In ventricular tachycardia, atrial electrical activity may be not dissociated from ventricular one if retrograde ventricular-atrial (V-A) conduction occurs, as it happens in about one half of cases. The V-A ratio may be 1 (every QRS complex is followed by a retrograde P wave) or less than 1 when some ventricular impulses are not conducted to the atria. In a wide QRS complex tachycardia, a QRS/P ratio >1 (more QRS complexes than P waves) demonstrates the ventricular origin of the arrhythmia [6–8, 12], (Fig. 6.3), whereas a 1:1 ratio does not permit any definite conclusion since P waves may (a) express a supraventricular tachycardia with 1:1 A-V conduction or (b) represent the retrograde atrial activation during ventricular tachycardia. If analysis of P wave configuration is possible, a main P vector directed inferiorly demonstrates supraventricular origin of the arrhythmia, whereas a P vector directed superiorly (negative P waves in the inferior leads) does not permit any conclusion since not only VT but also several supraventricular tachycardias share a retrograde activation of the atria. In some cases of VT, however, retrograde P waves appear as positive in the inferior leads, a phenomenon that has been called “the illusion of retrograde positive P waves” (Fig. 6.4) [8, 13, 14].

The presence of narrow, or relatively narrow, beats during wide QRS tachycardia suggests a diagnosis of VT, provided that narrow complexes are preceded by a P wave with an interval consistent with anterograde A-V conduction of the impulse. The narrow, or less wide, complexes are capture or fusion beats that occur whenever, during VT, a sinus or supraventricular impulse succeeds in reaching the ventricles, whose depolarization is due totally (capture) or partially (fusion) to that impulse (Fig. 6.5). Capture and fusion beats are reliable signs of VT, but they are rare (4 % in a study based on 96 cases of proved VT [10]) and can be observed only in the presence of A-V dissociation. Since the latter phenomenon is, in itself, a clear sign of VT, the further help provided by capture and fusion beats is trivial, provided that these never occur whenever the heart rate is very high, being the A-V node made always refractory by ectopic ventricular impulses [6, 8].

A “concordant” QRS morphology in all the precordial leads, namely, ventricular complexes totally negative (QS pattern) or positive (R or qR pattern), demonstrates a ventricular origin of tachycardia, since no intraventricular conduction disturbance can result in such a configuration [15]. Concordance, however, cannot be diagnosed if rS, Rs, or rs complexes occur even in one single precordial lead. In a study based on 232 electrocardiograms with bundle branch block analyzed during sinus rhythm, none showed precordial concordance, suggesting a 100 % specificity of this sign indicating the ventricular origin of the arrhythmia [16]. Negative concordance (QS morphology in all precordial leads, Fig. 6.6), however, is specific of VT, whereas positive concordance could be observed, although rarely, in a preexcited tachycardia due to a left-sided Kent bundle [7, 8]. Negative concordance in the bipolar limb leads (I, II, III) has also been proposed as a specific pattern suggesting VT; [17] such a configuration demonstrates an extreme right axis deviation, a phenomenon that never occurs in adults, apart from some cases of congenital heart disease or dextrocardia.

In several cases of VT, none of the precordial leads shows ventricular complexes with a configuration characterized by an R wave followed by an S wave (rs, RS, rS, or Rs). This sign, expressing in a slightly different manner the concept of “precordial concordance,” suggests a ventricular origin of the arrhythmia. The sign specificity was 100 % both in the original study [5] and in another research based on 133 patients with wide QRS tachycardia; [10] in a different series, however, specificity was 81 and 98 % in the presence of positive and negative precordial QRS complexes, respectively [16].

It has been observed that whenever, in a wide QRS complex tachycardia, the interval from QRS complex beginning to S wave nadir exceeds 100 ms in a precordial lead, tachycardia is ventricular in origin [5]. The above criterion was fulfilled in 41 % of patients with previous myocardial infarction and VT [18]. In subjects with slowed down intraventricular conduction, however, leads V4–V6 show at times the abovementioned sign even during sinus rhythm, particularly in the presence of left axis deviation. In a study based on electrocardiograms with left bundle branch block and sinus rhythm, 34 % of cases had an interval from QRS complex beginning to S wave nadir >100 ms [16], demonstrating a low specificity of this sign in revealing VT.

In the presence of QRS wide complex tachycardia, vagal stimulation can result in the following responses:

An ECG recorded in the absence of tachycardia can be at times helpful, since a conduction disturbance or preexcitation observed during sinus rhythm can be the key to recognize the mechanism underlying the wide QRS complexes. In the great majority of cases, however, no ECG recorded during sinus rhythm is available at the moment of the arrhythmic emergence. Whenever the ECG during tachycardia is identical to that obtained in sinus rhythm, the arrhythmia is supraventricular in origin, apart from a single exception: the bundle branch reentry tachycardia [19]. In the latter condition, a tracing in sinus rhythm can be misleading, since it suggests a supraventricular, rather than ventricular, origin of the arrhythmia [19].

Analysis of QRS complex configuration represents an important tool in wide QRS complex tachycardia. Whenever other diagnostic signs (A-V dissociation, capture and fusion beats, precordial concordance, etc.) are either absent or controversial, the distinction between supraventricular and ventricular tachycardia lies on morphologic analysis of ventricular complexes, taking particularly into account leads V1 and V6. The 1st step is tachycardia classification based on QRS morphology in lead V1: whenever ventricular complex is mainly positive in this lead, tachycardia will be defined as “RBBB type,” whereas if in that lead ventricular complexes are negative, tachycardia will be classified as “LBBB type.” In any situation, the leads to be analyzed are V1 and V6.