Background: The long QT syndrome is a genetically determined disease based on mutations of ion membrane channel genes. The resulting prolongation of repolarization increases a risk of malig- nant ventricular arrhythmias and sudden death. The diagnosis is problematic in individuals with bordeline or even normal corrected QT interval value. Ventricular arrhythmias in this syndrome are often provoked by exercise, therefore exercise testing is considered as a useful differential diagnostic method. Methods: In a 24-member family an occurence of the long QT syndrome was established clinically in 7 patients. In other 3 members bordeline corrected QT interval values were found (0,44 - 0,46 s). In these individuals a stress testing by bicycle ergometry was performed. Results: During exercise in a 28-year old man (patient III/6) the corrected QT interval prolonged to 0,53 s, in a 27-year old man (patient III/2) and in a 20-year old woman (patient III/9) physiologi- cal QT interval shortenings were observed (0,4 s during peak exercise). The diagnosis was confir- med by molecular genetic investigation. In patient III/6 (along with other symptomatic family members) a mutation in exon 7 of KCNQ1 (previously called KVLQT1) gene was found. In patients III/2 and III/9 no signs of KCNQ1 gene pathology were present. Conclusions: In families with clinically established diagnosis of long QT syndrome occurrence the exercise testing reveals previously asymptomatic individuals, at least in LQT1 type (which is the most common). In general stress testing and measurement of QT interval dynamics is a necessary part of arrhythmological investigation especially in young individuals with history of syncope.

        Key words: Arrhythmias - Exercise testing - KCNQ1 - Long QT syndrome - Mutation