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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