Lec 6 - Ventilation Perfusion Mismatch

Question 1

What are the consequences of inadequate ventilation of an alveolar unit?

Answer 1

Inadequate ventilation for perfusion (V/Q <1)

• PACO2 rises
• PAO2 falls.
• a new steady state is established.
• diffusion can continue at this level.
• blood equilibrates to the new alveolar PAO2 and PACO2.

Question 2

What are the causes for when Ventilation perfusion mismatch (V/Q) has a V/Q ratio of less than 1?

Answer 2

1. asthma - airway narrowing which is not uniform throughout the lungs.
2. COPD early stage s
3. Pneumonia - there is acute inflammatory exudate in the affected alveoli.
4. RDS is newborn - some alveoli are not expanded.
5. Pulmonary oedema - there is fluid in the alveoli.

Question 3

What is the normal V/Q ratio?

Answer 3

1

Question 4

What are the alveolar partial pressures when the V/Q ratio is <1?

Answer 4

• alveolar PO2 is lower and PCO2 is higher.

Question 5

What are the alveolar partial pressures when the V/Q ratio is >1?

Answer 5

• alveolar PO2 is higher and PCO2 is lower.

Question 6

What does the nature of the oxy-Hb curve mean in terms of compensating?

Answer 6

• units with V/Q >1 are unable to compensate for the low PO2 arising from units with a V/Q ratio <1.
• PCO2 arising from units, a V/Q ratio <1 is compensated for by units with V/Q >1.

Question 7

What do disorders with V/Q mismatch result in?

Answer 7

They result in low PO2 and a normal or low PCO2.