Increased V/Q Mismatch in Lung Disease causes Low PaO2 (PK5)

Question 1

Causes of increased O2 A-a difference

Answer 1

• Excessive V/Q inequality
• Increased a-v shunting
• Impaired diffusion

Question 2

Conditions affecting the whole lung that cause increased O2 A-a difference

Answer 2

• Pneumonia
• Surgery on single lung
• Congenital diaphragmatic hernia

Question 3

Conditions affecting the part(s) of a lung that cause increased O2 A-a difference

Answer 3

• Pneumonia (infection)
• Asthma/bronchitis/emphysema
• Aspiration (incl. stomach contents)
• Pulmonary embolism
• Septicaemia
• ARDS or RDS of the newborn

Question 4

20-year-old patient breathing room air at sea level has a PaO2 of 7kPa and a PaCO2 of 8kPa. R = 0.8. The patient is hypoventilating. Is the hypoxia due only to this reduced alveolar ventilation?

Answer 4

• Work out ideal PAO2 using alveolar gas equation
  > PIO2 - (PACO2 / R) = 19.5 - (8/0.8) = 9.5kPa
• Thus A-a = 9.5 - 7 = 2.5kPa
• This is excessive for his age
• Therefore he has increased V/Q mismatch/increased shunt/impaired diffusion