Ventilation-Perfusion Relationships

Question 1

What occurs when the V/Q ratio is less than 1?

Answer 1

V/Q ratio < 1 - perfusion exceeds ventilation

Question 2

What occurs when the V/Q ratio is greater than 1?

Answer 2

V/Q ratio > 1 - ventilation exceeds perfusion

Question 3

What is the normal V/Q value?

Answer 3

0.8

Question 4

What is the V/Q ratio in the perfect lung?

Answer 4

1

Question 5

What does the V/Q ratio represent at the level of the alveoli?

Answer 5

It is the ratio of alveolar ventilation to capillary flow

Question 6

What does the V/Q ratio represent at the level of the whole lung?

Answer 6

It is the ratio of total alveolar ventilation to cardiac output

Question 7

What happens to the V/Q ratio when ventilation is obstructed?

Answer 7

The V/Q will decrease.

PO2 levels in the lung will decrease and PCO2 will increase.

Question 8

What happens to the V/Q ratio when perfusion is obstructed?

Answer 8

The V/Q will increase.

PO2 levels in the lung will increase and the PCO2 will decrease.

Question 9

Which part of the lung receives the best ventilation??

Answer 9

Bottom

Question 10

Which part of the lung receives the best perfusion?

Answer 10

Bottom

Question 11

What happens to the V/Q ratio as you go from the bottom of the lung to the top of the lung?

Answer 11

The V/Q ratio will increase towards the top of the lung as the ventilation decreases more slowly that the perfusion from the base to the apex.

Question 12

Why is the PO2 of blood leaving the lung lower than that of inspired air?

Answer 12

Most of it comes from the base of the lungs which has a lower PO2 than the apex of the lungs.

Question 13

What is the normal arterial-alveolar PO2 difference (AaDO2)?

Answer 13

<15 mmHg

Question 14

What are some of the reasons for the AaDO2?

Answer 14

V/Q Inequality
Anatomic Shunt (Thebesian Vessels)

Question 15

What is the AaDO2 equation?

Answer 15

AaDO2 (alveolar) = [PiO2 (inspired) - (PCO2/R)] - PaO2 (measured)

Question 16

What is the PO2 level in hypoxemia?

Answer 16

<80 mmHg PO2

Question 17

What are the causes of hypoxemia?

Answer 17

Hypoventilation
Diffusion Limitation
Shunted Blood
Ventilation-Perfusion Inequality

Question 18

How does hypoventilation result in hypoxemia?

Answer 18

It will decrease PO2 and increase PCO2 which will result in a decrease of PAO2 (alveolar).

Question 19

How does hypoventilation affect the AaDO2?

Answer 19

It should not affect it as the gas is diffusing properly

Question 20

How does diffusion limitation result in hypoxemia?

Answer 20

It will increase the AaDO2 and make it more difficult for O2 to diffuse into the blood

Question 21

How does an anatomical or physiological shunt result in hypoxemia?

Answer 21

Blood can enter the arterial system being mixed with “dirty blood” or without going through ventilated areas.

Question 22

Can increasing the O2 help to increase the PO2 in shunt hypoxemia?

Answer 22

No because it will not affect the shunted area. This is the ONLY hypoxemia that cannot be helped with increased O2.

Question 23

What is the most frequent cause for hypoxemia?

Answer 23

Ventilation-Perfusion Inequality

Question 24

How does ventilation-perfusion inequality result in hypoxemia?

Answer 24

It causes either a lack of adequate ventilation for the blood flow, or a lack of adequate blood flow for the ventilation and not enough O2 reaches the blood.

Question 25

Can hypoxemia from ventilation-perfusion inequality be eliminated by increasing ventilation?

Answer 25

No.

Question 26

How are PCO2 levels affected by VP inequality?

Answer 26

CO2 levels can be fixed with increased ventilation from chemoreceptors, whereas O2 cannot. Therefore CO2 levels should be normal

O2 dissociation curve is flat so only low V/Q inequality ratios will benefit from increased ventilation.

Question 27

How are PO2 levels affected by VP inequality?

Answer 27

It will have decreased levels that cannot be fixed with hyperventilation. This is because O2 is near 100% saturation at normal breathing and increasing ventilation would not increase the O2 to the areas of inequality very much.

This is similar to the shunt problem.

Question 28

What are the dead space units?

Answer 28

They are regions at the top of the lung where it is over-ventilated

Question 29

What are the physiological shunt units?

Answer 29

They are regions at the base of the lung where turnover of alveolar gas is less frequent than normal