KCNH2 and torsades de pointes: However, it has been well documented that: 1) hERG inhibition on its own does not always translate to the occurrence of TdP, 2) action potential duration (APD) prolongation in vitro is not always associated with the clinical prolongation of the QTc interval and, 3) a prolonged QTc interval does not always linearly correlate to the occurrence of TdP.