Ventilation Perfusion Relationships

Question 1

What are the 4 mainc auses of hypoxemia?

Answer 1

1. hypoventilation
2. diffusion limitation
3. left to right shunts
4. ventilation-perfusion mismatch

Question 2

What is the oxygen level in air? in the alveoli? in tissues?

Answer 2

150 mmHg in air
100 in alveoli (since some has gone to the blood)
1-100 in tissues

Question 3

How do you maintain the 100 mmHg in the alveoli if O2 is being taken into the blood?

Answer 3

it’s constantly being replenished by inhalation of fresh air - if it’s not replenished fast enough in the alveoli, the alveolar pO2 will decline and you get hypoxia

Question 4

What are some causes of hypoventilation?

Answer 4

opiates, chest wall damage, paralysis

Question 5

pO2 will decrease in hypoventilation, but what happens fo pCO2?

Answer 5

will increase

Question 6

1

Answer 6

1

Question 7

1

Answer 7

1

Question 8

What is the respiratory quotient?

Answer 8

it’s produced CO2/consumed O2

it depends on substrate - 1 with carbohydrates as substrate, 0.7 with fatty acid substrate

Question 9

How do you impair diffusion?

Answer 9

reduce the area available for gas exchange or increase the diffusion distance

Question 10

Describe how a shunt can naturally occur.

Answer 10

Mixing of O2-depleted blood from the bronchial circulation with a small amount of blood from the thebesian veins of the heart.

Question 11

What are ways a shunt can occur abnormally?

Answer 11

congenital cardiac abnormalities and PDA

Question 12

How do you calculate shunt flow?

Answer 12

Qs/Qt = (CcO2 - CaO2 )/ (CcO2 - CVo2)

Question 13

Can 100% supplemental O2 correct the hypoxemia resulting from a shunt?

Answer 13

nope

Question 14

What’s the major cause of hypoxia in lung diseases?

Answer 14

mismatch of ventilation and perfusion

Question 15

If there is no perfusion of a unit but the alveoli are still ventilation, what will the pO2 and pCO2 of the alveoli be?

Answer 15

they will approach that of inspired gas = 100 mmHg O2 and 0 mmHg of CO2

Question 16

If there is perfusion but no ventilation, what will the pO2 and pCO2 of the alveoli be?

Answer 16

they will approach that of venous blood = 40 mmHg O2 and 46 mmHg CO2

Question 17

Describe how shunts are represented by oxygen/carbon dioxide curves

Answer 17

1

Question 18

1

Answer 18

1

Question 19

Describe the gradation of ventilation-perfusion differences thorugh the lung. Why do these occur?

Answer 19

The aprex has a high ventilation-perfusion ratio (pO2 132 pCO2 28) while the base has a low ventilation-perfusion ratio (PO2 89, pCO2 42)

this occurs because the apex has more oxygen, but lower perfusion because of gravity while the base has higher perfusion but less O2

Question 20

Why does pH dramatically change from apex to base?

Answer 20

because pH is largely influenced by CO2 and CO2 is higher at the base than the apex. More CO2, more buffering to increase [H], so the base has a lower pH than the apex

Question 21

Normally ventilation perfusion mismatches produce only a slight lowering of arterial pO2. WHen is it dramatic?

Answer 21

in lung disease

Question 22

Which can rapid breathing correct - the hypoxia or hypercapcnia?

Answer 22

can’t fix the hypoxia completely but it can fix the hypercapnia by flowing off the CO2 since CO2 elimination is a function of breathing rate

Question 23

Why are O2 and CO2 responses to rapid breathing different?

Answer 23

it’s a function of the different shapes of their dissociation curves:

CO2 dissodiation is linear in the physiological range
O2 dissociation is sigmoidal and almost flat at the top of the range

Question 24

How do you assess ventilation-perfusion abnormalities by calculation?

Answer 24

Use the alveolar gas equation to calculate what arterial pO2 should be and compare it to what is observed…

Question 25

What is the alveolar gas equation?

Answer 25

PaO2 = PiO2 - (PaCO2/R)

O2 in the alveolus = pressure of O2 in inspired air minus the pressure oc CO2 in the alveolus divided by the respiratory coefficiency