<s>
Second-degree	B-Algorithm
atrioventricular	I-Algorithm
block	I-Algorithm
(	O
AV	B-Algorithm
block	I-Algorithm
)	O
is	O
a	O
disease	O
of	O
the	O
electrical	B-Algorithm
conduction	I-Algorithm
system	I-Algorithm
of	I-Algorithm
the	I-Algorithm
heart	I-Algorithm
.	O
</s>
<s>
It	O
is	O
a	O
conduction	B-Algorithm
block	O
between	O
the	O
atria	O
and	O
ventricles	O
.	O
</s>
<s>
The	O
presence	O
of	O
second-degree	B-Algorithm
AV	I-Algorithm
block	I-Algorithm
is	O
diagnosed	O
when	O
one	O
or	O
more	O
(	O
but	O
not	O
all	O
)	O
of	O
the	O
atrial	O
impulses	O
fail	O
to	O
conduct	O
to	O
the	O
ventricles	O
due	O
to	O
impaired	O
conduction	B-Algorithm
.	O
</s>
<s>
It	O
is	O
classified	O
as	O
a	O
block	O
of	O
the	O
AV	O
node	O
and	O
is	O
categorized	O
in	O
between	O
first-degree	B-Algorithm
(	O
slowed	O
conduction	B-Algorithm
)	O
and	O
third	B-Architecture
degree	I-Architecture
blocks	I-Architecture
(	O
complete	O
block	O
)	O
.	O
</s>
<s>
There	O
are	O
two	O
non-distinct	O
types	O
of	O
second-degree	B-Algorithm
AV	I-Algorithm
block	I-Algorithm
,	O
called	O
Type	O
1	O
and	O
Type	O
2	O
.	O
</s>
<s>
In	O
both	O
types	O
,	O
a	O
P	O
wave	O
is	O
blocked	O
from	O
initiating	O
a	O
QRS	B-Algorithm
complex	I-Algorithm
;	O
but	O
,	O
in	O
Type	O
1	O
,	O
there	O
are	O
increasing	O
delays	O
in	O
each	O
cycle	O
before	O
the	O
omission	O
,	O
whereas	O
,	O
in	O
Type	O
2	O
,	O
there	O
is	O
no	O
such	O
pattern	O
.	O
</s>
<s>
Type	O
1	O
second-degree	B-Algorithm
heart	I-Algorithm
block	I-Algorithm
is	O
considered	O
a	O
more	O
benign	O
entity	O
than	O
type	O
2	O
second-degree	B-Algorithm
heart	I-Algorithm
block	I-Algorithm
with	O
type	O
1	O
not	O
having	O
structural	O
changes	O
found	O
on	O
histology	O
.	O
</s>
<s>
Type	O
1	O
Second-degree	B-Algorithm
AV	I-Algorithm
block	I-Algorithm
,	O
also	O
known	O
as	O
Mobitz	B-Algorithm
I	I-Algorithm
or	O
Wenckebach	O
periodicity	O
,	O
is	O
almost	O
always	O
a	O
disease	O
of	O
the	O
AV	O
node	O
.	O
</s>
<s>
In	O
Wenckebach	B-Algorithm
's	I-Algorithm
1906	O
paper	O
,	O
his	O
original	O
observations	O
were	O
from	O
increasing	O
delay	O
in	O
contraction	O
of	O
the	O
atria	O
and	O
ventricles	O
that	O
shortened	O
after	O
a	O
brief	O
pause	O
and	O
this	O
was	O
later	O
observed	O
on	O
ECG	B-Application
after	O
Einthoven	O
's	O
invention	O
in	O
1901	O
that	O
became	O
the	O
electrocardiogram	B-Application
(	O
ECG	B-Application
)	O
.	O
</s>
<s>
In	O
modern	O
practice	O
,	O
Mobitz	B-Algorithm
I	I-Algorithm
heart	O
block	O
is	O
characterized	O
by	O
progressive	O
prolongation	O
