Objective-To study the effect of sympathetic stimulation and increase in heart rate on the QT and QTc intervals. Design-Prospective non-randomised study of eight consecutive patients. Setting-Electrophysiology laboratory at a tertiary centre. Patients-Eight patients aged 10-20 years (median 12.5) undergoing repeat electrophysiological study after previously successful catheter ablation (n = 6) or presumed supraventricular tachycardia (n = 2) with negative studies. years) (six boys) were prospectively studied during electrophysiological (EP) evaluation. Six patients had previously undergone successful radiofrequency catheter ablation for supraventricular tachycardia, and the other two had EP studies for presumed supraventricular tachycardia but were found to be normal. All had normal cardiac structure by echocardiography. None had inducible arrhythmia at the time of the present study. Those who underwent ablations had their procedure performed through the atrial approach, and no lesions were placed on the ventricular myocardium. A previous study from our laboratory had shown no change in the repolarisation time (as measured by JT interval) in patients with WolffParkinson-White syndrome after successful ablation.5 Also, for this study, we used each patient as his or her own control. Informed consent was obtained from all parents of patients before the EP study.Twelve-lead electrocardiograms (ECG) were obtained at resting baseline and during right atrial pacing at 133 beats/min (cycle length 450 ms). An isoprenaline infusion was then begun at a dose of 0-025 ,g/kg/min. After an interval of about 5 min, when the heart rate was seen to have stabilised, a 12-lead ECG was obtained. Right atrial pacing was then performed again at the rate of 133 beats/min and, once again, after an interval of 5 min to allow stabilisation, another ECG was obtained. Finally, the dose of isoprenaline was increased to 0 05 ,ug/kg/min and, after stabilisation of heart rate, another ECG was performed. Pacing was not performed at the 0 05 ug/kg/min dose because the heart rate was often faster than 133 beats/min.The ECG recordings were obtained at normal filtering (0-100 Hz) run at 50 mm/s paper speed. In each recording, the RR interval and the QT interval were measured by hand, using calipers in six consecutive beats, and the mean value was calculated. The QT was measured in lead II as recommended by Moss et al.
A child with familial atrial standstill and a ventricular pacemaker had syncope due to atrial flutter that was treated by His-bundle ablation. Bradycardia protection alone may be insufficient in patients with atrial standstill.
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