QT interval variability in panic disorder patients after isoproterenol infusions

Anxiety symptoms are associated with a marked increase in sudden cardiac death, suggesting an abnormality in the autonomic control of the heart. We examined the effects of sympathetic stimulation on QT interval variability in panic disorder patients by infusing the β-adrenergic agonist isoproterenol in 6 panic disorder patients and 11 normal subjects. The ECG signal was analysed before the infusion and after 5 min after the infusion was started. The outcome measures were the QT variability normalized for mean QT interval (QTvm) and the QT variability index (QTvi), a measure of QT variability normalized by the concomitant heart rate variability. Patients with panic disorder had more variability in QT interval duration than normal controls and this variability was increased further by sympathetic stimulation with isoproterenol. The isoproterenol-associated increase in QT interval occurred in controls in the absence of significant anxiety. However, on one of two measures, the increase in QT interval variability was greater in patients with panic disorder, suggesting a greater sensitivity to isoproterenol or to isoproterenol-induced anxiety.