RESUMO
Alterations in the sequence of ventricular activation (left bundle branch block occurring spontaneously or induced by atrial pacing; right ventricular pacing) were studied and shown to induce two opposite changes in the order of ventricular repolarization. One, which causes classic secondary T wave changes, occurs and disappears instantaneously, and is proportional in magnitude to the QRS complex but of opposite direction. The other, which requires a long time to reach its maximal effect and to be dissipated, shows the same direction as the abnormal QRS forces but becomes apparent only when normal activation is restored. The former is a well known consequence of prolongation of the activation time, but the latter appears to be modulated by electrotonic interactions occurring during cardiac activation, in such a way that repolarization is delayed in sites where depolarization begins, and accelerated in sites where depolarization terminates. Our study suggests that electrotonically modulated T wave changes show accumulation and memory, and may persist for days or weeks after the provoking stimulus (the change in the activation sequence) is discontinued. The fact that any shift of the activation sequence may produce "pseudoprimary" T wave changes that may persist long after cessation of the triggering factor has wide clinical implications. Electrotonic modulation of ventricular repolarization is a basic electrophysiologic mechanism that may account for several features of normal and abnormal T waves, and for the possible existence of a heart memory.