
68-11Blood Chemistry Measurement
To account for COHb and MetHb accurately, four transmission wavelengths would be required, or for
ac
curate
S
a
O
2
readings the total Hb concentration needs to be known or measured [5,7,20].
68.3.3.2 Pulse Oximetry
Pulse
ox
imetry
is ba
sed
on th
e
tr
ansmission,
ab
sorption,
an
d
di
spersion
of li
ght
as it pa
sses
th
rough
he
moglobin
. Be
er’s
la
w,
as st
ated
in Eq
uation
68.1, de
termines
th
e
tr
ansmission
of li
ght
th
rough
a su
b-
stance
. Fo
r
pu
lse
ox
imetry,
th
e
li
ght
il
luminates
bo
th
ar
terial
an
d
ve
nous
bl
ood
an
d
th
e
li
ght
mu
st
tr
averse
al
l
ti
ssue
be
tween
li
ght
so
urce
an
d
re
ceiver
. Fi
gure
68.7 re
presents
th
e
li
ght
pa
th
an
d
in