PAO2 is calculated as follows: where FIO2 is the fraction of inspired oxygen (eg, 0.21 at room air), Patm is the ambient barometric pressure (eg, 760 mm Hg at sea level), PH2O is the partial pressure of water vapor (eg, usually 47 mm Hg), PaCO2 is the measured partial pressure of arterial carbon dioxide, and R is the respiratory quotient, which is assumed to be 0.8 in a resting patient eating a normal diet.