Formulas related to O2
transport
Fiona Campbell BS, RRT-NPS
Spring 2008
Oxygen content of arterial
blood (CaO2)


Calculated by adding the volume of oxygen
dissolved in the plasma and the volume of
combined oxygen
O2 Dissolved in Plasma



O2 combined with Hb



PaO2 X solubility coefficient of O2
100 mmHg X .003 vol%/mmHg = 0.3 vol%
Hb X 1.34 X SaO2
15 X 1.34 X .97 = 19.5 vol%
CaO2=O2 dissolved in Plasma+O2 combined with Hb

CaO2 = 0.3 vol % + 19.5 vol % = 19.8 vol %
Oxygen content of mixed
venous blood (CvO2)



Blood returning to the heart will have a
lower content
Normally PvO2 is approx 40 mmHg and
SvO2 is normally 75%
The most accurate PvO2 is drawn from
the pulmonary artery

CvO2 = (PvO2 X .003) + (Hb X 1.34 X SvO2)

CvO2 = (40 mmHg X .003)+(15 X 1.34 X .75)=15.2 vol%
Arterial-Venous O2 content
difference
C(a-v)O2

The C(a-v)O2 measures oxygen
consumption of the tissues

C(a-v)O2 = CaO2 – CvO2

C(a-v)O2 =20 vol% - 15 vol% = 5 vol%

C(a-v)O2 difference will
CvO2
& indicates a
output
when
cardiac
Factors that effect C(a-v)O2
INCREASE
Decreased Cardiac
Output
Periods of increased O2
consumption
i.e. - Exercise, Seizures
Shivering in Post-Op
Hyperthermia
DECREASE
Increased Cardiac Output
Skeletal relaxation (i.e.induced by drugs)
Peripheral shunting (i.e. –
sepsis, trauma)
Certain Poisons
(i.e. – cyanide prevents
cellular metabolism
Hypothermia
Alveolar Gas Equation (PAO2)
Partial Pressure of O2 in the Alveoli

PAO2=[(PB – PH2O)FiO2] –[PaCO2/(RQ)]

PAO2 = [(760 – 47).21] – [40 /(0.8)]
PAO2 = [149.73] – [50]
PAO2 = 99.73 mmHg


Alveolar-arterial oxygen
gradient
P(A-a)O2




The difference between the amount of O2
that enters the alveoli and the amount that
gets to the blood stream
Non-invasive bedside tool used to quantitate
the efficiency of oxygen loading
An increase in P(A-a)O2 indicates an
increase in shunt
PAO2 – PaO2 = P(A-a)O2
a/A ratio

Represents the percent of alveolar O2
that reaches the blood stream

a/A ratio = PaO2/PAO2
a/A ratio = 55/100 = 0.55 or 55%


An a/A ratio of 75% is considered
acceptable
Predicted PaO2

Comparisons of O2 in the lung and O2 in the
bloodstream can be formulated using P(A-a)O2 and
a/A ratio

These formulas can also be used to predict a PaO2
when the FiO2 needs to be altered

By understanding the relationship of the PAO2 and
PaO2, one can properly treat the patient
Selecting an incorrect FiO2 one can over-correct or
under-correct hypoxemia

Predicted PaO2

Formula:
Current PaO2 = Desired PaO2
Current FiO2 = Desired FiO2


This formula works only if other factors that
effect his ventilation or metabolism are not
involved
Solve for X
72 torr = 86 torr
.30
X
72X = .30(86)
X = .36 or 36% * The FiO2 must be increased to
achieve the desired PaO2 of 86 torr
Predicted PaO2

This formula can be proven with other formulas
learned:

With given PaO2 = 76 torr, PCO2 = 40 torr, FiO2 30% :

PAO2 = 163.9 torr

a/A = 72 torr/163.9 torr = 0.44 or 44%

Now with the FiO2 raised to 36% the PAO2 rises to
206.68 mmHg, 44% of O2 will get to the bloodstream and
the predicted PaO2 is approx. 90 mmHg

NOTE: The ratio is for estimations and not absolute, by being
aware of this relationship, one can quickly asses the oxygen
requirement for the patient