Wednesday, December 21, 2016

ECG Blog #133 (Aberrant Conduction – VT – Fusion – AV Dissociation - Sinus)

The lead II rhythm strip shown in Figure-1 begins with 3 sinus-conducted beats. There follows a run of a WCT (Wide-Complex Tachycardia). How certain are you that the run of WCT that begins with beat #4 is VT (Ventricular Tachycardia)?
Figure-1: Lead II rhythm strip. How certain are you that the run of wide beats beginning with beat #4 is VT? NOTE — Enlarge by clicking on the Figure.
Interpretation: As stated, the first 3 beats in Figure-1 are sinus-conducted. The PR interval is upper normal at 0.20 second. The P-P interval changes slightly — so there is underlying sinus arrhythmia. QRS morphology then abruptly changes beginning with beat #4. The QRS widens, and is oppositely directed (all positive) compared to the narrow rS complexes of the first 3 beats. There is no reason for aberrant conduction to occur beginning with beat #4. This is because beat #4 occurs late in the cycle at a time by which conduction properties that lead to aberrancy should have long ago resolved. Instead we can state with 100% certainty that the run of wide beats beginning with beat #4 is VT:
  • The first principle is that abrupt onset of a regular (or at least fairly regular) wide rhythm of different morphology than sinus-conducted beats predicts VT with greater than 90% likelihood. Consideration of clinical details (ie, history of underlying heart disease and/or prior documented VT episodes) — together with morphologic ECG features can often increase certainty of our diagnosis beyond this level (See Below for MORE on this Subject).
  • Beat #4 is a fusion beat. Note that the PR interval preceding beat #4 is shorter than the PR interval preceding each of the 3 sinus-conducted beats at the beginning of this tracing. This means that something else must have happened to produce the oppositely-directed upright QRS complex of beat #4 — because the on-time sinus P wave preceding beat #4 simply did not have enough time to complete its conduction through the ventricles.
  • Fusion beats manifest QRS and ST-T wave morphology intermediate between the QRS and ST-T wave morphology of sinus-conducted beats and ventricular beats. Depending on how deep in the ventricles the sinus P wave is able to penetrate — the resulting QRS and ST-T wave will look more like sinus beats or ventricular beats. Beat #4 is an upright QRS complex (like the wide run of beats that follow) — but beat #4 is not quite as wide, nor is its negative T wave as deep as the beats that follow because there is fusion (simultaneous occurrence) of supraventricular and ventricular activation.
  • AV dissociation is also seen on this tracing, at least at the beginning of the run of wide beats. The P wave preceding beat #4 is on time. Note that another on-time P wave is seen to notch the ST-T wave just after beat #5 (Figure-2). But since these last two P waves do not conduct normally — there is AV dissociation. The abrupt onset of a different-looking WCT rhythm with fusion beats and AV dissociation provides indisputable proof that the rhythm in the last part of Figure-2 is VT.
Figure-2: Use of calipers allows us to easily establish that 2 additional on-time P waves are seen after the first 3 sinus-conducted beats. The red arrow highlights the 2nd of these last two on-time P waves, which notches the ST segment of beat #5. Since this final P wave is not conducting — there is (by definition) at least transient AV dissociation. Together with the fusion beat (beat #4) that manifests a QRS complex and ST-T wave intermediate between that of the preceding sinus beat (beat #3) and subsequent wide beats (beat #6 and thereafter) — we can say with 100% certainty that the run of wide beats beginning with beat #4 is VT.
PEARL: The clinical utility of recognizing fusion beats and/or AV dissociation that occurs in association with a WCT rhythm — is that either of these findings proves that the rhythm must be VT. That said, neither finding is commonly seen with faster forms of VT — because the rapid rate of the tachycardia often obscures atrial activity that is hidden within QRS complexes or the ST-T wave. This case illustrates an exception in that we can clearly identify both a fusion beat and transient AV dissociation at the onset of the WCT — and this allows us to be 100% certain of our diagnosis!
For More on this Subject:
  • See ECG Blog #42 for full discussion on assessment of the wide tachycardia.
  • See ECG Blog #128 and Blog #129 that review Fusion Beats in detail.

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