A Mapleson D breathing system is used for spontaneous ventilation in a 70 kg adult. The fresh gas flow (FGF) required to prevent rebreathing during spontaneous ventilation is approximately:

• A 100–150 mL/kg/min (equal to minute ventilation)
• B 2–3 × minute ventilation (~15–20 L/min)
• C 250 mL/kg/min (2.5 × minute ventilation) ✓
• D 1 L/min (minimal flow)

Explanation

In a Mapleson D/Bain system during spontaneous ventilation, fresh gas flow (FGF) must equal approximately 2–3 times the minute ventilation to prevent rebreathing of CO2, which equates to approximately 200–250 mL/kg/min. For a 70 kg adult with minute ventilation ~7 L/min, FGF of 14–17.5 L/min (>2× MV) is required. During controlled (positive pressure) ventilation with Mapleson D, the FGF can be reduced to 70–100 mL/kg/min (about 1× MV) because expiratory time allows CO2-laden gas to be flushed out. The Mapleson D is efficient for controlled ventilation but wasteful for spontaneous breathing.

Reference: Morgan & Mikhail's Clinical Anesthesiology, 6th ed.

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