Lecture 4: VQ Relationships

Question 1

What is the alveolar gas equation?

Answer 1

PAO2 = PIO2 - PaCO2/R

PIO2 = inspired O2
PaCO2 = arterial CO2
R = 0.8

Question 2

How do you calculate PiO2?

How do you calculate respiratory quotient?

Answer 2

PiO2 = (barometric pressure/760 - partial pressure of water vapor/47 x oxygen % (21% at room air)

R = CO2 consumption/O2 consumption

Question 3

How do you calculate A-a O2 gradient and what is this used for?

Answer 3

-PAO2 - PaO2 (alveolar - arterial). Determines alveolar “health”

Question 4

What is the normal level for the A-a O2 gradient and what does this pathologically indicate?

Answer 4

<20 mm Hg. Increase indicates diffusion problem at alveoli level

Question 5

At the apex of the lung…
_____ intrapleural fluid
_____ pressure
_____ alveolar size

Answer 5

Less (gravity pulls it down)
More negative
Big

Question 6

At the base of the lung….

_____ intrapleural fluid
_____ pressure
_____ alveolar size

Answer 6

More (gravity causes pooling at the base)
Less negative
Small

Question 7

How does compliance compare in the apex vs. base of the lung? (use the intrapleural pressure vs. alveolar volume graph)

Answer 7

Apex: large alveoli can only expand so much > lower compliance and change in volume

Base: small alveoli can still expand by a lot > higher compliance and change in volume

Question 8

What is the V/Q ratio and what is the average?

Answer 8

Ratio between V (ventilation) and Q (perfusion)

0.8

Question 9

What happens at low V/Q ratio?

What does this mimic?

Answer 9

Means lower ventilation (air) compared to perfusion (blood). Some of the blood has no air to exchange with and leaves lungs without getting O2 and getting rid of CO2.

-R to L shunt, as if blood didn’t go to the lungs)

Question 10

What happens with a high V/Q ratio?

Answer 10

High ventilation (lots more air) vs. low perfusion (flowing blood). Air in alveolus cannot exchange since there is no blood flowing by

Question 11

At the apex of the lung…
V/Q ratio is \_\_\_
PaO2 is \_\_\_
PaCO2 is \_\_\_
pH is \_\_\_
Blood volume is \_\_\_

Answer 11

Increased
Increased
Decreased
Increased
Decreased

Question 12

At the base of the lung….
V/Q ratio is \_\_\_
PaO2 is \_\_\_
PaCO2 is \_\_\_
pH is \_\_\_
Blood volume is \_\_\_

Answer 12

Decreased
Decreased
Increased
Decreased
Increased

Question 13

How does the body minimize V/Q differences in the different zones of the lung?

Answer 13

Hypoxic vasoconstriction (shunt blood away from hypoxic regions)