitute AIVR at a rate of ~75/minute. 
• NOTE: Although we are given no clinical information about this patient — the finding of a bigeminal pattern of end-diastolic PVCs with 2 consecutive ventricular beats at a rate consistent with AIVR is characteristic enough to prompt consideration of the possibility that the rhythm in Figure-2 might represent a reperfusion rhythm!

What about Beat #8?

We save assessment of beat #8 for last — since the explanation for why this beat looks different from all others in Figure-2 might not initially be apparent.

• This illustrates the 4th Helpful Step we favor for interpretation of virtually any complex arrhythmia = iv) Save assessment of the more challenging part(s) of any given tracing for last, waiting until after you are able to explain the more easily interpretable parts of the tracing.
• We KNOW beats #1,3,6,7,9,11 and 13 in Figure-2 are normally-conducted sinus beats.
• We KNOW beats #2,4,5,10 and 12 are ventricular beats.
• What if a ventricular beat and a sinus-conducted QRS complex got together to “have children”. What would the QRS complex and the T wave of the children look like? Wouldn’t such a beat look like beat #8?
• Beat #8 is a Fusion Beat. Conditions for fusion are present — in that beat #8 is preceded by an on-time P wave of similar shape as other sinus P waves, but with a shorter preceding PR interval.

Illustration of what is happening with beat #8 is facilitated by use of a Laddergram (Figure-3):

Figure-3: Laddergram illustrating the reason for the short PR interval preceding beat #8 (See text).

Explanation of Figure-3: We discussed and illustrated the phenomenon of Fusion in our ECG Blog #128. The key to distinguishing a fusion beat from an aberrantly conducted PAC (Premature Atrial Contraction) — is that the P wave preceding a fusion beat is on-time and not premature.

• As can be seen in the laddergram shown in Figure-3 — the PR interval of the preceding P wave (RED arrow just before beat #8) is shorter than the PR interval preceding pure sinus-conducted beats. This is because this P wave is only able to partially penetrate through the ventricles before meeting up with an oppositely directed wavefront originating from a near-simultaneously occurring ventricular beat.
• The result is a fusion beat, with a QRS complex and T wave that looks intermediate between sinus-conducted beats and pure ventricular beats. Verify this yourself. Fusion beat #8 manifests a similar upright shape as do other ventricular beats — but the QRS is not as wide, and the negative ST-T wave not as deep because fusion with a narrow, predominantly negative sinus beat (similar to that seen for beat #7) counteracts the QRS/T wave appearance of ventricular beats.
• Bottom Line — Although we have already conclusively proven that beats #2,4,5,10 and 12 in Figure-3 are ventricular in etiology — recognition that beat #8 is a fusion beat provides yet one more sign of a definitive ventricular etiology.

A Few Final Points (Beyond-the-Core): It is interesting how QRS morphology of the ventricular beats seen in Figure-3 changes with regard to the single or double peaking of each R wave. We are not sure of the reason for this, other than our awareness that ventricular beats from the same focus may sometimes vary in morphology. This could be because the pathway through the ventricles is not always exactly the same — or perhaps in this case, by the fact that several ventricular beats in Figure-3 are seemingly altered by an on-time P wave that deforms the initial upslope of the R wave. Some degree of fusion might be occurring in a few of these beats, accounting for slight variation in QRS morphology. Practically speaking — none of this matters, since the “theme” of this arrhythmia remains clear — namely, an underlying sinus rhythm with an intermittent bigeminal pattern of late-cycle PVCs, as well as one ventricular couplet with a relatively long R-R interval.

• Although the P-P interval of sinus P waves is quite regular during the latter portion of this tracing (RED arrows in Figure-3) — we have to acknowledge that we lose track of P waves earlier in the rhythm strip — and, we do not see any P wave occurring during the relatively long R-R interval between beats #4-5. I do not know why. That said, this also is unimportant clinically — as the “theme” of this arrhythmia (just stated above) remains clear. One cannot always explain all findings that appear on every ECG — and, one does not always have to try ...
• Finally — despite what superficially looks like a short PR interval with initial delta wave slurring preceding some of the wide beats — this is not WPW! That’s because the QRS complex remains wide for beats such as #4 and 5, which are not preceded by either P waves or by any initial slurring. In addition, the coupling interval preceding beats #8 and 10 is different — whereas it should be the same in WPW with variable preexcitation (Concertina effect).

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Concluding NOTE:

This case provides a superb example of how to apply the 4 Helpful Steps I find most useful for facilitating interpretation of complex arrhythmias. Recognition that the underlying rhythm is sinus, with a late-cycle pattern of ventricular bigeminy plus one definite fusion beat — are the key points to appreciate.

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Additional Reading: — The links below may be of interest regarding selected concepts discussed in this case:

• For more on Fusion Beats — Please see my ECG Blog #128 —
• For more on Laddergrams — Please see my ECG Blog #69 —
• For more on the Concertina Effect with WPW — Please CLICK HERE for the link to the brief commentary by Singla et al. —

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