Abstract
Delay of QT interval adaptation to sudden heart rate change causes hysteresis in dynamic QT-RR relationship. We analyzed QT-RR plotting during and after exercise in a patient with genetically identified type 2 long QT syndrome before and after starting oral propranolol. Blunted QT shortening by exercise and augmented postexercise QT prolongation resulted in an open-loop, clockwise QT-RR hysteresis immediately before the onset of torsades de pointes before propranolol. However, this hysteresis was eliminated by propranolol. QT-RR analysis provided insight into the mechanisms of the onset of torsades de pointes at least in this case of type 2 long QT syndrome.
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