The lead II rhythm strip shown in the figure below begins with three sinus-conducted beats. There follows a run of a wide complex tachycardia (WCT). How certain are you that the run of WCT that begins with beat #4 is ventricular tachycardia (VT)?
The first three beats in the figure are sinus-conducted. The PR interval is upper normal at 0.20 seconds. The P-P interval changes slightly, which means 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 three beats. There is no reason for aberrant conduction to occur beginning with beat #4 because beat #4 occurs late in the cycle, at a time by which conduction properties that lead to aberrancy should have resolved. Instead, we can say 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 > 90% likelihood. Consideration of clinical details (i.e., history of underlying heart disease and/or prior documented VT episodes), together with morphologic ECG features, often can increase certainty of our diagnosis beyond this level.
Beat #4 is a fusion beat. Note that the PR interval preceding beat #4 is shorter than the PR interval preceding each of the three sinus-conducted beats. 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 upright like the wide run that follows, but this beat is not quite as wide, nor is its negative T wave as deep because there is fusion (simultaneous occurrence) of supraventricular and ventricular activation.
AV dissociation also appears 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 appears to notch the ST-T wave just after beat #5. But since these P waves do not conduct normally, there is AV dissociation. The abrupt onset of a different wide run with fusion beats and AV dissociation provides indisputable proof that the rhythm in the Figure is VT.