Alveolar Air Equation: Calculates partial pressure of O2 in Alveoli
PAO2 = (( - 47)) - (PaCO2/)
Shortcut: PAO2 = (7 x FiO2) - (PaCO2 +10)
A-a Gradient: Measures difference between aveolar and arterial O2
A-aDO2 = PAO2 - PaO2
25-65 on 100% O2 is 66 - 300 is mismatch >300 =
Arterial O2 Content (CaO2) Best measurement of O2 delivered to tissues
CaO2 = ( x x SaO2) + (PaO2 x ) O2 in red blood cells plus O2 in plasma
Normal value is - vol% (mL/dL)
Mixed Venous O2 Content (CvO2) Total O2 in venous blood
CvO2 = (Hb x x ) + x .003)
Normal - vol% CvO2 will when cardiac output decreases SvO2 will decrease also.
Arterial - Venous O2 Content Difference Ca-vO2 - Provides estimate of Tissue O2 consumption Ca-vO2 = CaO2 - CvO2
Normal is - vol %
Used in Fick equation to calculate Output
Cardiac Output Qt Modified Used to calculate Cardiac output when O2 consumption and O2 content differences are known Qt = ( / (Ca-vO2()) Normal value is 4-8 L/min VO2 = Qt x Ca-vO2 x 10
Shunt Equation Qs / Qt
Portion of cardiac output that is shunted and does not participate in gas exchange
Qs/Qt = ((A-aDO2) x .003) / (A-aDO2) x .003) + Ca-vO2
Normal is - % Can be estimated: Assuming 5% shunt increase for each 100 torr of A-aDO2
SaO2 Arterial O2 Saturation % of Hb that is bound by O2 calculated by ABG analyzer Measured by hemoximeter or co-ox
Large differences in calculated and measured could be because of increased carboxyhemoglobin levels
PaO2 is estimated by subtracting from
HB / Hct HB is normally - g/dl HCT is usually times HB in percent
Deadspace to Vt Ratio VD/Vt % of gas that doesn't participate in gas exchange Normal is -%
((PaCO2 - PeCo2) / ) x 100
Increased VD/VT indicates deadspace disease like a
Normal Values
PCO2 Arterial Range - venous
PO2 Arterial Range - Venous
pH Arterial Range - Venous SO2 Arterial % Range - % Venous - %
HCO3- Arterial Range - Venous Normal BE +- Hb Arteral/Venous Range -
CaO2 Arterial vol% Range - vol%
CvO2 Venous vol% Range - vol %
