Aerial blood gas of a 5 year old child done at sea level gives the following results: pH 7.41, Pa02100 mmHg, and PaCO2 40 mm Hg. The child is being ventilated with 80% oxygen. What is the (A-a) P02

D i.e. 420. 4 mm Hg PAO2 can be calculated by alveolar gas equation PAO2 = FIO2 x (Po - PH2O) - PaCO2 R Where: PA02 1,102 PB PH2O PaCO2 R = P02 of the alveolar air = Fraction of 02 in the air Barometric pressure (N-760 mm Hg) Water vapour pressure (N- 47 mm H20) Paial pressure of CO2 (N - 40 mm Hg) Respiratory quotient (CO2output= 200 ml 0 8 02 uptake 250 ml The data provided in the question is: Pa02 100 mm Hg PaCO2 * 40 mm Hg Fi02 * 8 mm Hg Fi02 - Fraction of inspired Oxygen At sea level (760 mm Hg pressure), the composition of air - 02 = 20%; N2 = 80% 20 So Fi02 is - = 0.2 100 In this question, the child is being ventilated with 80% 02 80 So Fi02 is ___ = 0.8 100 Substituting these values PAO2 = 0.8 x (760-47) - 40/0.8 = 520.4 mm Hg Therefore PAO2 - Pa02 = (520.4 mm Hg - 100 mm Hg) = 420.4 mm Hg.