<s>
A	O
premature	B-Error_Name
ventricular	I-Error_Name
contraction	I-Error_Name
(	O
PVC	O
)	O
is	O
a	O
common	O
event	O
where	O
the	O
heartbeat	O
is	O
initiated	O
by	O
Purkinje	B-Algorithm
fibers	I-Algorithm
in	O
the	O
ventricles	O
rather	O
than	O
by	O
the	O
sinoatrial	O
node	O
.	O
</s>
<s>
PVCs	O
may	O
cause	O
no	O
symptoms	O
or	O
may	O
be	O
perceived	O
as	O
a	O
"	O
skipped	B-Algorithm
beat	I-Algorithm
"	O
or	O
felt	O
as	O
palpitations	B-Algorithm
in	O
the	O
chest	O
.	O
</s>
<s>
The	O
electrical	O
events	O
of	O
the	O
heart	O
detected	O
by	O
the	O
electrocardiogram	B-Application
(	O
ECG	B-Application
)	O
allow	O
a	O
PVC	O
to	O
be	O
easily	O
distinguished	O
from	O
a	O
normal	O
heart	O
beat	O
.	O
</s>
<s>
However	O
,	O
very	O
frequent	O
PVCs	O
can	O
be	O
symptomatic	O
of	O
an	O
underlying	O
heart	O
condition	O
(	O
such	O
as	O
arrhythmogenic	B-Application
right	O
ventricular	O
cardiomyopathy	O
)	O
.	O
</s>
<s>
Furthermore	O
,	O
very	O
frequent	O
(	O
over	O
20%	O
of	O
all	O
heartbeats	O
)	O
PVCs	O
are	O
considered	O
a	O
risk	O
factor	O
for	O
arrhythmia-induced	O
cardiomyopathy	O
,	O
in	O
which	O
the	O
heart	O
muscle	O
becomes	O
less	O
effective	O
and	O
symptoms	O
of	O
heart	O
failure	O
may	O
develop	O
.	O
</s>
<s>
Ultrasound	B-Application
of	I-Application
the	I-Application
heart	I-Application
is	O
therefore	O
recommended	O
in	O
people	O
with	O
frequent	O
PVCs	O
.	O
</s>
<s>
PVCs	O
may	O
be	O
perceived	O
as	O
a	O
skipped	B-Algorithm
heart	I-Algorithm
beat	I-Algorithm
,	O
a	O
strong	O
beat	O
,	O
palpitations	B-Algorithm
,	O
or	O
lightheadedness	O
.	O
</s>
<s>
Premature	B-Error_Name
ventricular	I-Error_Name
contractions	I-Error_Name
may	O
be	O
associated	O
with	O
underlying	O
heart	O
disease	O
,	O
and	O
certain	O
characteristics	O
are	O
therefore	O
elicited	O
routinely	O
:	O
the	O
presence	O
of	O
signs	O
of	O
heart	O
disease	O
or	O
a	O
known	O
history	O
of	O
heart	O
disease	O
(	O
e.g.	O
</s>
<s>
PVCs	O
and	O
palpitation	B-Algorithm
associated	O
with	O
syncope	O
(	O
transient	O
loss	O
of	O
consciousness	O
)	O
or	O
provoked	O
by	O
exertion	O
are	O
also	O
concerning	O
.	O
</s>
<s>
Premature	B-Error_Name
ventricular	I-Error_Name
contractions	I-Error_Name
occur	O
in	O
healthy	O
persons	O
of	O
any	O
age	O
,	O
but	O
are	O
more	O
prevalent	O
in	O
the	O
elderly	O
and	O
in	O
men	O
.	O
</s>
<s>
Normally	O
,	O
impulses	O
pass	O
through	O
both	O
ventricles	O
almost	O
at	O
the	O
same	O
time	O
and	O
the	O
depolarization	O
waves	O
of	O
the	O
two	O
ventricles	O
partially	O
cancel	O
each	O
other	O
out	O
in	O
the	O
