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Notes
      3. Old stuff
          3.2. Old physio stuff (around 2005)
              3.2.3. Physiology
                  3.2.3.13. Respiratory
                      3.2.3.13.2. Ventilation and perfusion
                          3.2.3.13.2.5. Abnormality in ventilation and/or perfusion
 3.2.3.13.2.5.4. Respiratory failure 

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]

 

 

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