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 (DeltaAPD=left-right ventricular monophasic action potential duration: APD) is associated with TdP, therefore we tested the hypothesis that the ectopic beats contributes to DeltaAPD. METHODS: In 17 anaesthetized dogs with chronic AV-block, which showed spontaneous TdP after class III medication, APD was analyzed to 1. quantitate the alterations due to (multiple) ectopic beats on the left and right APD (measured with endocardial catheters) and 2. compare the DeltaAPD prior to the occurrence of premature beats (steady state) 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 ectopic beats, phase II: the beat(s) belonging to the dynamic phase, and phase III: the beat causing TdP. Because the coupling interval of premature beats in this condition often falls within the APD, the DeltaAPD(50) was validated as an alternative for the previously applied DeltaAPD(100) (r=0.51, P<0.01). RESULTS: In steady state (phase I) DeltaAPD(50) is longer in the sudden 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 heterogeneity in APD, i.e. LV-APD increases more than RV-APD leading to a DeltaAPD(50) increase to 130+/-100 ms (P<0.01) just preceding TdP (phase III). CONCLUSION: The synergism between ectopic beats (short-long-short sequence) and DeltaAPD create the circumstances for TdP initiation.