of	O
the	O
PR	O
interval	O
on	O
consecutive	O
beats	O
followed	O
by	O
a	O
blocked	O
P	O
wave	O
(	O
i.e.	O
,	O
a	O
dropped	O
QRS	B-Algorithm
complex	I-Algorithm
)	O
.	O
</s>
<s>
After	O
the	O
dropped	O
QRS	B-Algorithm
complex	I-Algorithm
,	O
the	O
PR	O
interval	O
resets	O
and	O
the	O
cycle	O
repeats	O
.	O
</s>
<s>
The	O
result	O
is	O
a	O
lengthening	O
of	O
the	O
R-R	O
intervals	O
as	O
each	O
subsequent	O
P-wave	O
reaches	O
an	O
increasingly	O
refractory	O
AV	O
node	O
until	O
the	O
impulse	O
fails	O
to	O
conduct	O
,	O
which	O
ultimately	O
results	O
in	O
a	O
blocked	O
QRS	B-Algorithm
complex	I-Algorithm
.	O
</s>
<s>
One	O
of	O
the	O
baseline	O
assumptions	O
when	O
determining	O
if	O
an	O
individual	O
has	O
Mobitz	B-Algorithm
I	I-Algorithm
heart	O
block	O
is	O
that	O
the	O
atrial	O
rhythm	O
has	O
to	O
be	O
regular	O
.	O
</s>
<s>
In	O
symptomatic	O
cases	O
,	O
intravenous	O
atropine	O
or	O
isoproterenol	O
may	O
transiently	O
improve	O
conduction	B-Algorithm
.	O
</s>
<s>
Type	O
2	O
Second-degree	B-Algorithm
AV	I-Algorithm
block	I-Algorithm
,	O
also	O
known	O
as	O
Mobitz	B-Algorithm
II	I-Algorithm
,	O
is	O
almost	O
always	O
a	O
disease	O
of	O
the	O
distal	O
conduction	B-Algorithm
system	I-Algorithm
(	O
His-Purkinje	B-Algorithm
System	I-Algorithm
)	O
.	O
</s>
<s>
Mobitz	B-Algorithm
II	I-Algorithm
heart	O
block	O
is	O
characterized	O
on	O
a	O
surface	O
ECG	B-Application
by	O
intermittently	O
non	O
conducted	O
P	O
waves	O
not	O
preceded	O
by	O
PR	O
prolongation	O
and	O
not	O
followed	O
by	O
PR	O
shortening	O
.	O
</s>
<s>
There	O
is	O
usually	O
a	O
fixed	O
number	O
of	O
non-conducted	O
P	O
waves	O
for	O
every	O
successfully	O
conducted	O
QRS	B-Algorithm
complex	I-Algorithm
,	O
and	O
this	O
ratio	O
is	O
often	O
specified	O
in	O
describing	O
Mobitz	B-Algorithm
II	I-Algorithm
blocks	O
.	O
</s>
<s>
For	O
example	O
,	O
Mobitz	B-Algorithm
II	I-Algorithm
block	O
in	O
which	O
there	O
are	O
two	O
P	O
waves	O
for	O
every	O
one	O
QRS	B-Algorithm
complex	I-Algorithm
may	O
be	O
referred	O
to	O
as	O
2:1	O
Mobitz	B-Algorithm
II	I-Algorithm
block	O
.	O
</s>
<s>
The	O
medical	O
significance	O
of	O
this	O
type	O
of	O
AV	B-Algorithm
block	I-Algorithm
is	O
that	O
it	O
may	O
progress	O
rapidly	O
to	O
complete	B-Architecture
heart	I-Architecture
block	I-Architecture
,	O
in	O
which	O
no	O
escape	B-Algorithm
rhythm	I-Algorithm
may	O
emerge	O
.	O
</s>
<s>
In	O
this	O
case	O
,	O
the	O
person	O
may	O
experience	O
a	O
Stokes-Adams	B-Algorithm
attack	I-Algorithm
,	O
cardiac	O
arrest	O
,	O
or	O
sudden	O
cardiac	O
