BACKGROUND: Pathologic prolongation of QT interval is related to increased risk of arrhythmias. Changes of this parameter are influenced by many conditions, the most important is heart rate. Several formulas have been proposed for mathematical description of QT interval/heart rate relationship. The aim of this study was comparison of different QT interval correction formulas in families with congenital long QT syndrome (LQTS).

METHODS: In 28 members of 6 families with LQTS occurrence bicycle ergometry testings were performed. QT and RR intervals were measured before exercise, at peak exercise and in the 1st and the 6th minute of restitution. For QT interval correction single-parameter formulas by Bazett, Fridericia, Malik and Framingham study were used. In 3 families the results could be correlated with genetically proved diagnosis (KCNQ1 gene mutations in 2 families, HERG-KCNH2 gene mutation in the other).

RESULTS: In the described group the genetically established diagnosis of LQTS correlated at best with values obtained with correction by Bazett. All the mutation carriers were correctly identified only by this method. The Fridericia, Malik and Framingham formulas failed to identify 2 patients--mutation carriers (both KCNQ1 and HERG-KCNH2 mutations).

DISCUSSION: Because of simplicity the Bazett formula remains the most common method of QT interval correction. Moreover, in our study this formula appeared to be the most sensitive for clinical diagnosis of LQTS.

Keywords: Algorithms; Electrocardiography; Exercise Test; Heart Rate; Humans; Long QT Syndrome /genetics/; Mutation