Alveolar gas equation

Question 1

Alveolar gas equation

Answer 1

PAO2= PIO2 - PaCO2/R

PIO2= PO2 in inspired air (mmHg)
PaCO2= arterial PCO2 (mmHg)
R= respiratory quotient = CO2 produced/O2 consumed.

Question 2

Alveolar gas equation: approximation

Answer 2

Normally approximated by

PAO2= 150-PaCO2/0.8

Question 3

A-a gradient

Answer 3

A-a gradient= PAO2-PaO2 = 10-15 mmHg

Question 4

Increased A-a gradient

Answer 4

Hypoxemia;

caused by shunting, V/Q mismatch, fibrosis (impairs diffusion)

Question 5

Oxygen deprivation:

hypoxemia (low PaO2)

Answer 5

Normal A-a gradient:

1) high altitude
2) hypoventilation

Increased A-a gradient:

3) V/Q mismatch
4) diffusion limitation
5) R to L shunt

Question 6

Oxygen deprivation:

hypoxia (low O2 delivery to tissues)

Answer 6

1) low cardiac output
2) hypoxemia
3) anemia
4) CO poisoning

Question 7

Oxygen deprivation:

ischemia (loss of blood flow)

Answer 7

Impeded arterial flow

Reduced venous drainage