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Background Congenital long QT syndrome (LQTS) is an inherited ion channel disorder resulting in abnormal cardiac repolarization that can cause syncope and sudden death associated with a prolonged rate-corrected QT interval and polymorphic ventricular tachycardia. Several studies in adults showed that LQTS patients have altered QT adaptation to heart rate changes compared with normal subjects which forming a "hysteresis loop" in the QT-circle length plot. This study was to observe the QT interval changing during exercise testing in children long QT syndrome (LQTS) patients,explore the new diagnosis methods of LQTS.Methods The subjects were divided into 3 groups according to 1993 LQTS diagnostic criteria. Group 1: LQTS group(n=17) who scored ＞ or = 4 points indicating definite LQTS. Group 2: Middle group (n=16), patients who have prolonged QT interval but scored 1.5 to 3.5. Group 3: Normal control group (n=18). The average age of all study population is (12.3±5.8) years. No case had beta-adrenergic antagonists administration before exercise testing. All subjects were underwent tread mill exercise testing and electrocardiograph in whole exercise testing and recovery were recorded. QT and heart rate changing during whole exercise testing period were recorded. △QT, the QT interval at 1, 2, 4, 6 minutes into recovery subtract from the QT interval at a similar heart rate during exercise, were calculated.Results In all three groups, QT intervals were shortening with the increasing of heart rate, but QTc had no significant change. △QT at 1 minute ((45±11) ms), 2 minutes ((37±15) ms), 4 minutes ((23±12) ms) into recovery in LQTS group were significantly greater than that of the other two groups (P＜0.05, P＜0.01, P＜0.01, respectively). There was no△QT significant difference between middle group and normal control group at recovery time. During the recovery phase in LQTS group, the QT interval remained shortened despite a decelerating heart rate, forming a hysteresis "loop" in the curve relating the QT interval to the cycle length.Conclusions In children LQTS patients, there is significant QT hysteresis loop in the relation of QT interval with heart rate during recovery of exercise testing, which could be useful to the early diagnosis for LQTS.