Abnormal Response to Exercise, Face Immersion, and Isoproterenol in Children with the Long QT Syndrome

The present study was performed to observe the change of QT interval by sympathetic stimulations in patients with the long QT syndrome (LQTS). The study group consisted of 6 children with LQTS and 6 healthy children without QT prolongation. All LQTS patients had syncopal episodes. The QT_c and ΔQT_c% ([QT_C interval after examination - QT_c interval at rest]/QT_c interval at rest × 100) by treadmill testing, face immersion, and isoproterenol were examined. One minute after peak exercise of treadmill testing, the changes in the QT_r, interval were not significant in either group, but ΔQT_C% was larger in the LQTS group than in the control group (+ 11.0 ± 12.1% vs -2.6 ± 3.2%;P= 0.02). The QT_c interval at the shortest RR interval during face immersion was prolonged in the LQTS group (0.47 ± 0.01 s to 0.51 ± 0.04 s; P = 0.02), but there were no significant changes in the control group (0.40 ± 0.03 s to 0.41 ± 0.03 s; P = NS). ΔQT_C% was larger in the LQTS group than in the control group (+ 10.0 ± 7.3% vs +1.1 ± 5.5%; P = 0.04). In the LQTS group, the RR interval was shortened (P = 0.009) and QT_c interval was prolonged (P = 0.0008) after isoproterenol infusion. These sympathetic stimulations amplified the TU abnormality in the LQTS group. By observing the TU changes caused by face immersion, we hoped to find a possible new method with which to diagnose LQTS. The combination of these examinations may be helpful in screening the borderline cases of TU abnormalities.