Question

When calculating the partial pressure of oxygen (P_AO₂) in the alveoli, why do we remove the water vapor pressure and the P_ACO2.

If a patient is receiving an FIO2 of .70 on a day when the barometric pressure is 748 mm Hg and if the PaCO2 is 50 mm Hg, what is the patient’s alveolar oxygen tension (PAO2).

Answer 1

When the inspired air reaches the alveoli, there is an addition reduction of PO₂( partial pressure of oxygen) due to the humidification and alveolar pressure of CO​​​​​​₂ .That is why, we remove the partial pressure of water vapour and carbon dioxide while calculating partial pressure of oxygen in the alveoli( PACO₂) .

To find the alveolar oxygen tension of the patient( PAO₂) , the given values are:

FiO₂ ( fractional concentration of oxygen) = 0.70

P_B ( barometric pressure) =748 mm Hgthe patient's

PaCO₂  ​​​​​​( partial pressure of CO₂)=50 mm hg

We know that ,

P_H2O ( partial pressure of water vapour) =47 mmHg

R ( respiratory quotient)=0.8( even though R=0.825)

The standard equation for finding alveolar partial pressure,

PAO₂ =FiO₂(P_B -P_H2O) -(PACO₂/R)

Applying values of the patient,

PAO₂ =0.70(748-47) -(50/0.8)

=(0.70×701) -62. 5

=491-62.5

​​​​​​​ =428.5 mmHg

Therefore, the patient's alveolar oxygen tension is 428.5mmHg