<s>
QT	B-Algorithm
interval	I-Algorithm
variability	I-Algorithm
(	O
QTV	O
)	O
refers	O
to	O
the	O
physiological	O
phenomenon	O
of	O
beat-to-beat	O
fluctuations	O
in	O
QT	B-Algorithm
interval	I-Algorithm
of	O
electrocardiograms	B-Application
.	O
</s>
<s>
Under	O
normal	O
resting	O
conditions	O
,	O
beat-to-beat	O
changes	O
in	O
QT	B-Algorithm
interval	I-Algorithm
are	O
very	O
small	O
with	O
a	O
standard	O
deviation	O
typically	O
below	O
5	O
ms.	O
Digital	O
high	O
resolution	O
ECG	B-Application
sampled	O
at	O
300	O
Hz	O
or	O
higher	O
and	O
dedicated	O
computer	O
algorithms	O
are	O
required	O
for	O
QTV	O
assessment	O
.	O
</s>
<s>
Template-based	O
algorithms	O
that	O
use	O
parts	O
of	O
,	O
or	O
the	O
entire	O
ECG	B-Application
waveform	O
usually	O
deliver	O
good	O
results	O
;	O
template	O
stretching	O
or	O
warping	O
techniques	O
perform	O
comparably	O
well	O
in	O
the	O
presence	O
of	O
noise	O
.	O
</s>
<s>
,	O
where	O
,	O
,	O
,	O
and	O
denote	O
standard	O
deviation	O
and	O
mean	O
of	O
QT	B-Algorithm
interval	I-Algorithm
and	O
heart	O
rate	O
time	O
series	O
,	O
respectively	O
.	O
</s>
