If PAO2 is 380 mmHg and PaO2 is 85 mmHg, what is the A-a gradient?

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If PAO2 is 380 mmHg and PaO2 is 85 mmHg, what is the A-a gradient?

The A-a gradient shows how well oxygen moves from the alveoli into the arterial blood. It’s found by subtracting the arterial O2 tension from the alveolar O2 tension: PAO2 minus PaO2. Here, PAO2 is 380 mmHg and PaO2 is 85 mmHg, so the gradient is 380 − 85 = 295 mmHg. A gradient this large indicates a significant problem with oxygen transfer (ventilation-perfusion mismatch, diffusion limitation, or shunt), since the arterial oxygen remains much lower than what’s available in the alveoli. In contrast, a normal gradient is relatively small (roughly age-related, around 10–15 mmHg in young adults). The main point is the subtraction to get a wide difference, which explains the markedly reduced PaO2 despite high PAO2.

The A-a gradient shows how well oxygen moves from the alveoli into the arterial blood. It’s found by subtracting the arterial O2 tension from the alveolar O2 tension: PAO2 minus PaO2. Here, PAO2 is 380 mmHg and PaO2 is 85 mmHg, so the gradient is 380 − 85 = 295 mmHg. A gradient this large indicates a significant problem with oxygen transfer (ventilation-perfusion mismatch, diffusion limitation, or shunt), since the arterial oxygen remains much lower than what’s available in the alveoli. In contrast, a normal gradient is relatively small (roughly age-related, around 10–15 mmHg in young adults). The main point is the subtraction to get a wide difference, which explains the markedly reduced PaO2 despite high PAO2.

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