Atrial activity and ventricular tachycardia
The comparison between ventricular activation and atrial activation is a defining element for the interpretation of a wide QRS tachycardia tracing and requires careful analysis because of the frequent difficulty in identifying atrial depolarization, which can be superimposed on the ventricular signals (QRS complexes, T waves). In a significant number of cases, atrial activity is indistinguishable (tracing 1). During ventricular tachycardia, the relationship between ventricular activation and atrial activation is dependent on their respective rate and the possibilities for atrioventricular and ventriculoatrial conduction. Atrial activation can be completely independent of that of the ventricles or may be connected by retrograde conduction. We can thus observe:
- an atrioventricular dissociation with atrial sinus activity (tracing 2): this is the most common modality since it is observed in 80% of cases. The diagnosis of ventricular tachycardia is very probable, although it will be necessary to eliminate certain relatively rare diagnoses (Hisian or nodal tachycardia with ventricular aberration and retrograde block, tachycardia due to Mahaim fibers in anterograde direction and nodal block). The P waves are completely independent of the QRS complexes, are of sinus origin (positive in leads I, II, negative in aVR), and often have a relatively rapid rate (reflex sinus tachycardia as a result of the decrease in cardiac output generated by the ventricular tachycardia). The block responsible for the dissociation is usually nodal and functional even if potentially organic and permanent (absence of possible retrograde conduction). This block results from the collision of the ascending impulse propagating from the ventricle and the descending impulse propagating from the atrium and from a hidden conduction prohibiting the passage through the atrioventricular node both in anterograde and retrograde manner. An impulse is sometimes able to cross the atrial junction and partially (fusion complex) or totally (capture) activates the ventricles. The presence of ventricular or fusion captures is a formal criterion of ventricular tachycardia. A ventricular capture is reflected on the electrocardiogram by a premature QRS complex relative to the tachycardia cycle, following a sinusoidal P wave with a PR interval > 120 ms and usually reproducing the sinus QRS pattern. The pattern may be misleading if a conduction aberration occurs, especially if the bundle branch block is homolateral relative to the ventricular tachycardia circuit (complex with morphology similar to that of the tachycardia). When the capture is partial (fusion), the QRS complex has an intermediate pattern between that of the sinus complexes and that of the tachycardia complexes.
- atrioventricular dissociation and bitachycardia (tracing 3): the atria are the primary site of a tachycardia independent of that of the ventricles. Atrial activity (atrial fibrillation, atrial flutter) is often faster than that of the ventricle and is independent (lack of fixed relationship between atrial and ventricular activity). This type of presentation renders the differential diagnosis between bitachycardia and supraventricular tachycardia with conduction aberration difficult, the analysis being primarily based on the QRS pattern.
- retrograde atrial activation: corresponds to approximately 20% of ventricular tachycardias documented due to a rhythm previously sinus. Retrograde conduction may be present regardless of the rate of the tachycardia although rare for extremely rapid tachycardias and appears to be more frequent for short-lived tachycardias. When the conduction is of the 1:1 type (tracing 4), a P' wave (negative in the inferior leads) is identifiable behind each QRS with a P'R interval exceeding 120 ms and which can be prolonged. Retrograde conduction can also be carried out in 2:1 mode (tracing 5) or with retrograde Wenckebach periods (gradual prolongation of the P'R interval leading to a QRS complex not followed by a P' wave). When the retrograde conduction is relatively slow (RP' > 200 ms) or in the presence of a retrograde Wenckebach, there is sometimes an echo phenomenon with the succession of a large QRS complex, a retrograde P' wave with a long RP' interval and finally a second QRS complex of different morphology corresponding to a complete capture or a fusion.
If the retrograde conduction is of the 1:1 type, the differential dianosis with supraventricular tachycardia with broad QRS may often prove difficult, vagal maneuvers or injection of nodal conduction-depressant drugs potentially allowing the diagnosis if they induce dissociation.
- the modalities of atrial activation are sometimes mixed within a same patient with coexistence between passages with retrograde conduction and passages with dissociation.