Sinus nodal dysfunction in young patients with long QT syndrome. Although sinus bradycardia and low heart rate with exercise have been found in some patients with long QT syndrome, systematic evaluation, including intracardiac electrophysiologic (EPS) tests of sinus node function, has not been reported. Records were reviewed of 14 children and adolescents (age 3 to 16 years) with long QT syndrome (mean QTc 0.51 second) who underwent noninvasive testing and intracardiac EPS because of syncope or cardiac arrest. The resting electrocardiographic (ECG) sinus heart rate was low for age in only 1 of 13 patients, while the lowest Holter-monitored sinus heart rate was abnormal in 4 of 12. The maximum exercise heart rate was abnormally low in 6 of the 12 who underwent exercise testing. For the EPS tests, the corrected sinus node recovery time (CSNRT) was long in 8 of 14 and the sinoatrial conduction time (SACT) was long in six of the nine in whom it was calculable. When both noninvasive and EPS indices are considered, 13 of the 14 patients had some type of sinus node dysfunction. 