Ventilation and Perfusion

Question 1

Increased V / Q will […] CO2 and

[…]O2 partial pressure.

Answer 1

Increased V / Q will drop CO2 and raise O2 partial pressure.

•In this case ventilation is removing carbon dioxide faster than perfusion is delivering it to the alveolar capillary unit. Also ventilation is delivering fresh oxygen faster than perfusion is removing it from the alveolar capillary unit.

Question 2

Decreased V / Q will

[…] CO2 and

[…] O2 partial pressure.

Answer 2

Decreased V / Q will raise CO2 and drop O2 partial pressure.

•In this case perfusion is delivering carbon dioxide faster than ventilation is removing it from the alveolar capillary unit. Also perfusion is removing oxygen faster than ventilation can deliver it to the alveolar capillary unit.

Question 3

Answer 3

Question 4

[…] and […] tensions are usually in equilibrium and are determined in large part by their V’/ Q’relationship.

Answer 4

Alveolar gas and end-pulmonary capillary gas tensions are usually in equilibrium and are determined in large part by their V’/ Q’relationship.

Question 5

In the upright lung gravity causes a […] gradient to alter distribution of ventilation and a […] to alter distribution of bloodflow.

Answer 5

In the upright lung gravity causes a pleural pressure gradient to alter distribution of ventilation and a hydrostatic pressure gradient to alter distribution of bloodflow.

Question 6

The bottom of the lung both […] more than the top and receives […] than the top thereby matching ventilation.

Answer 6

The bottom of the lung both ventilates more than the top and receives more cardiac output than the top thereby matching ventilation.

Question 7

The lung’s overall V’/Q’ is about 0.8; however the ventilation at the top of the lung is relatively greater than the perfusion (increased V’/Q’). How will this affect alveolar gas tensions?

Answer 7

Alveolar gas at the apex will be higher in oxygen and lower in carbon dioxide than then at the base.

Question 8

At the bottom of the lung the V’/Q’ is 0.6. What does this tell us about the relative change in ventilation and perfusion in the base of the lung? How will it affect alveolar gas tension?

Answer 8

Perfusion increases more rapidly from the top to the bottom then does ventilation. The base is relatively over-perfused and thus will have lower oxygen and higher carbon dioxide gas tensions.

Question 9

Normal P_AO₂

Answer 9

100 mmHg

Question 10

Normal P_ACO₂

Answer 10

40 mmHg

Question 11

Normal P_IO₂

Answer 11

150 mmHg

Question 12

Normal P_aO₂

Answer 12

95 mmHg

Question 13

Normal P_aCO₂

Answer 13

40 mmHg

Question 14

F_IO₂

Answer 14

0.21

fractional O₂ inspired

Question 15

Answer 15

Question 16

Mixed Venous Blood PO₂

Answer 16

40 mmHg

Question 17

Mixed Venous Blood PCO₂

Answer 17

46 mmHg

Question 18

Answer 18

Question 19

Answer 19

Question 20

Answer 20

Question 21

4 Causes of Arterial Hypoxemia

Answer 21

1. Hypoventilation
2. Diffusion Defects
3. V’/Q’ Mismatch
4. Shunt (V’/Q’ = 0)

Question 22

Hypoventilation

Answer 22

• In this case the lung is being globally hypoventilated, but has relatively uniform ventilation/perfusion distribution. This occurs in disruption of central respiratory control as is seen in certain sedative/narcotic overdoses, or in conditions where the respiratory pump is failing such as muscular dystrophy.
• Giving supplemental O₂ can very effectively overcome severe carbon dioxide retention (hyoercapnea or hypercarbia)
• If the A-a Difference in PaO2 is increased, then something other than hypoventilation is causing it;i.e. diffusion defect, V’/Q’ mismatch, or shunt.

Question 23

Diffusion Defects

Answer 23

•Thickening of the alveolar capillary membrane can cause hypoxemia. Causes include pulmonary fibrosis and interstitial inflammation. Hypoxemia due to diffusion block is worsened at high altitude and during exercise.

Question 24

V’/Q’ Mismatch

Answer 24

•This occurs when some regions of the lung are hypoventilated so that their alveolar oxygen drops and the blood is not adequately oxygenated. This occurs in most lung disease with airway obstruction and/or alveolar damage. This term is usually used to describe conditions where ventilation is not zero.

Question 25

Shunt (V’/Q’ = 0)

Answer 25

•In this case some units of the lung receive no ventilation at all. Shunting of blood completely bypasses ventilation and can occur in the lung (intrapulmonary shunt) or due to cardiovascular malformation (extrapulmonary or cardiac shunt).

Question 26

How to Find the Cause of Hypoxemia

Answer 26

• First, calculate the PAO2 and decide if hypoventilation is the cause of the hypoxia.
• If it is not hypoventilation, then we could measure a diffusion capacity (DLCO) to see if there is diffusion limitation causing the hypoxia.
• If it is not diffusion limitation then we are down to V’/Q’ mismatch and shunt. We could give the patient 100% oxygen to breathe. In V’/Q’ mismatch the hypoxia will reverse. In Right to Left shunt, there will be no impact of the supplemental oxygen.