4 - Ventilation-Perfusion Relationships

Question 1

What is the alveolar gas equation?

Answer 1

PAO2 = PIO2 - (PaCO2/R)

PAO2 = alveolar partial pressure of oxygen 
PIO2 = inspired partial pressure of oxygen (have to account for water vapor) 
PaCO2 = arterial CO2
R = 0.8 with a mix of fuels (200/250)

Question 2

PIO2 is the inspired partial pressure of oxygen, and for this you have to account for the water vapor. How do you calculate PIO2?

Answer 2

Barometric Pressure (760 mmHg) - Water Vapor (47 mmHg) x FiO2 (21%)

Question 3

T/F. The alveolar gas equation is used to calculate what inspired O2 needs to be to produce a desired alveolar (and therefore arterial) O2 level.

Answer 3

True

Question 4

On its own, being able to estimate the PAO2 is nice but not terribly useful. However, it allows us to calculate the ______ O2 gradient. This is a very useful number for determining the ‘health’ o the alveoli.

Answer 4

Alveolar-arterial

Question 5

Calculate the Alveolar-arterial gradient by PAO2 - PaO2. This number should normally be what?

Answer 5

< 20 mmHg

***It would be 0 in absolutely perfect lungs.

Question 6

An increase in the A-a O2 gradient indicates what?

Answer 6

Diffusion impairment – something is wrong at the alveoli

Question 7

If the A-a gradient is normal, but the PaO2 and the PAO2 are both low, the problem is elsewhere. The patient may be…

Answer 7

Hypoventilating

Question 8

At the apex of the lung, the blood pressure is reduced because it is slightly above the level of the heart. In middle regions (at level of the heart) blood flow is ________. At the base of the lungs, the blood flow is ________ because the pressure is high (we’re below the heart).

Answer 8

Normal

High

Question 9

The intrapleural fluid is also acted on by gravity and affects the intrapleural pressure. At the apex, there is less intrapleural fluid so the intrapleural pressure is more _________. At the base, there is more intrapleural fluid so the pressure is less _________ than you expect.

Answer 9

Negative (up to -10 cm H2O)

Negative (usually -2 to -3 cm H2O)

Question 10

The intrapleural pressure is also acted on by gravity. At the apex, the intrapleural pressure is more negative so the alveoli are ________ at rest. At the base, the pressure is less negative than you expect so those alveoli are ________ at rest (not as expanded by the pressure).

Answer 10

Large

Small

Question 11

At the apex, the intrapleural pressure is more negative, so the alveoli are large at rest. With _________, the alveoli can only get a little larger (think of a balloon that is already mostly inflated).

Answer 11

Inspiration

***Top of compliance curve

Question 12

At the base, the pressure is less negative than you expect so those alveoli are small at rest. With inspiration, the alveoli can get much (SMALLER/LARGER).

Answer 12

Larger

***Low on compliance curve

Question 13

Over the entire lung, what is the average ventilation/perfusion ratio (V/Q)?

Answer 13

V = 4 L/min
Q = 5 L/min

4/5 = 0.8

Question 14

What is high V/Q?

Answer 14

Ventilation is high relative to perfusion

***No blood coming to alveolus

Question 15

What is low V/Q?

Answer 15

Ventilation is low compared to perfusion

***Airway is blocked by something