from one foci spreads varyingly.
Locating the ectopic foci causing ventricular tachycardia
The ECG provides valuable information regarding the location of the ectopic foci causing the tachycardia. This is done by classifying ventricular tachycardias broadly as either “left bundle branch appearance” or “right bundle branch appearance”. Ventricular tachycardias with ECG waveforms reminding of a left bundle branch block (dominant S-wave in V1) originate in the right ventricle. The opposite is also true, namely that ventricular tachycardias reminding of right bundle branch block (dominant R-wave in V1) originates in the left ventricle. This might be useful in trying to decipher what the cause of the ventricular tachycardia may be. Figure 4 and Figure 5 below shows examples.
Distinguishing ventricular tachycardia from supraventricular tachycardias with wide QRS complexes
Occasionally supraventricular tachycardias (which mostly have normal QRS complexes, i.e QRS duration <0.12 seconds) may display wide QRS complexes. This might be due to concomitant bundle branch block, aberration, hyperkalemia, pre-excitation or side effect of drugs (tricyclic antidepressants, antiarrhythmic drugs class I). It is fundamental to be able to differentiate supraventricular tachycardias with wide QRS from VT and the reason for this is simple: VT is potentially life-threatening, whereas supraventricular arrhythmias rarely are. Hence, wide QRS complexes do not guarantee that the rhythm is ventricular in origin.
Fortunately, there are several characteristics that separate ventricular tachycardia from supraventricular tachycardias (SVT). These characteristics can be used separately or in algorithms (Which are easy to use) to determine whether a tachycardia with wide QRS complexes (often called wide complex tachycardia) is a ventricular tachycardia or an SVT. Before dwelling into these characteristics and algorithm it should be noted that 90% of all wide complex tachycardias are ventricular tachycardias! If the patient suffers from any of the conditions 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 ventr