Question

You decide to stand on the roof of the local hospital where the barometric pressure is 747 mm Hg. You then place a breathing mask over your face that contains 50% oxygen and 50% nitrogen. What is your approximate arterial PO2 (PaO2) under these conditions?

a) 350 mm hg

b) 300 mm hg

c) 160 mm hg

d) 100 mm hg

Answer 1

Option B-300 mm Hg

Give,Barometric Pressure=747 mm Hg

Breathing Mask contains 50%O2 and 50% N2.That is  fractional concentration of inspired oxygen,FiO2=0.5

According to  alveolar gas equation,PaO2=FiO2(PB-PH2O)-(PaCO2/RQ)

where,PAO2 - alveolar oxygen partial pressure

FiO2 - fraction of inspired oxygen=0.5

PB - barometric pressure=747 mm Hg

PH2O -partial pressure of water approximatelly Equall to 47 mm Hg

PaCO2 - partial pressure of carbon dioxide Approximately = 40 mm Hg

  RQ is the respiratory quotient,In Rest it is Approximately equalls to 0.8(The rate of CO2 production to O2 use)

So PaO2=0.5(747-47)-(40/0.8)

=0.5*700-50=350-50=300 mm Hg.