<s>
Those	O
large	O
TWAs	O
were	O
associated	O
with	O
increased	O
susceptibility	O
to	O
lethal	O
ventricular	B-Application
tachycardias	I-Application
.	O
</s>
<s>
Most	O
modern	O
references	O
to	O
TWA	O
refer	O
to	O
microvolt	O
T	B-Algorithm
wave	I-Algorithm
alternans	I-Algorithm
(	O
MTWA	O
)	O
,	O
a	O
non-invasive	O
heart	O
test	O
that	O
can	O
identify	O
patients	O
who	O
are	O
at	O
increased	O
risk	O
of	O
sudden	O
cardiac	O
death	O
.	O
</s>
<s>
It	O
is	O
most	O
often	O
used	O
in	O
patients	O
who	O
have	O
had	O
myocardial	O
infarctions	O
(	O
heart	O
attacks	O
)	O
or	O
other	O
heart	O
damage	O
to	O
see	O
if	O
they	O
are	O
at	O
high	O
risk	O
of	O
developing	O
a	O
potentially	O
lethal	O
cardiac	B-Application
arrhythmia	I-Application
.	O
</s>
<s>
Those	O
who	O
are	O
found	O
to	O
be	O
at	O
high	O
risk	O
would	O
therefore	O
benefit	O
from	O
the	O
placement	O
of	O
a	O
defibrillator	O
device	O
which	O
can	O
stop	O
an	O
arrhythmia	B-Application
and	O
save	O
the	O
patient	O
's	O
life	O
.	O
</s>
<s>
The	O
TWA	O
test	O
uses	O
an	O
electrocardiogram	B-Application
(	O
ECG	B-Application
)	O
measurement	O
of	O
the	O
heart	O
's	O
electrical	O
conduction	O
using	O
electrodes	O
attached	O
to	O
one	O
's	O
torso	O
.	O
</s>
<s>
The	O
test	O
looks	O
for	O
the	O
presence	O
of	O
repolarization	O
alternans	O
(	O
T-wave	B-Algorithm
alternans	I-Algorithm
)	O
,	O
which	O
is	O
variation	O
in	O
the	O
vector	O
and	O
amplitude	B-Application
of	O
the	O
T	B-Algorithm
wave	I-Algorithm
component	O
of	O
the	O
EKG	B-Application
.	O
</s>
<s>
Microvolt	O
T	B-Algorithm
wave	I-Algorithm
alternans	I-Algorithm
is	O
a	O
variant	O
of	O
T	B-Algorithm
wave	I-Algorithm
alternans	I-Algorithm
that	O
detects	O
T	B-Algorithm
wave	I-Algorithm
alternans	I-Algorithm
signals	O
as	O
small	O
as	O
one-millionth	O
of	O
a	O
volt	O
.	O
</s>
<s>
Microvolt	O
T	B-Algorithm
wave	I-Algorithm
alternans	I-Algorithm
is	O
defined	O
as	O
an	O
alternation	O
in	O
the	O
morphology	O
of	O
the	O
T	B-Algorithm
wave	I-Algorithm
in	O
an	O
every	O
other	O
beat	O
or	O
AB-AB	O
pattern	O
.	O
</s>
<s>
It	O
has	O
long	O
been	O
associated	O
with	O
ventricular	O
arrhythmias	B-Application
and	O
sudden	O
death	O
.	O
</s>
<s>
First	O
recognized	O
nearly	O
a	O
century	O
ago	O
,	O
visually	O
discernible	O
alternans	O
were	O
linked	O
to	O
the	O
rapid	O
onset	O
of	O
ventricular	B-Application
tachyarrhythmias	I-Application
.	O
</s>
<s>
These	O
efforts	O
established	O
a	O
link	O
between	O
visually	O
imperceptible	O
alternans	O
at	O
the	O
microvolt	O
level	O
and	O
susceptibility	O
to	O
arrhythmias	B-Application
and	O
showed	O
alternans	O
to	O
be	O
a	O
heart	O
rate	O
dependent	O
phenomenon	O
.	O
</s>
<s>
Microvolt	O
T	B-Algorithm
wave	I-Algorithm
alternans	I-Algorithm
testing	O
acts	O
as	O
a	O
risk	O
stratifier	O
between	O
patients	O
who	O
need	O
implantable	O
cardiac	O
defibrillators	O
(	O
ICDs	O
)	O
and	O
those	O
who	O
do	O
not	O
.	O
</s>
<s>
The	O
MMA	O
method	O
uses	O
routine	O
,	O
symptom-limited	O
exercise	O
stress	O
testing	O
or	O
ambulatory	O
ECG	B-Application
monitoring	O
and	O
standard	O
electrodes	O
and	O
requires	O
that	O
chronic	O
medications	O
be	O
retained	O
.	O
</s>
<s>
The	O
remaining	O
1500	O
patients	O
were	O
studied	O
during	O
ambulatory	O
ECG	B-Application
monitoring	O
.	O
</s>
<s>
MTWA	O
testing	O
has	O
been	O
recommended	O
for	O
ventricular	O
arrhythmia	B-Application
risk	O
assessment	O
by	O
the	O
American	O
College	O
of	O
Cardiology	O
,	O
American	O
Heart	O
Association	O
and	O
European	O
Society	O
of	O
Cardiology	O
and	O
by	O
CMS	O
in	O
National	O
Coverage	O
Analysis	O
for	O
Implantable	O
Cardioverter	O
Defibrillators	O
(	O
CAG-00157N	O
)	O
.	O
</s>
<s>
Use	O
of	O
the	O
spectral	O
method	O
for	O
measuring	O
Microvolt	O
T-wave	B-Algorithm
Alternans	I-Algorithm
has	O
been	O
approved	O
for	O
reimbursement	O
by	O
Medicare	O
,	O
as	O
well	O
as	O
major	O
insurers	O
such	O
as	O
Aetna	O
,	O
Cigna	O
,	O
and	O
Humana	O
.	O
</s>
