A-a gradient interpretation and response to supplemental oxygen
Core concept
- Alveolar gas equation: PAO2 = FiO2 x (Patm - PH2O) - PaCO2/R
- Room air, sea level: PAO2 = 150 - (PaCO2 / 0.8)
- A-a gradient = PAO2 - PaO2
- Normal (room air) = (age/4) + 4, or roughly <15 mmHg young, <25 elderly
- Rises with age and with FiO2 - a gradient measured on high-flow O2 cannot be compared with the room-air normal
- The gradient answers one question: is the lung the problem, or is it upstream of the lung?
- Normal gradient -> the alveolus is fine; the problem is hypoventilation or low inspired PO2
- Wide gradient -> V/Q mismatch, shunt, or diffusion limitation
3 more sections, plus exam facts
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