The phenomenon of “QT stunning”: The abnormal QT prolongation provoked by standing persists even as the heart rate returns to normal in patients with long QT syndrome

Background Patients with long QT syndrome (LQTS) have inadequate shortening of the QT interval in response to the sudden heart rate accelerations provoked by standing—a phenomenon of diagnostic value. We now validate our original observations in a cohort twice as large. We also describe that this abnormal QT-interval response persists as the heart rate acceleration returns to baseline. Objectives To describe a novel observation, termed “QT stunning” and to validate previous observations regarding the “QT-stretching” phenomenon in patients with LQTS by using our recently described “standing test.” Methods The electrocardiograms of 108 patients with LQTS and 112 healthy subjects were recorded in the supine position. Subjects were then instructed to stand up quickly and remain standing for 5 minutes during continuous electrocardiographic recording. The corrected QT interval was measured at baseline (QTcbase ), when heart rate acceleration without appropriate QT-interval shortening leads to maximal QT stretching (QTcstretch ) and upon return of heart rate to baseline (QTcreturn ). Results QTcstretch lengthened significantly more in patients with LQTS (103 ± 80 ms vs 66 ± 40 ms in controls; P