ECG	B-Application
.	O
</s>
<s>
However	O
,	O
when	O
a	O
PVC	O
occurs	O
the	O
impulse	O
nearly	O
always	O
travels	O
through	O
only	O
one	O
bundle	O
fiber	O
,	O
so	O
there	O
is	O
no	O
neutralization	O
effect	O
;	O
this	O
results	O
in	O
the	O
high	O
voltage	O
QRS	O
wave	O
in	O
the	O
electrocardiograph	B-Application
.	O
</s>
<s>
There	O
are	O
three	O
main	O
physiological	O
explanations	O
for	O
premature	B-Error_Name
ventricular	I-Error_Name
contractions	I-Error_Name
:	O
enhanced	O
ectopic	O
nodal	O
automaticity	O
,	O
re-entry	O
signaling	O
,	O
and	O
toxic/reperfusion	O
triggered	O
.	O
</s>
<s>
This	O
process	O
is	O
the	O
underlying	O
mechanism	O
for	O
arrhythmias	B-Application
due	O
to	O
excess	O
catecholamines	O
and	O
some	O
electrolyte	O
deficiencies	O
,	O
particularly	O
low	O
blood	O
potassium	O
,	O
known	O
as	O
hypokalemia	O
.	O
</s>
<s>
Reentry	O
occurs	O
when	O
an	O
area	O
of	O
1-way	O
block	O
in	O
the	O
Purkinje	B-Algorithm
fibers	I-Algorithm
and	O
a	O
second	O
area	O
of	O
slow	O
conduction	B-Algorithm
are	O
present	O
.	O
</s>
<s>
This	O
condition	O
is	O
frequently	O
seen	O
in	O
patients	O
with	O
underlying	O
heart	O
disease	O
that	O
creates	O
areas	O
of	O
differential	O
conduction	B-Algorithm
and	O
recovery	O
due	O
to	O
myocardial	O
scarring	O
or	O
ischemia	O
.	O
</s>
<s>
During	O
ventricular	O
activation	O
,	O
one	O
bundle	O
tract	O
's	O
area	O
of	O
slow	O
conduction	B-Algorithm
activates	O
the	O
other	O
tract	O
's	O
bundle	O
fibers	O
post	O
block	O
after	O
the	O
rest	O
of	O
the	O
ventricle	O
has	O
recovered	O
.	O
</s>
<s>
These	O
are	O
often	O
seen	O
in	O
patients	O
with	O
ventricular	O
arrhythmias	B-Application
due	O
to	O
digoxin	O
toxicity	O
and	O
reperfusion	O
therapy	O
after	O
myocardial	O
infarction	O
(	O
MI	O
)	O
.	O
</s>
<s>
Other	O
sympathomimetic	O
molecules	O
such	O
as	O
amphetamines	O
and	O
cocaine	B-Application
will	O
also	O
cause	O
this	O
effect	O
.	O
</s>
<s>
Phosphodiesterase	O
inhibitors	O
such	O
as	O
caffeine	B-Application
directly	O
affect	O
the	O
G-coupled	O
signal	O
transduction	O
cascade	O
by	O
inhibiting	O
the	O
enzyme	O
that	O
catalyzes	O
the	O
breakdown	O
of	O
cAMP	O
,	O
again	O
leading	O
to	O
the	O
increased	O
concentration	O
of	O
calcium	O
ions	O
in	O
the	O
cytosol	O
.	O
</s>
<s>
The	O
myocardial	O
scarring	O
that	O
occurs	O
in	O
myocardial	O
infarction	O
and	O
also	O
in	O
the	O
surgical	O
repair	O
of	O
congenital	O
heart	O
disease	O
can	O
disrupt	O
the	O
conduction	B-Algorithm
system	I-Algorithm
of	I-Algorithm
the	I-Algorithm
heart	I-Algorithm
and	O
may	O
also	O
