Assessment of dynamic relationship between QT dispersion and RR interval in normal subjects

Little is known to what extent heart rate and its circadian variations can influence QT dispersion (QTd). Thus we studied the relationship between RR and QTd in 10 healthy subjects (6 M, 4 F mean age 32/spl plusmn/11 years). Beat-to-beat automated measurement of RR and QT was performed on a home-made analyzer. QTd was automatically measured for each beat on X, Y and Z leads. Mean hourly RR and QTd values were calculated for each 60 min period of 24-hour Holter recordings. A significant linear relationship was found between mean hourly RR and QTd values (r=.47; p=0.02). Hourly median RR values showed the expected circadian variation, with a nocturnal peak between 3 and 6 A.M. Similarly, there was a circadian rhythm in hourly median QTd values, with a prominent peak between the hours 2 and 3 A.M. In conclusion, in healthy subjects QTd is weakly correlated to RR and shows a circadian variation.