Arterial Alveolar Oxygen Tension Ratio (a/A)
The arterial alveolar oxygen tension ratio is useful to predict
oxygen tension in alveolar gas and to provide an index of
oxygenation which remains relatively stable when FIO
2
changes.
a/A =
**
PO
2
/A Equation 28
** Temperature corrected gas value
Arterial-Mixed Venous O
2
Content Difference ( a – v
–
DO
2
)
When the oxygenated blood from the lungs comes into
contact with tissues, it releases oxygen and takes up carbon
dioxide. The quantity of oxygen donated depends on 2 factors:
• The speed of blood ow
• The consumption by tissues
The rst factor can be detected from the cardiac output, and
the second depends on the metabolic rate of the patient.
The a – v
–
DO
2
is the measurement of the difference in oxygen
content between the arterial blood and the mixed venous
blood (the amount of oxygen donated to the tissues). This
parameter is not an indication of the basic metabolism or
of the cardiac output. It is an nonspecic indication; even if,
in some cases completely relaxed patients with a constant
metabolic rate, it can be related to the cardiac output.
a – v
–
DO
2
= CaO2 – Cv
–
O
2
Equation 29
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