If FiO2 increases from 0.60 to 0.80 while PaO2 remains at 90 mmHg, the PaO2/FiO2 ratio will:

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Multiple Choice

If FiO2 increases from 0.60 to 0.80 while PaO2 remains at 90 mmHg, the PaO2/FiO2 ratio will:

The key idea is that the PaO2/FiO2 ratio reflects oxygen transfer efficiency and depends on both the oxygen tension in blood and the inspired oxygen fraction. If PaO2 stays the same while FiO2 increases, the ratio will fall because you’re dividing the same PaO2 by a larger FiO2. Calculate it to see the change: initially, 90 mmHg divided by 0.60 gives 150. After increasing FiO2 to 0.80 with the same PaO2, 90 divided by 0.80 equals 112.5. So the ratio decreases from 150 to 112.5, indicating a drop in the efficiency of oxygen transfer despite more oxygen being delivered.

The key idea is that the PaO2/FiO2 ratio reflects oxygen transfer efficiency and depends on both the oxygen tension in blood and the inspired oxygen fraction. If PaO2 stays the same while FiO2 increases, the ratio will fall because you’re dividing the same PaO2 by a larger FiO2.

Calculate it to see the change: initially, 90 mmHg divided by 0.60 gives 150. After increasing FiO2 to 0.80 with the same PaO2, 90 divided by 0.80 equals 112.5. So the ratio decreases from 150 to 112.5, indicating a drop in the efficiency of oxygen transfer despite more oxygen being delivered.

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