death	O
.	O
</s>
<s>
The	O
definitive	O
treatment	O
for	O
this	O
form	O
of	O
AV	B-Algorithm
Block	I-Algorithm
is	O
an	O
implanted	B-Device
pacemaker	I-Device
.	O
</s>
<s>
Although	O
the	O
terms	O
infranodal	O
block	O
or	O
infrahisian	O
block	O
are	O
often	O
applied	O
to	O
this	O
disorder	O
,	O
they	O
refer	O
to	O
the	O
anatomic	O
location	O
of	O
the	O
block	O
,	O
whereas	O
Mobitz	B-Algorithm
II	I-Algorithm
refers	O
to	O
an	O
electrocardiographic	B-Application
pattern	O
.	O
</s>
<s>
Because	O
type	O
I	O
Mobitz	O
block	O
occurs	O
in	O
regular	O
cycles	O
,	O
there	O
is	O
always	O
a	O
fixed	O
ratio	O
between	O
the	O
number	O
of	O
P	O
waves	O
and	O
the	O
number	O
of	O
QRS	B-Algorithm
complexes	I-Algorithm
per	O
cycle	O
.	O
</s>
<s>
For	O
example	O
,	O
a	O
Mobitz	O
type	O
I	O
block	O
which	O
has	O
4	O
P	O
waves	O
and	O
3	O
QRS	B-Algorithm
complexes	I-Algorithm
per	O
cycle	O
may	O
be	O
referred	O
to	O
as	O
4:3	O
Mobitz	O
Type	O
I	O
block	O
.	O
</s>
<s>
Type	O
II	O
Mobitz	O
block	O
also	O
usually	O
occurs	O
with	O
a	O
fixed	O
P:QRS	O
ratio	O
,	O
with	O
a	O
set	O
number	O
of	O
P	O
waves	O
for	O
every	O
successfully	O
elicited	O
QRS	B-Algorithm
.	O
</s>
<s>
Higher	O
numbers	O
of	O
P	O
waves	O
for	O
every	O
QRS	B-Algorithm
indicate	O
more	O
severe	O
block:181	O
and	O
ratios	O
of	O
3:1	O
and	O
above	O
are	O
also	O
referred	O
to	O
as	O
"	O
High	O
Grade	O
"	O
AV	O
Blocke60	O
.	O
</s>
<s>
Of	O
course	O
,	O
because	O
type	O
II	O
Mobitz	O
block	O
is	O
unstable	O
by	O
nature	O
,	O
it	O
is	O
common	O
for	O
the	O
P:QRS	O
ratio	O
in	O
Mobitz	O
type	O
II	O
block	O
to	O
change	O
over	O
time	O
.	O
</s>
<s>
The	O
P:QRS	O
ratio	O
is	O
always	O
of	O
the	O
form	O
X	O
:	O
(	O
X−1	O
)	O
in	O
type	O
I	O
Mobitz	O
block	O
and	O
of	O
the	O
form	O
X:1	O
in	O
type	O
2	O
Mobitz	O
block	O
because	O
of	O
the	O
nature	O
of	O
the	O
pattern	O
of	O
each	O
.	O
</s>
<s>
In	O
the	O
case	O
of	O
2:1	O
block	O
(	O
2	O
P	O
waves	O
for	O
every	O
QRS	B-Algorithm
complex	I-Algorithm
)	O
it	O
is	O
impossible	O
to	O
differentiate	O
type	O
I	O
from	O
type	O
II	O
Mobitz	O
block-based	O
solely	O
on	O
the	O
P:QRS	O
ratio	O
or	O
on	O
a	O
pattern	O
of	O
lengthening	O
PR	O
intervals	O
.	O
</s>
<s>
:182	O
In	O
this	O
case	O
,	O
a	O
lengthened	O
PR	O
interval	O
with	O
a	O
normal	O
QRS	B-Algorithm
width	O
is	O
most	O
likely	O
indicative	O
of	O
a	O
type	O
I-like	O
pathology	O
,	O
and	O
a	O
normal	O
PR	O
interval	O
with	O
a	O
widened	O
QRS	B-Algorithm
is	O
most	O
likely	O
indicative	O
of	O
a	O
type	O
II-like	O
pathology	O
.	O
</s>
