Spiral wave activity: a possible common mechanism for polymorphic and monomorphic ventricular tachycardias.

Several mechanisms have been proposed to explain the electrocardiographic patterns observed during various forms of polymorphic ventricular tachycardias, including torsades de pointes. Such mechanisms include the coexistence of either multiple foci or multiple exit pathways from single foci giving rise to various forms of aberrant ventricular activation sequences. For example, the simultaneous firing of two widely spaced foci at slightly different frequencies has been used to explain the undulating electrocardiogram that is characteristic of torsades de pointes. However, in spite of some supporting experimental evidence, such an idea remains conjectural from the clinical point of view. Here I discuss a mechanism that has been proposed recently to explain both monomorphic and polymorphic patterns (including undulating patterns) of ventricular tachycardia. The hypothesis is derived from the theory of spiral wave activity in excitable media, and from recent experiments using high resolution optical mapping in isolated two-dimensional ventricular muscle preparations that demonstrate that spiral wave activity may account for self-sustaining reentrant activation. Such studies have led to the observation that the behavior of the spiral center, the core, plays a key role in determining the electrocardiographic manifestation of the arrhythmia. Indeed, a stationary position of the core results in a monomorphic pattern of activation. On the other hand, beat-to-beat changes in the core position (i.e., drifting) leads to irregular patterns of activation. In fact, when drifting occurs in one direction, it gives rise to a Doppler shift in the excitation period in such a way that two coexisting frequencies are manifest, one ahead of and one behind the drifting core. The activation frequency in the region ahead of the core is always higher than that behind the core. Under such conditions, electrocardiographic recordings of the activity demonstrate an undulating pattern, which resembles that of torsades de pointes. When the core drifts in various directions, a polymorphic pattern is manifest. Thus, depending on spiral core dynamics, monomorphic, undulating, or completely irregular patterns may be observed. Moreover, transitions between such patterns can also occur. For example, drifting spirals giving rise to polymorphic activation can become stationary and result in monomorphic activation as a result of anchoring of the core to a small discontinuity (e.g., an artery or small scar) in the tissue. Direct extrapolation of such results to clinical cases is not appropriate. However, the observations discussed in this article offer a new testable hypothesis in which a common mechanism is postulated for the electrocardiographic patterns associated with monomorphic and polymorphic tachycardias.