Chp. 5: Ventilation-Perfusion Relationships

Question 1

Rate of removal of O2 from lung

Answer 1

Governed by O2 consumption of tissues and varies little under resting conditions. Alveolar PO2 is largely determined by level of alveolar ventilation.

Question 2

Normal alveolar PCO2

Answer 2

40mmHg

Question 3

Causes of hypoxemia

Answer 3

Hypoventilation
Shunt
Diffusion impairment
V/Q inequality

Question 4

Hypoventilation

Answer 4

If alveolar ventilation is abnormally low, alveolar PO2 falls and PCO2 rises.

Question 5

Causes of hypoventilation

Answer 5

Drugs, damage to chest wall, weakness or paralysis of respiratory muscles, high resistance to breathing (eg. asthma)

Question 6

Is the fall of PO2 or rise of PCO2 greater during hypoventilation?

Answer 6

Fall of PO2

CO2 stores are much greater than O2 stores because of large amount of CO2 in form of bicarbonate in the blood and interstitial fluid

Question 7

When does the PO2 difference between alveolar gas and end-capillary blood become larger?

Answer 7

Exercise or when blood-gas barrier is thickened or if low O2 mixture is inhaled

Diffusion limitation RARELY causes hypoxemia at rest at sea level

Question 8

Shunt

Answer 8

Refers to blood that enters the arterial system without going through ventilated areas of the lung

Bronchial artery blood, coronary venous blood through thebesian veins

Question 9

Shunt fraction

Answer 9

QS/QT = (CcO2 - CaO2) / (CcO2 - CvO2)

Question 10

Normal shunt fraction

Answer 10

5%

Question 11

Why is hypoxemia from shunt not abolished by giving 100% O2?

Answer 11

Shunted blood that bypasses ventilated alveoli is never exposed to the higher alveolar PO2, so it continues to depress the arterial PO2.

HOWEVER, some elevation of the arterial PO2 occurs because of the O2 added to the capillary blood of ventilated lung

Question 12

Why does shunt usually not result in elevated PCO2 in arterial blood?

Answer 12

Peripheral chemoreceptors respond by increasing ventilation. In some patients with shunt, the arterial PCO2 is low because the hypoxemia increases respiratory drive.

Question 13

Useful diagnostic test for shunt

Answer 13

When 100% O2 is inspired, arterial PO2 does not rise to expected level

Question 14

Most common cause of hypoxemia

Answer 14

V/Q inequality

Question 15

Low V/Q

Answer 15

Perfusion without ventilation

Question 16

High V/Q

Answer 16

Ventilation without perfusion

Question 17

How do BF and ventilation change from top to bottom of lung?

Answer 17

Ventilation increases slowly from top to bottom, blood flow increases more rapidly

Question 18

Change in V/Q ratio down lung

Answer 18

Decreases

Ventilation is less at top of lung than bottom, but differences in blood flow are more marked

Question 19

Wasted ventilation

Answer 19

Ventilation in excess of that normally required, necessary because lung units with abnormally high V/Q ratios are inefficient at eliminating CO2

Question 20

Hypoventilation: A-a difference and O2 response

Answer 20

None
Good

Question 21

Diffusion limitation: A-a difference and O2 response

Answer 21

Increased
Good

Question 22

Shunt: A-a difference and O2 response

Answer 22

Increased
Small but often useful

Question 23

V/Q inequality: A-a difference and O2 response

Answer 23

Increased
Good

Question 24

Ideal alveolar PO2

Answer 24

PO2 lung would have if there were no ventilation-perfusion inequality

Question 25

KEY CONCEPTS for V/Q Relationships

Answer 25

1) Four causes of hypoxemia are hypoventilation, shunt, diffusion limitation, and V/Q inequality 2) Two causes of hypercapnia are hypoventilation and V/Q inequality 3) Shunt is the only cause of hypoxemia in which the arterial PO2 does not rise to the expected level when a patient is given 100% oxygen 4) V/Q ratio determines PO2 and PCO2 in any lung unit. Because the ratio is high at the top of the lung, PO2 is high there and PCO2 is low 5) V/Q inequality reduces the gas exchange efficiency of the lung for all gases. However, many patients with V/Q inequality have a normal PCO2 because they increase the ventilation to their alveoli. Arterial PO2 is always low. Different behavior is attributable to their dissociation curves. 6) A-a difference is a useful measure of V/Q inequality. Alveolar PO2 is calculated from the alveolar gas equation using the arterial PCO2

Question 26

Normal A-a difference

Answer 26

10-15mmHg