ECG Blog #136 — Run of Narrow, then Wide Beats

The rhythm in Figure-1 was observed as a previously healthy young adult was being treated for his “palpitations”. He was hemodynamically stable at the time. Interpret the rhythm.

• What happened? Should what we see be cause for alarm?

Figure-1: Lead II rhythm strip during treatment (obtained from a patient with palpitations). Should what you see be cause for alarm? 

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Note: Discussion of this tracing is far easier if beats are labeled (Figure-2).

Figure-2: We have labeled all beats in Figure-1.

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Interpretation: The patient is hemodynamically stable. The first 9 beats show a regular SVT rhythm (SupraVentricular Tachycardia) at a rate between 185-190/minute. No atrial activity is seen during this run. The rhythm changes beginning with beat #10.

• After the run of SVT — it is easiest to look next at beats #11,12. Both beats are clearly ventricular in etiology — since the QRS complex is wider and completely different in appearance from QRS complexes during the SVT run at the beginning of the tracing.
• Beat #10 manifests an intermediate morphology between the narrow beats before it — and the ventricular couplet that follows. Beat #10 is a fusion beat — which means it is due to simultaneous occurrence of a supraventricular and ventricular beat. Therefore, beats #10-through-12 constitute a 3-beat salvo of ventricular tachycardia. PEARL: Recognition of fusion beats can be facilitated by looking not only to see if QRS appearance is intermediate between beats occurring before and beats that come after — but also by looking to see if ST-T wave appearance is intermediate! This clearly is the case here — as T wave shape and amplitude of beat #10 is indeed intermediate between T wave shape and amplitude of beats #9 and #11.
• Note that there is conversion to sinus rhythm beginning with beat #13. Another ventricular couplet follows (beats #14,15) — with the tracing ending in a regular sinus rhythm at a normal rate.

IMPRESSION: The rhythm in Figure-2 begins with a 9-beat run of AVNRT (AV Nodal Reentry Tachycardia). The rate of this SVT rhythm is too fast for atrial flutter with 2:1 conduction. It is also must faster than the usual rate range of sinus tachycardia. Abrupt conversion to sinus rhythm (beat #13) supports the diagnosis of AVNRT, which is a common cause of “palpitations” in the young adult age group.

• We do not know if conversion to sinus rhythm was achieved by a vagal maneuver, by medication — or by a combination of the two. Regardless — the point to emphasize is that it is a common and normal phenomenon to see PVCs (including ventricular couplets or salvos) at the time of conversion from a reentry tachycardia to sinus rhythm. Therefore, there is no cause for alarm from the rhythm in Figure-2, and assuming no other concerning features, additional workup would not be indicated at this time.

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For More on this Subject:

• See ECG Blog #128 and Blog #129 that review Fusion Beats in detail.
• See also these Sections from our ACLS-2013-ePub — Section 13.0 on SVTs of Uncertain Etiology — and Section 14.3 on SVT/PSVT/AVNRT.

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