Ventilation - Perfusion Ratio

Question 1

What are the oxygen gradients of oxygen from air to tissue?

Answer 1

150mmHg in humidified air
100mmHg in alveoli
1-100 mmHg in tissues

this means that oxygen is constantly being removed by blood if alveolus is perfused

Question 2

What happens during hypoventilation?

Answer 2

O2 is not replenished fast enough in the alveoli, the alveolar PO2 declines

Also causes increased PCO2

Question 3

What conditions cause hypoventilation?

Answer 3

drugs (opiates), chest wall damage, paralysis etc.

Question 4

What happens to gas pressure if there is no perfusion (blocked blood vessel) but there is still ventilation?

Answer 4

the PO2 and PCO2 approach that of inspired gas (150 & 0 mmHg)

Question 5

What happens to gas pressure if there is no ventilation (blocked alveolus) but there is still perfusion?

Answer 5

the PO2 and PCO2 approach that of venous blood (40 & 46 mmHg)

Question 6

What is the ventilation-perfusion ratio of the apex?

Answer 6

Apex has high ventilation-perfusion ratio
PO2 of 132 mmHg
PCO2 of 28 mmHg

Question 7

What is the ventilation-perfusion ratio of the base?

Answer 7

Base has a low ventilation-perfusion ratio
PO2 of 89 mmHg
PCO2 of 42 mmHg

Question 8

What normally happens to pCO2 in lung disease?

Answer 8

Normally the PCO2 will be close to normal even in lung disease because alterations in arterial PCO2 will result in more rapid breathing

-increases work of breathing

Question 9

How do you calculate ventilation perfusion abnormalities?

Answer 9

Use the alveolar gas equation to calculate what arterial PO2 should be and compare it to what is observed

Question 10

What is the alveolar gas equation and how are its normal values?

Answer 10

PAO2 = PIO2 – (PACO2/R)

Usually should give a value of 100 mmHg (PCO2 of 40 & R = 0.8, PIO2 = 149)