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  PaO2 X solubility coefficient of O2  100 mmHg X.003 vol%/mmHg = 0.3 vol% O2 combined with Hb  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 % 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 when CvO2 & indicates a cardiac output

Factors that effect C(a-v)O2 INCREASEDECREASE Decreased Cardiac Output Increased Cardiac Output Periods of increased O2 consumption Skeletal relaxation (i.e.- induced by drugs) i.e. - Exercise, SeizuresPeripheral shunting (i.e. – sepsis, trauma) Shivering in Post-OpCertain Poisons (i.e. – cyanide prevents cellular metabolism HyperthermiaHypothermia

Alveolar Gas Equation (PAO2) Partial Pressure of O2 in the Alveoli PAO2=[(P B – P H2O )FiO2] –[PaCO2/(RQ )] PAO2 = [(760 – 47).21] – [40 /(0.8)] PAO2 = [149.73] – [50] PAO2 = 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 = torr  a/A = 72 torr/163.9 torr = 0.44 or 44%  Now with the FiO2 raised to 36% the PAO2 rises to 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