Observations on the onset of Torsade de Pointes arrhythmias in the acquired long QT syndrome

Objective: Premature ectopic beats may create a specific sequence of events (e.g, short-long-short) preceding Torsade de Pointes arrhythmias (TdP) in the long QT syndrome. The relevance of this sequence for the initiation of TdP is not clear. In our dog model of TdP, interventricular dispersion (Del ta APD=left-right ventricular monophasic action potential duration: APD) is associated with TdP, therefore we tested the hypothesis that the ectopic b eats contributes to Delta APD. Methods: In 17 anaesthetized dogs with chron ic AV-block, which showed spontaneous TdP after class III medication, APD w as analyzed to 1. quantitate the alterations due to (multiple) ectopic beat s on the left and right APD (measured with endocardial catheters) and 2. co mpare the Delta APD prior to the occurrence of premature beats (steady stat e) in dogs with non-sudden onset of TdP (n=10) and sudden onset TdP (n=7). Three phases were distinguished: phase 1: steady state beats prior to ectop ic beats, phase II: the beat(s) belonging to the dynamic phase, and phase I II: the beat causing TdP. Because the coupling interval of premature beats in this condition often falls within the APD, the Delta APD(50) was validat ed as an alternative for the previously applied Delta APD(100) (r=0.51, P<0 .01). Results: In steady state (phase I) <Delta>APD(50) is longer in the su dden onset TdP (130+/-35 ms) as in the non-sudden onset TdP (65+-40 ms). In the non-sudden TdP group the dynamic phase II contribute to the heterogene ity in APD, i.e. LV-APD increases more than RV-APD leading to a Delta APD(5 0) increase to 130+/-100 ms (P < 0.01) just preceding TdP (phase III). Conc lusion: The synergism between ectopic beats (short-long-short sequence) and <Delta>APD create the circumstances for TdP initiation. (C) 2000 Elsevier Science B.V. All rights reserved.