16- Ventilation Perfusion Ratio

Question 1

Air (V) and blood flow (Q) and V/Q are not uniform throughout the lung

Answer 1

• blood flow and ventilation and higher at the bottom of the lung
• V/Q is low at the bottom then gradually curves up as you go to the top of the lungs

Question 2

range of ventilation/perfusion ratios

Answer 2

O2:CO2

ideal for gas exchange
100:40
V/Q = 1.0

no exchange, venous admixture with occluded airway
40:45
V/Q = 0

no exchange, dead space ventilation with blocked vasculature
150:0
V/Q = infinity

-alveolar-capillary units must receive same proportion of alveolar ventilation and cardiac output for optimal gas exchange

Question 3

ventilation/perfusion ratio

Answer 3

V/Q determines alveolar gases

Note: with complete obstruction of the airway, PCO2 and PO2 approximate values in mixed venous blood (V)

with complete block of perfusion PCO2 and PO2 approximate values in inspired air

Question 4

Summary of regional differences in ventilation (left) and perfusion (right) in the normal upright lung.

Answer 4

TOP

• pleural pressure more negative, so alveoli are more filled but less ventilated b/c lung compliance is not linear
• less intravascular pressure than bottom
• higher resistance at top b/c of expanded alveoli, thus perfusion is less on the top than on the bottom

BOTTOM

• pleural pressure is less negative, so alveoli are less filled but ventilation is greater b/c of the steep lung compliance
• greater perfusion pressure and less resistance than at the top thus greater flow

Question 5

normal physiologic shunt

Answer 5

normal physiologic shunt (venous admixture) results in slight difference between alveolar and arterial PO2

-shunts in one lung cause hypoxemia (ventilated lung cannot compensate for unventilated lung)

Question 6

can a hyperventilated lung compensate for a hypoventilated lung?

Answer 6

No, you cannot maintain normal PaO2 but it can to maintain a normal PaCO2

Question 7

whats the effect of increasing PO2 in alveoli with normal V/Q?

Answer 7

it will not add sufficient O2 to compensate for alveoli with a V/Q of zero

Question 8

what happens to PaO2 as you increase in the % shunt?

Answer 8

as you increase the shunts (increasing non-ventilation to perfusion) then you have an increasing level of hypoxemia

Question 9

possible reasons for low PO2

Answer 9

• hypoventilation (not bringing in enough O2)

- can be due to exchange b/w alveoli and capillary

Question 10

How to tell the difference between low PO2 as a result of either diffusion problem or shunt problem?

Answer 10

if you give the patient 100% O2…

• if its a diffusion problem then PO2 will increase considerably
• if its a shunt problem then you wont get in increase in PO2

Question 11

abnormal alveolar-capillary gas exchange…

Answer 11

increases difference b/w alveolar and arterial PO2

Question 12

abnormal alveolar-capillary gas exchange and hypoventilation cause

Answer 12

hypoxemia and hypercapnia