Frequency Domain Heart Period and QT Interval Variability Markers Are Linked to Arrhythmic Risk in Long QT Syndrome Type 2

Long QT syndrome type 2 (LQT2) is an inherited disease leading to arrhythmias and sudden death often in response to an abrupt sympathetic stimulus. Heart period (HP) and QT interval variability analyses were carried out over asymptomatic (ASYMP, n=10, age 39.0±11.0 yrs, 9 males) and symptomatic (SYMP, n=7, age 41.7±12.8 yrs, 4 males) LQT2 patients. Beat-to-beat series were derived from 24h Holter ECG recordings during daytime and nighttime. In addition to HP and QT mean and variance, we calculated HP power in high frequency (HF, from 0.15 to 0.5 Hz) band as a marker of vagal modulation directed to sinus node and QT power in low frequency (LF, from 0.04 to 0.15 Hz) band as a marker of sympathetic modulation directed to ventricles. HP and QT means exhibited significant day-night changes in both groups. ASYMP showed a shorter corrected QT and a more reactive vagal modulation leading to significant day-night variations of respiratory sinus arrhythmia. By contrast, SYMP had no significant day-night variations of the vagal control. Conversely, SYMP showed a higher sympathetic control during day compared to ASYMP, having a larger QT variance. These findings suggest that frequency domain HP and QT variability markers can account for the different arrhythmic risk of LQT2 ASYMP and SYMP patients.