irritate	O
surrounding	O
viable	O
ventricular	O
myocytes	O
,	O
make	O
them	O
more	O
likely	O
to	O
depolarize	O
spontaneously	O
.	O
</s>
<s>
PVCs	O
may	O
be	O
found	O
incidentally	O
on	O
cardiac	O
tests	O
such	O
as	O
a	O
12-lead	O
electrocardiogram	B-Application
(	O
ECG/EKG	O
)	O
performed	O
for	O
another	O
reason	O
.	O
</s>
<s>
In	O
those	O
with	O
symptoms	O
suggestive	O
of	O
premature	B-Error_Name
ventricular	I-Error_Name
complexes	I-Error_Name
,	O
the	O
ECG/EKG	O
is	O
the	O
first	O
investigation	O
that	O
may	O
identify	O
PVCs	O
as	O
well	O
as	O
other	O
cardiac	B-Algorithm
rhythm	I-Algorithm
issues	O
that	O
may	O
cause	O
similar	O
symptoms	O
.	O
</s>
<s>
The	O
advantage	O
of	O
these	O
monitors	O
is	O
that	O
they	O
allow	O
a	O
quantification	O
of	O
the	O
amount	O
of	O
abnormal	O
beats	O
(	O
"	O
burden	O
"	O
)	O
and	O
ensure	O
that	O
there	O
are	O
no	O
heart	B-Application
arrhythmias	I-Application
present	O
that	O
might	O
require	O
attention	O
,	O
such	O
as	O
ventricular	B-Application
tachycardia	I-Application
.	O
</s>
<s>
Specifically	O
,	O
if	O
this	O
shows	O
exercise-induced	O
ventricular	B-Application
tachycardia	I-Application
this	O
would	O
require	O
specific	O
treatment	O
.	O
</s>
<s>
On	O
electrocardiography	B-Application
(	O
ECG	B-Application
or	O
Holter	O
)	O
premature	B-Error_Name
ventricular	I-Error_Name
contractions	I-Error_Name
have	O
a	O
specific	O
appearance	O
of	O
the	O
QRS	O
complexes	O
and	O
T	O
waves	O
,	O
which	O
are	O
different	O
from	O
normal	O
readings	O
.	O
</s>
<s>
PVCs	O
can	O
be	O
distinguished	O
from	O
premature	B-Algorithm
atrial	I-Algorithm
contractions	I-Algorithm
because	O
the	O
compensatory	O
pause	O
is	O
longer	O
following	O
premature	B-Error_Name
ventricular	I-Error_Name
contractions	I-Error_Name
,	O
in	O
addition	O
to	O
a	O
difference	O
in	O
QRS	O
appearance	O
.	O
</s>
<s>
Depending	O
whether	O
there	O
are	O
one	O
,	O
two	O
,	O
or	O
three	O
normal	O
(	O
sinus	O
)	O
beats	O
between	O
each	O
PVC	O
,	O
the	O
rhythm	O
is	O
called	O
bigeminy	B-Algorithm
,	O
trigeminy	O
,	O
or	O
quadrigeminy	O
.	O
</s>
<s>
If	O
3	O
or	O
more	O
consecutive	O
PVCs	O
occur	O
in	O
a	O
row	O
it	O
may	O
be	O
called	O
ventricular	B-Application
tachycardia	I-Application
.	O
</s>
<s>
The	O
most	O
effective	O
treatment	O
is	O
the	O
elimination	O
of	O
triggers	O
(	O
particularly	O
stopping	O
the	O
use	O
of	O
substances	O
such	O
as	O
caffeine	B-Application
and	O
certain	O
drugs	O
,	O
like	O
tobacco	O
)	O
.	O
</s>
<s>
Antiarrhythmics	B-Algorithm
:	O
these	O
agents	O
alter	O
the	O
electrophysiologic	O
