onditions stated above as risk factors for ventricular tachycardia, one should be very prone to assume that it is ventricular tachycardia.
Characteristics of ventricular tachycardia are now discussed.
Atrioventricular (AV) dissociation
AV dissociation means that atria and ventricles function independently of each other. On the ECG this manifests as P-waves having no relation to QRS complexes (P-P intervals are different from R-R intervals, PR intervals vary and there is no relation between P and QRS). Note that it is often difficult to discern P-waves during VT (esophagus ECG may be very helpful). If AV dissociation can be verified, VT is very likely to be the cause of the arrhythmia. However, occasionally the ventricular impulses may be conducted retrogradely through the His bundle and AV node to the atria and depolarize the atria synchronously with the ventricles; thus VT may actually display synchronized P-waves. The following ECG shows VT with AV dissociation (the arrows point at P-waves).
Initiation of the tachyarrhythmia
If the start of the tachycardia is recorded it is valuable to assess the initial beats. If the R-R intervals during the start of the tachycardia were irregular, it suggests ventricular tachycardia. This is called warp-up phenomenon and is characteristic of ventricular tachycardia. Supraventricular tachycardias do not display warm-up phenomenon (with the exception of atrial tachycardia).
Initiation by premature atrial beats
Ventricular tachycardia is not induced by premature atrial beats, but supraventricular tachycardias typically do. If the start of the tachycardia is recorded, one must examine whether it was preceded by a premature atrial beat.
Fusion beats & capture beats
If a ventricular impulse is discharged simultaneously as the atrial impulse enters the His-Purkinje system, the ventricles will be depolarized by both. The resulting QRS complex will have an appearance resembling both a normal QRS and a wide QRS. Such beats are called fusion beats, and such beats are diagnostic of ventricular tachycardia. Figure 6 shows an example.
Occasionally during a ventricular tachycardia, the atrial impulse will break through and manage to depolarize the ventricles. This is seen as the occurrence of a normal beat in the midst of the tachycardia. Such beats are called capture beats and they are also diagnostic of ventricular tachycardia.
Ventricular tachycardia is mostly regular, although the R-R intervals may vary somewhat. Discrete variability in R-R intervals actually suggest ventricular tachycardia. However, polymorphic ventricular tachycardia may be irregular. Supraventricular tachycardias may also be irregular; the most common being atrial fibrillation. Note that pre-excitation during atrial fibrillation causes an irregular wide complex tachcyardia, with heart rate >190 beats per minute in most cases.
Previously existing bundle branch block
Individuals with previously existing conduction defects (right or left bundle branch block) or other causes of wide QRS complexes (pre-excitation, drugs, hyperkalemia) should have their ECGs during tachyarrhythmia compared with the ECG during sinus rhythm (or any earlier ECG). If the QRS morphology during the tachyarrhythmia is similar to the QRS complex in sinus rhythm, it is likely to be an SVT. Moreover, if the patient has recently had premature ventricular complexes, and the QRS during tachyarrhythmia resembles that of the premature ventricular complexes, then it is likely to be ventricular tachycardia.
Electrical axis between –90° and –180° strongly suggest ventricular tachycardia (although antidromic AVRT is a differential diagnosis). If the electrical axis during tachycardia differs >40° from the electrical axis durign sinus rhtyhm, it also suggest ventricular tachycardia. If the tachyarrhythmia has a right bundle branch block pattern but the electrical axis is more negative than –30° it suggests ventricular tachycardia. If the tachyarrhythmia has a left bundle branch block pattern but the electrical axis is more positive than 90° it suggests ventricular ta