- How certain are you of your diagnosis?
- Are the P waves preceding beats #6 and #7 conducting?
- Challenge Question: How many different-shaped beats are there on this tracing?
|Figure-1: Long lead II rhythm strip showing a changing rhythm. Can you explain what is happening? NOTE — Enlarge by clicking on the Figure.|
- To do this — Mentally block out the first 7 beats on this tracing. If ALL you had to worry about was the last 6 beats in Figure-1 (ie, beats #8-thru-13) — How would you interpret the rhythm in Figure-2?
|Figure-2: The first 7 beats from Figure-1 have been blocked out. How would you interpret the arrhythmia represented by beats #8-thru-13?|
- Now mentally block out the last 8 beats on this tracing. If all you had to worry about were the initial 5 beats — How would you interpret the arrhythmia in Figure-3?
|Figure-3: The last 8 beats (#6-thru-13) from Figure-1 have been blocked out. How would you interpret the arrhythmia represented by beats #1-thru-5?|
- Return to Figure-1. Now focus on the more difficult part of the tracing = beats #5-thru-8. Can you figure out what is going on in Figure-4?
|Figure-4: Beats #1-thru-4 and #9-thru-13 from Figure-1 have been blocked out. Can you figure out what is happening with the remaining beats #5-to-8?|
- What kind of beat is beat #8? (See Answer to Figure-2).
- What kind of beat is beat #5? (See Answer to Figure-3).
- Would you expect the P wave preceding beat #6 to be able to conduct normally? If not — Why not?
- Think of beats #5 and #8 as “parent beats”. If these parent beats (#5 and #8) were to mate (ie, combine) and “have children” — What would you expect the children to look like?
- Note that the PR interval preceding beat #6 is shorter than the PR interval preceding other sinus-conducted beats (beats #8-thru-13 in Figure-1). It is too short to conduct normally.
- Note also that although the QRS complex of beat #6 is entirely upright — it is not nearly as wide as the other upright (ventricular) beats (beats #1-thru-5 in Figure-1). Beat #6 is a Fusion Beat.
- Panel A in Figure-5 — schematically shows the pathway of normal conduction (SA Node–to–AV Node – to bundle branches). This results in a sinus-conducted beat (S) with a normal PR interval and a narrow QRS complex.
- In contrast, Panel B — begins in the ventricles (V). This results in a wide QRS complex without preceding atrial activity.
- The phenomenon of Fusion is represented in Panel C — in which there is simultaneous (or near simultaneous) occurrence of a supraventricular and ventricular complex. Depolarization wavefronts meet before they are able to complete their path, and the ECG appearance of the resultant fusion beat takes on characteristics of both the supraventricular and ventricular complex (F).
|Figure-5: Illustration of the concept of fusion beats. Panel A — Sinus-conducted beat (S). Panel B — Ventricular beat (V). Panel C — Fusion beats (F1 and F2).|
- Clinically — the reason recognition of fusion beats is important, is that it proves anomalous complexes in a tracing must be of ventricular etiology!
- Close inspection just before widened beat #5 reveals a subtle-but-definite small hump at the onset of the R wave of this beat. This small hump is almost certainly one more P wave (BLUE arrow) — that occurs right on time (ie, at the appropriate P-P interval distance just before the last RED arrow). No sinus P waves are seen before this blue arrow ...
|Figure-6: Long lead II rhythm strip taken from Figure-1. We have labeled the regularly-occurring sinus P waves that are clearly seen with RED arrows. The BLUE arrow indicates yet one more on-time P wave that deforms the initial part of beat #5.|
- The appearance of beats #6 and #7 in Figure-6 is as might be anticipated considering the PR interval that precedes each of these fusion beats. That is, the very short PR interval preceding beat #6 would not be expected to allow sufficient time for deep penetration of the supraventricular impulse (P wave) into the ventricles. Thus, beat #6 much more closely resembles the beats of ventricular etiology.
- In contrast — the PR interval preceding beat #7 is almost normal. As a result, this supraventricular impulse (P wave) should have had time to travel relatively far down the conduction system before fusion occurred (explaining why the beat more closely resembles the morphology of supraventricular beats).
- KEY POINT — Clinically, recognition that beats #6 and #7 in this tracing are fusion beats confirms the ventricular etiology of beats #1-thru-5.
- PEARL — One looks for fusion beats not only by examining the QRS complex — but also by close inspection of each T wave!
- Careful inspection of beat #5 reveals that its R wave is not quite as tall and its T wave not quite as deep as the other ventricular beats. Note also that the very initial portion of the upstroke of this R wave is deformed. A P wave is hiding here — and accounts for the slight degree of fusion that this beat manifests (BLUE arrow in Figure-6).
- Beats #8 and #9 are also fusion beats. Careful comparison of these beats with beats #10-thru-13 reveals that they have a slightly narrower QRS complex and, a T wave that is smaller and less peaked.