mechanisms	O
responsible	O
for	O
PVCs	O
.	O
</s>
<s>
In	O
meta-analysis	O
of	O
11	O
studies	O
,	O
people	O
with	O
frequent	O
PVCs	O
( ≥	O
once	O
during	O
a	O
standard	O
electrocardiographic	B-Application
recording	O
or	O
≥30	O
times	O
over	O
a	O
1-hour	O
recording	O
)	O
had	O
risk	O
of	O
cardiac	O
death	O
twice	O
as	O
great	O
as	O
that	O
of	O
participants	O
with	O
occasional	O
PVCs	O
.	O
</s>
<s>
In	O
this	O
study	O
absence	O
of	O
heart	O
of	O
disease	O
was	O
established	O
by	O
echocardiography	B-Application
,	O
cardiac	O
magnetic	O
resonance	O
imaging	O
in	O
63	O
persons	O
and	O
Holter	O
monitoring	O
.	O
</s>
<s>
In	O
men	O
with	O
coronary	O
heart	O
disease	O
and	O
in	O
women	O
with	O
or	O
without	O
coronary	O
heart	O
disease	O
,	O
complex	O
or	O
frequent	O
arrhythmias	B-Application
were	O
not	O
associated	O
with	O
an	O
increased	O
risk	O
.	O
</s>
<s>
In	O
the	O
ARIC	O
study	O
of	O
14,783	O
people	O
followed	O
for	O
15	O
to	O
17	O
years	O
those	O
with	O
detected	O
PVC	O
during	O
2	O
minute	O
ECG	B-Application
,	O
and	O
without	O
hypertension	O
or	O
diabetes	O
on	O
the	O
beginning	O
,	O
had	O
risk	O
of	O
stroke	O
increased	O
by	O
109%	O
.	O
</s>
<s>
In	O
the	O
Niigata	O
study	O
of	O
63,386	O
people	O
with	O
a	O
10-year	O
follow-up	O
period	O
,	O
subjects	O
with	O
PVC	O
during	O
a	O
10-second	O
recording	O
had	O
triple	O
the	O
risk	O
of	O
atrial	B-Application
fibrillation	I-Application
of	O
those	O
without	O
PVCs	O
,	O
independently	O
of	O
these	O
risk	O
factors	O
:	O
age	O
;	O
male	O
sex	O
;	O
high	O
simple	O
body	O
mass	O
index	O
(	O
a	O
possible	O
signifier	O
of	O
obesity	O
)	O
;	O
hypertension	O
(	O
systolic	O
and	O
diastolic	O
blood	O
pressure	O
within	O
certain	O
abnormal	O
limits	O
)	O
;	O
and	O
diabetes	O
.	O
</s>
<s>
Reducing	O
very	O
frequent	O
PVC	O
(	O
>20	O
%	O
)	O
by	O
antiarrhythmic	B-Algorithm
drugs	I-Algorithm
or	O
by	O
catheter	O
ablation	O
significantly	O
improves	O
heart	O
performance	O
.	O
</s>
<s>
Heart	O
disease	O
was	O
excluded	O
after	O
physical	O
examination	O
,	O
chest	O
x-ray	O
,	O
ECG	B-Application
,	O
echocardiography	B-Application
,	O
maximal	O
exercise	O
stress	O
test	O
,	O
right	O
-	O
and	O
left-heart	O
catheterization	O
and	O
coronary	O
angiography	O
.	O
</s>
<s>
In	O
122,043	O
United	O
States	O
Air	O
Force	O
flyers	O
and	O
cadet	O
applicants	O
during	O
approximately	O
48	O
seconds	O
of	O
ECG	B-Application
0.78	O
%	O
(	O
952	O
males	O
)	O
had	O
PVC	O
within	O
all	O
age	O
groups	O
,	O
but	O
with	O
increased	O
incidence	O
with	O
increasing	O
age	O
.	O
</s>
