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1

alveolar gas equation

PAO2 = PIO2-PaCO2/R = 150 mmHg -PaCO2 /0.8

PAO2 = alveolar PO2
PIO2 = PO2 in inspired air
PaCO2 = arterial PCO2
R = respiratory quotient = CO2 produced / O2 CONSUMED

2

what is R in alveolar gas equation

R = respiratory quotient = CO2 produced / O2 CONSUMED
normally is 0.8

3

PO2 in inspired air in sea level ( and why)

mm Hg
Ptotal (760) - correction because of trachea (47)
=713
713 x0.21 = 150 mmHg

4

A-a gradient equation

A-a gradient = alveolar PO - arterial PO

5

A-a gradient - normal values

10-15 mmHg

6

increased A-a gradient may occur in ....
situations

hypoxemia
causes include : shunting , V/Q mismatch, fibrosis ( impairs diffusion

7

• Define the variables of the full alveolar gas equation:

PAO2= PIO2 - PaCO2/R.
PAO2 = alveolar PO2, PIO2 = PO2 in inspired air, PaCO2 = arterial PCO2, R = respiratory quotient = CO2 produced/O2 consumed

8

• How can the alveolar gas equation be simplified and approximated (assuming that the patient is breathing ambient air)?

PAO2 = 150 - (PaCO2/0.8)

9

• By using the alveolar gas equation, what important measure of pulmonary function can be determined?


The alveolar-arterial gradient

10


• What is the normal alveolar-arterial gradient?


10–15 mmHg

11

• Explain why the alveolar-arterial gradient might be elevated. What three pathologic processes can cause this?

V/Q mismatches, diffusion limitations (fibrosis), and shunting (a rise in the A-a gradient may occur in hypoxemia)

12

• A patient has a respiratory quotient (R) of 1.0. What does this tell you about current metabolic state?

The respiratory quotient (R) is a ratio of carbon dioxide produced to oxygen consumed

13

• A patient breathes air with a PO2 of 100 mmHg. PaCO2 is 30 mmHg. With a respiratory quotient of 0.6, what is alveolar O2?

50 mmHg, as PAO2 = PIO2 - PaCO2/R = 100 mmHg - (30 mmHg/0.6)

14

• A patient's alveolar PO2 is 100 mmHg. Air PO2 is 150 mmHg. Assuming a respiratory quotient of 0.8, what is their arterial CO2?

40 mmHg, as = PAO2 = PlCO2 - PaCO2/R = 100 mmHg = 150 mmHg - (× mmHg/0.8)