The Mapleson D circuit (Bain coaxial modification) is commonly used for controlled ventilation. What is the minimum fresh gas flow (FGF) required to prevent rebreathing during IPPV in an adult?

• A 2 times minute ventilation (approximately 150–200 mL/kg/min)
• B 30 mL/kg/min
• C Equal to dead space ventilation only
• D 70 mL/kg/min ✓

Explanation

For the Mapleson D/Bain circuit during IPPV, the FGF required to prevent CO2 rebreathing is approximately 70 mL/kg/min. This is considerably less than that required for spontaneous ventilation (200 mL/kg/min), because during controlled ventilation the FGF pushes exhaled gas towards the APL valve during the expiratory phase before the next inspiration. For spontaneous breathing with Mapleson A (Magill circuit), FGF equal to minute ventilation (~70 mL/kg/min) is sufficient. The Mapleson A is most efficient for spontaneous breathing; Mapleson D (Bain) is most efficient for controlled ventilation.

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

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