Which setting would give the highest arterial oxygen concentration during inhalation induction with sevoflurane?

Which setting would give the highest arterial oxygen concentration during inhalation induction with sevoflurane?

• A. L/min O2: 1, Air: 2, N2O: 0
• B. L/min O2: 2, Air: 0, N2O: 2
• C. L/min O2: 2, Air: 2, N2O: 2
• D. L/min O2: 4, Air: 10, N2O: 1

Answer: (B)

The main idea here is FiO2—the fraction of inspired oxygen. Arterial oxygen concentration during inhalation induction tracks how much oxygen is delivered in the fresh gas mix. Nitrous oxide doesn’t add oxygen, and room air provides about 21% O2, so the oxygen fraction depends on the O2 flow, the amount of air, and any N2O present. Calculate FiO2 as (O2 flow × 1.0 + air flow × 0.21) divided by the total fresh gas flow (O2 + air + N2O): - Setting with 1 L/min O2, 2 L/min air, 0 N2O: FiO2 = (1.0 + 0.21×2) / 3 = 1.42/3 ≈ 0.473 (47.3%). - Setting with 2 L/min O2, 0 air, 2 L/min N2O: FiO2 = (2.0 + 0) / 4 = 2/4 = 0.50 (50%). - Setting with 2 L/min O2, 2 L/min air, 2 L/min N2O: FiO2 = (2.0 + 0.21×2) / 6 = 2.42/6 ≈ 0.403 (40.3%). - Setting with 4 L/min O2, 10 L/min air, 1 L/min N2O: FiO2 = (4.0 + 0.21×10) / 15 = 6.1/15 ≈ 0.407 (40.7%). The highest FiO2—and thus the highest arterial oxygen concentration—occurs with the second setting: 50% oxygen delivered (2 L/min O2 with 2 L/min N2O, no diluting air).

The main idea here is FiO2—the fraction of inspired oxygen. Arterial oxygen concentration during inhalation induction tracks how much oxygen is delivered in the fresh gas mix. Nitrous oxide doesn’t add oxygen, and room air provides about 21% O2, so the oxygen fraction depends on the O2 flow, the amount of air, and any N2O present.

Calculate FiO2 as (O2 flow × 1.0 + air flow × 0.21) divided by the total fresh gas flow (O2 + air + N2O):

• Setting with 1 L/min O2, 2 L/min air, 0 N2O: FiO2 = (1.0 + 0.21×2) / 3 = 1.42/3 ≈ 0.473 (47.3%).

• Setting with 2 L/min O2, 0 air, 2 L/min N2O: FiO2 = (2.0 + 0) / 4 = 2/4 = 0.50 (50%).

• Setting with 2 L/min O2, 2 L/min air, 2 L/min N2O: FiO2 = (2.0 + 0.21×2) / 6 = 2.42/6 ≈ 0.403 (40.3%).

• Setting with 4 L/min O2, 10 L/min air, 1 L/min N2O: FiO2 = (4.0 + 0.21×10) / 15 = 6.1/15 ≈ 0.407 (40.7%).

The highest FiO2—and thus the highest arterial oxygen concentration—occurs with the second setting: 50% oxygen delivered (2 L/min O2 with 2 L/min N2O, no diluting air).