Hypoxemia
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Hypoxia vs. Hypoxemia
Hypoxia is when end organs don't have enough oxygen to meet demand (often difficult to prove). Hypoxemia is when the oxygen content of the blood is abnormally low (easy to prove). Hypoxemia is the most common cause of hypoxia.
Calculating A-a gradient
Get an ABG. Then use the alveolar gas equation:
- PAO2 - PaO2
- PAO2 = ([Patm - PH2O] x FiO2 - [PaCO2/Q])
- PAO2 = ([Patm - 47] x FiO2 - [PaCO2/0.8])
- PAO2 = (150 - [PaCO2/0.8]) (when at sea level with a patient breathing room air)
Normal A-a gradient increases with age, use the equation: (Age+10)/4.
Differential
https://onepagericu.com/hypoxia
- Increased A-a gradient
- V/Q mismatch (most common cause)
- Shunt (only one which does NOT resolve with supplemental O2)
- Diffusion barrier (worse with increased cardiac output)
- Normal A-a gradient
- Low inspired O2 (high altitude, low FiO2)
- Hypoventilation (increased PaCO2)
- Low SvO2 (low cardiac output, high O2 consumption)
Special Situations
- CCBs can be a 'gas-fire on atelectasis' due to pulmonary vasodilation and disruption of V/Q matching leading to worsened shunt and hypoxemia