Transfer Entropy between RR and QT Intervals in Long QT Syndrome

Healthy subjects exhibit strong information transfer from
$RR$
to
$QT$
intervals. In this study, we assess the dynamic coupling between
$RR$
and
$QT$
intervals for subjects with long
$QT$
syndrome (LQTS) - a potentially fatal genetic cardiac disease characterized by delayed myocardial repolarization. We use transfer entropy
$(TE)$
to quantify the magnitude and direction of the information exchange between the
$RR$
and
$QT$
intervals. Using 24-hour Holter ECGs, we calculate
$TE$
as a function of
$RR$
and
$QT$
histories up to 50 beats and compare the results between the healthy controls and the subjects of LQTS type 1 and 2. Asymmetry between
$TE_{RR\rightarrow QT}$
and
$TE_{QT\rightarrow RR}$
was observed for the LQTS subjects. Compared to the healthy, the LQTS subjects had significantly smaller
$TE_{RR\rightarrow QT}$
at
$RR$
history lengths longer than about 18 beats, while
$TE_{QT\rightarrow RR}$
was significantly larger at all
$RR \ and \ QT$
history lengths. The average characteristic
$RR$
history length that maximized
$TE_{RR\rightarrow QT}$
was 27 beats for the healthy subjects, which was significantly longer than 21 and 22 beats for LQTl and LQT2, respectively. Aging had no significant effect on
$TE_{RR\rightarrow QT}$
, but reduced
$TE_{QT\rightarrow RR}$
for the LQTS subjects, whereas the gender and beta blocker were found to have relatively small effects on the
$TE$
. In conclusion, the dynamic coupling between
$RR$
and
$QT$
intervals is altered by LQTS.