Siemens Rapidlab 1200, Operator'S Manual, Page: 78 / 448

1-58 Rapidlab 1200 Operator’s Guide: System Overview and Intended Use
02087462 Rev. V
p50
Half saturation of hemoglobin by oxygen ( p 50) indicates the partial pressure of oxygen
when oxygen has saturated 50% of the available hemoglobin. The p 50 value indicates the
position of the oxygen-hemoglobin dissociation curve:
21
• Low p 50 shifts the curve to the left and indicates increased oxygen-hemoglobin
affinity
• High p 50 shifts the curve to the right and indicates decreased oxygen-hemoglobin
affinity
The p 50 value is useful in indicating the presence of abnormal hemoglobin that affects the
oxygen transport mechanism, and as an indirect measure of the 2,3 DPG concentration.
The
p 50 can also indicate changes in pH, p CO
2
, and temperature.
19,21
The p 50 value is reported for s O
2
values between 20% and 90% and is determined using
the following equation:
p 50 = 26.6 x ( p O
2
c / p O
2
s)
where
p O
2
c = p O
2
× 10
–[0.48 x (7.4–pH(37)) + 0.0013BE(B)]
and p O
2
s is calculated with an interactive program.
31
Oxygen Saturation (Estimated)
Oxygen saturation is a ratio, expressed as a percentage, of the volume of oxygen carried to
the maximum volume of oxygen that the hemoglobin can carry. When combined with
knowledge of oxygen content, oxygen saturation is useful for evaluating the amount of
oxygen actually available for the tissues. Oxygen saturation can also be used to evaluate
the effectiveness of oxygen therapy.
The system estimates oxygen saturation using the relationship described by Kelman
32
and
Thomas
33
as follows:
( )
100 *
10 4 . 2 31100 24000 15
2000 2045 15
6 2 3 4
2 3 4
2
× + − + −
+ + −
=
N N N N
N N N N
est SAT O
where
N = p O
2
× 10
[0.48(pH(37)–7.4)–0.0013 BE(B)]