using Aa gradient to determine reason for hypoxemia

• related: pathophysiology of hypoxia and hypoxemia
• tags: #permanent

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Aa gradient is difference between oxygen in the alveoli and oxygen diffused in the blood:

Normal Aa gradient is 10-15 mmHg.

Hypoxemia is decreased $O_2$ saturation, which is directly affected by $P_{aO_2}$ (alveolar gas equation calculates oxygen content of alveoli).

This could occur as result of either decreased available oxygen prior to oxygen exchange in the lung (low $P_{AO_2}$) or from decreased oxygen diffusion (low $P_{aO_2}$):

• decreased available oxygen prior to exchange: this will have normal $P_{AO_2}$ to $P_{aO_2}$ gradient
  • hypoventilation
  • high altitude
• decreased oxygen diffusion: this will have elevated Aa gradient:
  • fibrosis
  • low V/Q: can treat with O2
  • dead space: V/Q = infinity: can treat with O2
  • shunt: V/Q = 0: cannot treat with O2¹

Links to this note

• pathophysiology of hypoxia and hypoxemia
  • using Aa gradient to determine reason for hypoxemia
• nocturnal hypoxemia has various pathophysiology
  • patients with OSA/CSA have hypoxemia from apnea (using Aa gradient to determine reason for hypoxemia, sleep disordered breathing is characterized by apnea, hyponea, hypoventilation, RERA)

Footnotes

1. 05 Hypoxia - Boards and Beyond ↩