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PAO2 = Alveolar partial pressure of O2 [mm Hg]
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PaO2 = Arterial partial pressure of O2 [mm Hg]
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FiO2 = Fraction of inspired O2 (0.21 in room air)
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Patm = Atmospheric pressure (about 760 mm Hg at sea level)
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= Partial pressure of water vapor (about 47 mm Hg at 37°C)
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PaCO2 = Arterial partial pressure of CO2 [mm Hg]
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R = Respiratory quotient, reflecting basal metabolic rate describing amount of CO2 production for a given O2 consumption (ranges from 0.7 to 1; usually 0.8 is used)
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Measure of the efficiency of the oxygenation of blood
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The normal A-a gradient varies with age. A helpful calculation is [age(years)+10]/4
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Normal A-a gradient ranges from 7 to 14 mm Hg when breathing room air; 20–65 mm Hg when on 100% O2. As the gradient value increases, it is reflective of a worsening condition
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Gradient is affected by ventilation/perfusion (V/Q) or diffusion abnormalities but unaffected by hyper- or hypoventilation
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If PaO2 corrects with supplemental O2, consider asthma or other conditions associated with V/Q mismatch
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If PaO2 does not correct with supplemental O2, consider shunt