Aerial blood gas of a 5 year old child done at sea level gives the following results: pH 7.41, PaO2, 100 mmHg, and PaCO2, 40 mmHg. The child is being ventilated with 80% oxygen. What is the (A-a) DO2.-

Alveolar air equation: Using the alveolar air equation - PAO2 = - Data: Fraction of O2 in the inspired air = 80% Barometric pressure = 760 mm Hg (sea level) Water vapor pressure = 47 mm Hg Respiratory quotient = 0.8 Hence, Alveolar PO2 = - = 570.4 - 50 = 520.4 mm Hg Aerial PO2 is given = 100 mm Hg Hence, (A - a) DO2 = 520.4 - 100 = 420.4 mm Hg