Respiratory failure

Hypoxemia can be due to 4 lung-related causes

1. V/Q mismatch
2. Shunt
3. Hypoventilation
4. Diffusion limitation

Hypoxaemia and A-a gradient

Normal A-a gradient hypoxaemia

• Alveolar hypoventilaton
  * i.e. PACO2 increased
• Low FiO2
  * FiO2 <0.21

Raised A-a gradient hypoxaemia

• Diffusion limitation
  * Rare
• V/Q mismatch
• Shunt
  * Intrapulmonary or cardiac
• Increased arterio-venous oxygen extraction

Drawback of A-a gradient

• A-a gradient is both FiO2 and age dependent

Normal value of A-a gradient

FiO2	Young	Old
0.21	7 mmHg	14 mmHg
1.0	31 mmHg	56 mmHg

• When there is intrapulmonary shunt or V/Q mismatch, A-a gradient varies greatly and variably with FiO2
  * [TO5:p96-97]