A-aDO2 gradient is best determined after the patient has been on 100% oxygen for how long?

A-aDO2 gradient is best determined after the patient has been on 100% oxygen for how long?

• A. 5 minutes
• B. 10 minutes
• C. 20 minutes
• D. 60 minutes

Answer: (C)

The A-a gradient tells you how well oxygen is moving from the alveoli into the arterial blood. To calculate PAO2 for this gradient when the patient is on 100% oxygen, you use the alveolar gas equation, which depends on FiO2 and the patient’s PaCO2. After changing to 100% O2, it takes time for the alveolar gas tensions and the arterial gas values to reach a steady state that truly reflect the current FiO2. About 20 minutes is the typical period needed for this equilibrium, giving a reliable A-a gradient. Shorter times won’t yet reflect the new oxygen level, while much longer times add no benefit.

The A-a gradient tells you how well oxygen is moving from the alveoli into the arterial blood. To calculate PAO2 for this gradient when the patient is on 100% oxygen, you use the alveolar gas equation, which depends on FiO2 and the patient’s PaCO2. After changing to 100% O2, it takes time for the alveolar gas tensions and the arterial gas values to reach a steady state that truly reflect the current FiO2. About 20 minutes is the typical period needed for this equilibrium, giving a reliable A-a gradient. Shorter times won’t yet reflect the new oxygen level, while much longer times add no benefit.