Assessment of V/Q

Question 1

A V /Q ratio of _____ is generally thought to be sufficient.

Answer 1

0.8 - 1.0

Question 2

Example: pH = 7.40, PCO2 = 32, PO2 = 65. What is his A-a gradient?

Answer 2

PAO2 = 122- (32/.8) = 82mmHg 82-65 = 17mmHg

Question 2

What is a normal A-a gradient value?

Answer 2

less than 10mmHg

Question 3

What are the 5 causes of hypoxemia?

Answer 3

1. Low ambient PO2 (altitude)
2. Hypoventilation (Ondine’s curse)
3. V /Q mismatch (Pneumonia, COPD, asthma, etc)
4. Shunt (intracardiac or intrapulmonary)
5. Diffusion limitations (ARDS, RDS, ILD, etc..)

Question 4

Hypoxemia due to altitude and hypoventilation give a ______ A-a gradient.

Answer 4

normal

Question 6

PAO2 = PIO2 - (PaCO2/R)

Answer 6

PaCO2 normally = 40; use PIO2 = 122 for Denver; R = 0.8

Question 7

Hypoxemia due to _____ and ______ give a normal A-a gradient

Answer 7

altitude; hypoventilation

Question 8

What pathologic process causes alveolar air spaces to become filled with exudate?

Answer 8

• pneumonia
• ARDS

Question 8

Hypoxemia due to shunt, V/Q mismatch, and diffusion limitations give a _____ A-a gradient.

Answer 8

widened

Question 10

What pathologic process causes alveolar air spaces to become filled with transudate?

Answer 10

heart failure

Question 11

Hypoxemia due to ____, ____, and ____ give a widened A-a gradient.

Answer 11

shunt, V/Q mismatch, and diffusion limitations

Question 12

What is shunt?

Answer 12

blood perfusion where there is no ventilation

Question 13

How can you tell the difference between a shunt and V/Q mismatch clinically?

Answer 13

V/Q mismatch will improve PO2 with supplemental 100% O2

Question 14

____ is blood perfusion where there is no ventilation.

Answer 14

Shunt

Question 15

What is a normal value for physiologic dead space?

Answer 15

less than 30%

Question 16

You are having difficulty ventilating a patient in the ICU and you wish to determine the patient’s dead space. His arterial PCO2 is 60 mmHg. After obtaining a 5 minute sample of expired air it is determined to have a PCO2 (PECO2) of 20 mmHg. What is his dead space?

Answer 16

VD/VT = (arterial - expired) / arterial = (60-20)/60 = .67