Phys: Test 3: Lecture 7. Ventilation-Perfusion Relationships

Front 1

How does ventilation vary within the lung?

Back 1

greater at the base than the apex

Front 2

how does blood flow vary within the lung

Back 2

greater at the base than apex

Front 3

do ventilation and blood flow vary by the same amount?

Back 3

NO! blood flow increases more than ventilation as you move down the lung

Front 4

List the processes that determine arterial PO2

Back 4

o Ventilation
 Air into lung
o Diffusion
 Lung to blood
o Shunt
 Lung that passes through the lung from venous to arteriole side but does not get Oxygenated (adding deoxygenated blood)
o V/Q
 Matching of ventilation to perfusion is not ideal in the lung
 Reduces arteriole PO2 (slightly lower than alveolar)

Front 5

List the four causes of hypoxemia.

Back 5

• Low arteriole PO2 caused by any of the 4 following
o Reduced ventilation
o Diffusion impairment
o Increased amount of shunt
o Increased V/Q inequality

Front 6

What is the ideal alveolar gas equation?

Back 6

PAO2= PIO2 + (PaCO2 / .8)

Front 7

Be able to calculate the A-a gradient.

Back 7

o A-a= PAO2-PaO2
o Note: A-alveolar, a-arteriole

Front 8

• In a healthy patient, what should the A-a gradient look like?

Back 8

o It should be very small
o In breathing problems, there will be a large difference

Front 9

Explain why hypoventilation decreases arterial PO2 but does not increase the A-a gradient.
Describe what happens to arterial PCO2 in hypoventilation.

Back 9

• Alveolar PO2 is decreased
o But not interfering with shunt, or diffusion, etc.
 So the arteriole PO2 also drops, making the A-a gradient not change
• You will have a lower overall PaO2 and PAO2, but again the A-a gradient stays the same

Front 10

how do you treat hypoventilation

Back 10

with O2

Front 11

Does diffusion impairment often result in hypoxemia? how do u treat?

Back 11

• Rarely results in hypoxemia
• But in severe cases it would decrease the PaO2
• Treat with Oxygen

Front 12

What do V/Q inequality and shunt do to PaO2 and the A-a gradient?

Back 12

• V/Q inequality lowers the PaO2
o So does increased shunt
 This will lead to increased A-a gradient
• Alveolar PO2 will be the same, arteriole will be lowered
• They interfere with oxygen transfer

Front 13

• If you have a V/Q inequality how do you treat?

Back 13

o with oxygen

Front 14

Can you treat a pt with shunt successfully with O2?

Back 14

hell no. you out your stars?

Front 15

Kindergarten

Back 15

유치원

Front 16

How does the V/Q ratio change in different parts of the lung?

Back 16

• V/Q ratio in upright lung DECREASES apex to base

Front 17

Normal sources of shunt?

Back 17

• Some Bronchial artery blood
• Some Coronary venous blood

Front 18

at the base of the lung is blood flow or ventilation greater?

Back 18

blood flow

Front 19

at the apex of the lung is blood flow or ventilation greater?

Back 19

ventilation

Front 20

What occurs with perfusion/ventilation in SHUNT? what is the V/Q ratio? what will the gas composition of that alveoli look like?

Back 20

Perfusion with no ventilation

so V/Q=0

gas composition will represent pulmonary arteriole blood

Front 21

What occurs with perfusion/ventilation in DEAD SPACE? what is the V/Q ratio? what will the gas composition of that alveoli look like?

Back 21

Ventilation with no perfusion

V/Q = infinity

no addition of O2 or removal of CO2

gas will be that of atmospheric

Front 22

Describe two reasons why ventilation-perfusion inequality present in a normal healthy
subject depresses arterial PO2 slightly.

Back 22

• Most blood goes through areas where PAO2 is low
o So more blood will go to the base where PO2 is about 89
o Less blood will go to the apex where PO2 is around 132
 But because more goes to the base, you end up having an average of about 97 for your PaO2 whereas it was around 100 when it came in

Front 23

. Describe how the ventilation-perfusion ratio affects alveolar PO2 and PCO2. Describe the
variation in alveolar PO2 and PCO2 in an upright lung.

Back 23

o Upper part of the lung
 Resembles that of inspired air (high PO2, low PCO2)
o Base of lung
 Profusion is higher than ventilation
 So values represent more of venous return (low PO2, High PCO2)

Front 24

Why is arterial PCO2 often normal with V/Q inequality?

Back 24

• Lung units with high V/Q ratios don’t add much O2 to the blood, BUT can remove blood CO2

Front 25

if PaCO2 is up, and you have no increase in A-a gradient what do you have?

Back 25

Hypoventilation

Front 26

If PaCO2 is up and there is no change in A-a gradient what do you have?

Back 26

hypoventilation

Front 27

in PaCO2 is not up but your patient is hypoxic, and the A-a gradient is not changed, what do you have?

Back 27

Decreased PIO2

Front 28

in PaCO2 is not up but your patient is hypoxic, and the A-a gradient is changed, what do you have?

Back 28

you have to treat with O2 and see if they have shunt or V/Q inequality