• PAO2 = 150 – (PaCO2/0.8) = 150 – (56/0.8) = 150 – 70 = 80
• A-a gradient = 80 – 70 = 10 < 12 –> Normal A-a gradient consistent with hypoventilation or decreased FiO2 (high altitude
• Key idea: Common causes of hypoventilation include neuromuscular weakness, drug intoxication (particularly opioids) and obesity hypoventilation syndrome (which is most likely the case here)

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