Diurnal variations of QT interval after cardiac transplantation

The QT interval decreases with increasing heart rate (HR) induced either by exercise or atrial pacing.1,2 Autonomic innervation appears to have a direct influence on the QT interval independent of the effect on HR, as the QT interval shortens after autonomic blockade during atrial pacing.3,4 The denervated transplanted heart offers a unique model for further clarifying QT changes in the absence of autonomic innervation. In a previous study of the QT variations in cardiac transplants, the Bazett formula was used to correct for HR. This application may be misleading after interventions with influence on both the QT interval and the HR.3 Browne et al5 have described a direct method for QT comparison under states of different autonomic tone and reported QT prolongation during sleep in normal subjects independent of HR change. This study defines the diurnal variation of the QT interval in the denervated transplanted human heart without the use of correcting formulas, compares directly the QT interval of the denervated heart with the normal heart during similar activity and assesses if factors other than HR and autonomic innervation influence diurnal changes in QT interval.