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Physiological dead space
Dead space
Definition -­‐
the
volume
occupied
by
gas
which
does
not
participate
in
gas
exchange
in
lung.
A few different types, including:
anatomical
dead
space
physiological
dead
space
alveolar
dead
space
apparatus
dead
space
Anatomical dead space
Anatomical
dead
space
is
the
volume
of
the
conducting
airways.
=>
about 150mL in an average adult
=>
or 2.2mLs/kg
Anatomical dead space is constant regardless of circulation.
Physiological dead space

2.
Physiological
dead
space
is
the
part
of
the
tidal
volume
which
does
not
participate
in
gas
exchange.
Includes:
anatomical dead space
alveoli with no perfusion (i.e. infinite V/Q) (e.g. West's zone
1)
The
difference
between
anatomical
dead
space
and
physiological
dead
space
is
alveolar
dead
space.
With increased cardiac output (e.g. during exercise), physiological dead space is
reduced (due to reduction in alveolar dead space).
Alveolar dead space
Alveolar
dead
space
is
the
part
of
the
inspired
gas
which
passes
through
the
anatomical
dead
space
to
mix
with
gas
at
the
alveolar
level,
but
does
not
participate
in
gas
exchange.
(i.e.
infinite
V/Q)
Apparatus dead space
When
using
mask
or
anaesthetic
circuit
tubing,
this
adds
to
the
conducting
zone.
Measurement
Measurement
of
anatomical
dead
space

3.
By using Fowler's method.
Fowler's
method
Based
on
rapid
dilution
of
gas
already
in
lung
(N2
or
CO2)
by
inspired
gas
(100%
O2).
1.
Single
breath
of
100%
O2
2.
During
the
following
expiration,
[N2]
increases
from
0%
(pure
dead
space
gas)
to
equilibrium
(pure
alveolar
gas)
(i.e.
plateau)
=>
as
per
[N2]
vs
time
graph
3.Using
[N2]
vs
expired
volume
graph,
anatomical
dead
space
is
taken
to
be
at
the
mid-­‐point
of
the
transition
from
conducting
zone
to
gas
exchange
zon
Measurement of physiological dead space
By
using
Bohr's
equation
and
Bohr's
method
Bohr's method
Based
on
"all
expired
CO2
comes
from
alveolar
gas",
and
dead space doesn't
eliminate CO2.
VT
x
FECO2
=
VA
x
FACO2
Also,
VT
=
VA
+
VD
=>
VT
x
FECO2
=
(VT
-­‐
VD)
x
FACO2

4.
=>
VT
x
(FACO2
-­‐
FECO2)
=
VD
x
FACO2
=>
VD/VT
=
(FACO2
-­‐
FECO2)/FACO2
VA
=
ventilated
alveolar
volume
VT
=
tidal
volume
VD
=
dead
space
volume
FECO2
=
fractional
concentration
of
CO2
in
mixed
expired
air
FACO2
=
fractional
concentration
of
CO2
in
alveolus
Bohr
equation:
VD/VT = (PACO2 - PECO2)/PACO2
Normal
value:
0.2~0.35
NB:
PECO2
is
the
partial
pressure
in
MIXED
expired
gas,
NOT
end-­‐tidal
gas
Enghoff modification -­‐
using
measured
arterial
PaCO2
as
an
estimate
of
the
ideal
alveolar
PACO2
=>
modified:
VD/VT = (PaCO2 - PECO2)/PaCO2
Additional notes
Factors influencing anatomical dead space

5.
Size of subject
=>
increases
with
body
size
Age
=>
at
infancy,
anatomical
dead
space
is
higher
for
body
weight
(3.3mL/kg)
Posture
=>
sitting
147mL,
supine
101mL
Position of neck and jaw
Lung
volume
at
the
end
of
inspiration
=>
anatomical dead space increases by 20mL for each L of
lung volume
Drugs
e.g. bronchodilator will increase dead space
Factors influencing alveolar dead space
1.Low
cardiac
output
can
increase
alveolar
dead
space
(increasing
West's
zone
1)
2.
Pulmonary
embolism