Siemens RAPIDLAB 248 - Page 134

Gas Exchange Indices
Gas exchange indices are a quick way to estimate the relationship
between pulmonary dysfunction and hypoxia, and to quantitatively
determine the degree of pulmonary shunting. However, they do not
have a high level of correlation with the actual measurement of arterial
and mixed venous blood and should be used with discretion. The gas
exchange indices are provided for convenience. Final judgment of their
use is in the hands of the physician. The gas exchange indices require
an arterial sample and use measured values at patient temperature.
Alveolar O
2
Alveolar O
2
, referred to as pO
2
(A) or p
A
O
2
, is the partial pressure of
oxygen in alveolar gas. It is a primary component in the detection of gas
exchange indices. The following equation
14,19
is used to estimate the
alveolar O
2
:
pO
2
(A)(T) = F
I
O
2
/100 x (pAtm – pH
2
O) – pCO
2
(T) x (1.25 – 0.25 x F
I
O
2
/100)
where T = patient temperature (°C)
pH
2
O = 10
(0.0244 x T + 0.7655)
+ 0.4
Arterial-Alveolar Oxygen Tension Difference
The arterial-alveolar oxygen tension difference (pO
2
(A–a)), (or A–aDO
2
)
is useful as an index of gas exchange within the lungs if ctO
2
measurements are not available. The following equation
14,19
is used:
pO
2
(A–a)(T) = pO
2
(A)(T) – pO
2
(a)(T)
where pO
2
(A)(T) is the temperature corrected oxygen tension of alveolar
gas, and pO
2
(a)(T) is the temperature corrected oxygen tension of
arterial blood.
Arterial-Alveolar Oxygen Tension Ratio
The arterial-alveolar oxygen tension ratio (pO
2
(a/A)), (or a/A ratio)
provides an index of oxygenation that remains relatively stable when
F
I
O
2
changes. It is useful in predicting oxygen tension in alveolar gas.
The following equation
20
is used:
pO
2
(a)(T)
pO
2
(a/A)(T) =
pO
2
(A)(T)
where pO
2
(A)(T) is the temperature corrected oxygen tension of alveolar
gas, and pO
2
(a)(T) is the temperature corrected oxygen tension of
arterial blood.
NOTE: If F
I
O
2
value is not entered, the gas exchange indices will not be
calculated.
Page G-10 248 Operator’s Manual