EFFECT OF HEART-RATE ON QT INTERVAL IN CHILDREN AND ADOLESCENTS

Objective-To study the effect of sympathetic stimulation and increase in heart rate on the QT and QTc intervals. Design-Prospective non-rand omised study of eight consecutive patients. Setting-Electrophysiology laboratory at a tertiary centre. Patients-Eight patients aged 10-20 ye ars (median 12 . 5) undergoing repeat electrophysiological study after previously successful catheter ablation (n = 6) or presumed supravent ricular tachycardia (n = 2) with negative studies. Interventions-Elect rocardiograms were obtained (a) at baseline, (b) during atrial pacing at 450 ms cycle length, (c) during isoprenaline infusion at 0 . 05 mu g/kg/min, (d) adding atrial pacing (450 ms cycle length) to isoprenali ne at 0 . 025 mu g/kg/min, and (e) isoprenaline at 0 . 05 mu g/kg/min. Main outcome measures-QT and QTc intervals at each of the above menti oned stages. Results-The QT interval was reduced from a mean value of 350 ms to around 315-325 ms by each of the above manoeuvres. Correspon dingly, the QTc increased from a mean of 407 ms to around 445-470 ms. Pacing was as effective as isoprenaline in shortening the QT interval and prolonging the QTc intervals. Conclusions-Heart rate directly infl uences QT and QTc intervals in children and adolescents. The QT is sho rtened, but QTc is prolonged. Hence, reliance on the QTc alone could l ead to mistaken diagnosis of long QT syndrome.