Q4. A patient with acute hypoxemia is placed on a non‑rebreather mask at 15 L/min. What FiO₂ is expected?

Question

BlackTom AI · Accepted Answer

Non‑rebreather masks with a full reservoir bag and one‑way valves can deliver almost 100% oxygen when supplied at high flow (15 L/min).

1) Relevant knowledge points and concepts
- A non‑rebreather mask (NRB) has a reservoir bag and one‑way valves that prevent room air entrainment and rebreathing of exhaled gas when functioning properly.
- FiO₂ delivered depends on oxygen flow, mask fit, valve integrity, and patient minute ventilation. If flow into the reservoir exceeds the patient’s inspiratory demand and valves prevent ambient air entry, inspired gas is essentially pure O₂ (FiO₂ ≈ 1.0 or 100%).

2) Step‑by‑step reasoning
- Typical adult minute ventilation (MV) = tidal volume × respiratory rate. Example: TV = 500 mL, RR = 12 → MV = 0.5 L × 12 = 6 L/min.
- Oxygen flow to the NRB is 15 L/min, which is substantially greater than MV (15 L/min > 6 L/min). Thus the reservoir bag remains filled with oxygen throughout the respiratory cycle.
- During inspiration, the patient draws gas from the reservoir (nearly 100% O₂) rather than entraining room air.

3) Intermediate calculations
- MV example: 0.5 L × 12 = 6 L/min.
- Supply flow 15 L/min ÷ MV 6 L/min = 2.5 times the patient’s demand — sufficient to keep reservoir full each breath.
- Reservoir volumes (commonly 1–2 L) exceed single breath volume (≈0.5 L), so each tidal inspiration can be supplied from a predominantly O₂ reservoir.

4) Why the correct answer is correct
- Because the NRB at 15 L/min fills and maintains the reservoir with near‑pure oxygen and one‑way valves prevent room air entry, inspired gas approaches 100% oxygen (FiO₂ ≈ 1.0) in ideal conditions.

5) Why the other options are incorrect
- 0.21: This is room air (no supplemental oxygen), not applicable with a NRB at high flow.
- 0.3–0.6: Typical for low‑flow devices like nasal cannula or simple masks, not a fully functioning NRB at 15 L/min.
- 0.6–0.8: Possible for partial rebreather or NRB with leaks/insufficient flow, but not for an NRB correctly supplied at 15 L/min with intact valves.

6) Conclusion
Given that 15 L/min maintains a filled reservoir and prevents entrainment of room air, the expected FiO₂ from a properly functioning non‑rebreather mask is 1.0. This matches the provided correct answer: 1.0.

Q4. A patient with acute hypoxemia is placed on a non‑rebreather mask at 15 L/min. What FiO₂ is expected? 单项选择题

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